domain: sde sdv

text:
Julia Lorène

Michèle Marchand

Thomas Tarayre

Yann Dorey

Françoise Rantier

Hennion

Lorène Julia Marchand
email: lorene.marchand@univ-rennes1.fr

Michèle Tarayre

Thomas Dorey

Yann Rantier

Françoise Hennion

Marchand

geography in the Kerguelen Islands: implications for cushion necrosis and climate change

Keywords: cushion plant, Kerguelen Islands, morphological variability, necrosis, photointerpretation, sub-Antarctic

Morphological variability of cushion plant Lyallia kerguelensis (Caryophyllales) in relation to environmental conditions and

Introduction

In recent decades climate change has been faster in various parts of the world (IPCC 2013). Islands have been particularly concerned which threatened a large part of the insular floras [START_REF] Harter | Impacts of global climate change on the floras of oceanic islands -Projections, implications and current knowledge[END_REF][START_REF] Veron | Vulnerability to climate change of islands worldwide and its impact on the tree of life[END_REF]. Indeed, within species, to counter rapid climate changes, shift of geographical area, individuals' plastic responses or populations' genetic adaptation might occur [START_REF] Hoffmann | Climate change and evolutionary adaptation[END_REF][START_REF] Noble | Plastic responses to novel environments are biased towards phenotype dimensions with high additive genetic variation[END_REF]. However geographical shift of area will be limited for native insular plant species (Chen et al. 2011). Insular plants will thus have to respond and keep up with the speed of climate change, which will be particularly challenging if they are living close to their physiological limits [START_REF] Moritz | The Future of Species Under Climate Change: Resilience or Decline?[END_REF]. This is eminently interesting in the sub-Antarctic islands where climate change is extremely fast [START_REF] Smith | Climate Change in the Sub-Antarctic: An Illustration from Marion Island[END_REF][START_REF] Convey | Influences on and origins of terrestrial biodiversity of the sub-Antarctic islands[END_REF][START_REF] Lebouvier | The significance of the sub-Antarctic Kerguelen Islands for the assessment of the vulnerability of native communities to climate change, alien insect invasions and plant viruses[END_REF]) and plants are living close to such physiological limits (Hennion et al. 2006b). In the near future, global climate change is predicted to have particularly strong effects at higher latitudes in the (sub-)polar regions (IPCC 2013(IPCC , 2014)). For instance, according to the Representative Concentration Pathway scenario (RCP) 6.0, an increase in minimum temperatures of about +1 to +1.5°C is expected before the end of C o r r e c t e d M a n u s c r i p t 3 the current century, along with up to 10.0% change in precipitation, which may cause considerable stress for plants restricted to cold habitats [START_REF] Harter | Impacts of global climate change on the floras of oceanic islands -Projections, implications and current knowledge[END_REF]. Temperature seasonality will also be affected, inducing an earlier and longer growth period, lower frequency of freeze-thaw cycles, and higher frequency of droughts [START_REF] Boelhouwers | The maritime Subantarctic: a distinct periglacial environment[END_REF][START_REF] Lebouvier | The significance of the sub-Antarctic Kerguelen Islands for the assessment of the vulnerability of native communities to climate change, alien insect invasions and plant viruses[END_REF]IPCC 2013;[START_REF] Harter | Impacts of global climate change on the floras of oceanic islands -Projections, implications and current knowledge[END_REF]. Such is the case of the Kerguelen Islands, where the climate is chronically cool and windy year-round, as is typical for the sub-Antarctic islands generally [START_REF] Convey | Overwintering strategies of terrestrial invertebrates from Antarctica -the significance of flexibility in extremely seasonal environments[END_REF], with a mean annual temperature of 4.84°C for the period 1976-2008 [START_REF] Lebouvier | The significance of the sub-Antarctic Kerguelen Islands for the assessment of the vulnerability of native communities to climate change, alien insect invasions and plant viruses[END_REF]. Overall since 1950 mean annual temperature increased (+0.016 °C.year -1 ). Rainfall tends to decrease across time with two long periods of rainfall below the average (1962-1976 and 1991-2011) and two short periods of rainfall around the average (1977-1991 and 2011 until now) . These climatic changes have led to the retreat of the Cook ice cap which is among one of the most drastic glaciers retreats on Earth [START_REF] Verfaillie | Recent glacier decline in the Kerguelen Islands (49°S, 69°E) derived from modeling, field observations, and satellite data[END_REF][START_REF] Favier | Atmospheric drying as the main driver of dramatic glacier wastage in the southern Indian Ocean[END_REF].

The sub-Antarctic Islands form a biogeographical unit, even if there are variations in their origins and locations [START_REF] Shaw | Taxonomic homogenization and differentiation across Southern Ocean Islands differ among insects and vascular plants[END_REF]). Kerguelen Islands with three other archipelagos (Prince Edward Islands, Crozet archipelago and Heard and Mc Donald Islands) form the South Indian Ocean Province with unique biodiversity and a high degree of endemism [START_REF] Hennion | Ecology and seed morphology of endemic species from Kerguelen Phytogeographic Zone[END_REF][START_REF] Van Der Putten | Subantarctic flowering plants: preglacial survivors or post-glacial immigrants?[END_REF]. Impacts of climate change on vegetation have rapidly become apparent in the sub-Antarctic Islands with for instance occasional wilting of native plants, or dieback in several cushion plants [START_REF] Frenot | Human activities, ecosystem disturbance and plant invasions in subantarctic Crozet, Kerguelen and Amsterdam Islands[END_REF]Hennion et al. 2006a;[START_REF] Roux | Rapid range expansion and community reorganization in response to warming[END_REF][START_REF] Bergstrom | Rapid collapse of a sub-Antarctic alpine ecosystem: the role of climate and pathogens[END_REF]). Here we focus on the cushion plant Lyallia kerguelensis Hook.f. (Montiaceae), strictly endemic to Kerguelen, a monotypic genus and phylogenetically isolated in its family [START_REF] Wagstaff | Evolution and biogeography of Lyallia and Hectorella (Portulacaceae), geographically isolated sisters from the Southern Hemisphere[END_REF][START_REF] Nyffeler | Disintegrating Portulacaceae: a new familial classification of the suborder Portulacineae (Caryophyllales) based on molecular and morphological data[END_REF]. Partial necrosis of L. kerguelensis cushions was first described by [START_REF] Hennion | Etude des caracteristiques biologiques et génétiques de la flore endémique des ïles Kerguelen[END_REF], being absent from all previous descriptions of the species [START_REF] Hooker | The Botany of the Antarctic Voyage of H.M. discovery ships Erebus and Terror in the years 1839-1843 under the command of Captain Sir James Clarke Ross[END_REF][START_REF] Werth | Spatial and temporal variation in damage and dieback in a threatened subantarctic cushion species[END_REF][START_REF] Chastain | La flore et la végétation des îles Kerguelen. Polymorphisme des espèces australes[END_REF], and thereby suggesting it might be a recent phenomenon in this species (Fig. 1). Furthermore, as partly necrotic cushions were mostly observed in the drier parts of the island it was suggested that this necrosis may be induced by dry conditions [START_REF] Hennion | Etude des caracteristiques biologiques et génétiques de la flore endémique des ïles Kerguelen[END_REF][START_REF] Wagstaff | Evolution and biogeography of Lyallia and Hectorella (Portulacaceae), geographically isolated sisters from the Southern Hemisphere[END_REF]. Therefore, necrosis in L. kerguelensis cushions appeared variable and possibly related to local climatic conditions.

Cushion plants are often typical of areas that experience cold or cool climates, low precipitation and/or strong winds in various regions of the Earth [START_REF] Körner | Alpine plant life: functional plant ecology of high mountain ecosystems[END_REF][START_REF] Körner | Plant adaptation to cold climates[END_REF][START_REF] Boucher | Evolution and biogeography of the cushion life form in angiosperms[END_REF]. Such growth forms have a low surface area to volume ratio, which aids in reduction of heat and water losses [START_REF] Körner | Alpine plant life: functional plant ecology of high mountain ecosystems[END_REF][START_REF] Aubert | 1914-2014: A revised worldwide catalogue of cushion plants 100 years after Hauri and Schröter[END_REF]. Typically, in plants morphology is among the functional traits that vary in response to environmental, including climatic conditions. A few examples show that morphology of cushion plants may respond to environment. Indeed cushion size was in relation to altitudinal gradient, a proxy of temperature and precipitation in alpine environments [START_REF] Kleier | Size class structure, growth rates, and orientation of the central Andean cushion Azorella compacta[END_REF]Zhao et al. 2018). Azorella compacta (Apiaceae) plants present a larger surface area on the cushion part with northern exposure side which maximizes solar radiation [START_REF] Kleier | Size class structure, growth rates, and orientation of the central Andean cushion Azorella compacta[END_REF].

C o r r e c t e d

M a n u s c r i p t 4 Moreover, air circulation and windblown sediment around cushions may explain the initial development of the crescent shape in Azorella selago [START_REF] Combrinck | A computational fluid dynamic analysis of the airflow over the keystone plant species, Azorella selago, on subantarctic marion island[END_REF][START_REF] Combrinck | Wind and seed: a conceptual model of shape-formation in the cushion plant Azorella selago[END_REF].

Hence morphological variation in size and shape can be considered as a proxy of the response capacity of cushion plants to environmental, including climate variation [START_REF] Körner | Plant adaptation to cold climates[END_REF]. In contrast, when changes are fast low response capacity is expected. Direct drought sensitivity was demonstrated experimentally in Marion Island for the cushion plant A. selago [START_REF] Roux | Effects of a short-term climate change experiment on a sub-Antarctic keystone plant species[END_REF], with earlier senescence (i.e. stem death) apparent in cushions with lower water availability. Dieback, or large scale necrosis, is defined as a process during which portions of the cushion, or even the entire cushion, can no longer regenerate new foliage [START_REF] Armesto | Altitudinal distribution, cover and size structure of umbelliferous cushion plants in the high Andes of Central Chile[END_REF]. Studies of the spatial dynamics of dieback in the congeneric plant A. macquariensis from Macquarie Island highlight a relation of the dieback to a potential decrease in soil water availability under recent climate change rather than a pathogenic origin. Nevertheless, stressed plants might be more sensitive to infections [START_REF] Werth | Spatial and temporal variation in damage and dieback in a threatened subantarctic cushion species[END_REF][START_REF] Bergstrom | Rapid collapse of a sub-Antarctic alpine ecosystem: the role of climate and pathogens[END_REF][START_REF] Dickson | Spatial variation in the ongoing and widespread decline of a keystone plant species[END_REF]. Finally, necrosis can be related to wind intensity, with more necrosis on the more exposed part of the cushion [START_REF] Kleier | Microsite requirements, population structure and growth of the cushion plant[END_REF]. These observations suggest that necrosis may be due to or enhanced by climate change.

Measuring the variability of necrosis extent and morphology in L. kerguelensis more generally could provide a proxy for the response capacity of the plant to climate change. L. kerguelensis could be considered as a model case to study response capacity of an endemic plant species to climatic changes.

In the current study, we assumed that the morphological traits of L. kerguelensis co-vary so as to maintain a hemispherical shape that is optimal in harsh environments. We then predicted that morphological variation, including necrosis extent would be present across individuals and populations. We hypothesized that water availability, wind exposure and slope aspect were likely primary drivers of necrosis. To test these hypotheses, we (i) searched relations between growth traits and the entire morphology (further called allometry) in L. kerguelensis to select appropriate traits for assessing inter-population variability, and (ii) sought to identify relationships between this interpopulation variability and environmental conditions on one hand and geographical distances on the other hand. Finally, we (iii) studied the influence of necrosis on the cushion morphology and attempted to identify which environmental conditions are related to necrosis.

Materials and methods

Lyallia kerguelensis Hook.f. [START_REF] Nyffeler | Disintegrating Portulacaceae: a new familial classification of the suborder Portulacineae (Caryophyllales) based on molecular and morphological data[END_REF], Montiaceae, is a long-lived perennial herb, persisting at least 16 years [START_REF] Wagstaff | Evolution and biogeography of Lyallia and Hectorella (Portulacaceae), geographically isolated sisters from the Southern Hemisphere[END_REF]. It forms roughly round-shaped cushions (Fig. 1) 20 -40 cm across, exceptionally up to 1 m, in small and sparse populations [START_REF] Hennion | Ecology and seed morphology of endemic species from Kerguelen Phytogeographic Zone[END_REF][START_REF] Wagstaff | Evolution and biogeography of Lyallia and Hectorella (Portulacaceae), geographically isolated sisters from the Southern Hemisphere[END_REF]). The root system is strong and deep, composed of a main thick deep root branching out with numerous fine roots [START_REF] Werth | Spatial and temporal variation in damage and dieback in a threatened subantarctic cushion species[END_REF][START_REF] Chastain | La flore et la végétation des îles Kerguelen. Polymorphisme des espèces australes[END_REF]. Furthermore, we often observed aerial and slightly flexible roots that radiate from cushions and penetrate the soil at a C o r r e c t e d M a n u s c r i p t distance, likely functioning as guys to anchor the plant in the face of strong winds. L. kerguelensis distribution in Kerguelen is sparse, with an altitude range from sea level to around 500 m a.s.l. [START_REF] Hennion | Ecology and seed morphology of endemic species from Kerguelen Phytogeographic Zone[END_REF][START_REF] Wagstaff | Evolution and biogeography of Lyallia and Hectorella (Portulacaceae), geographically isolated sisters from the Southern Hemisphere[END_REF]; Réserve Naturelle TAF, IPEV Subanteco and PlantEvol, unpublished) (Fig. 1). The species is restricted to windswept stony habitats in fellfields or moraines [START_REF] Wagstaff | Evolution and biogeography of Lyallia and Hectorella (Portulacaceae), geographically isolated sisters from the Southern Hemisphere[END_REF]. Fellfields (or "wind deserts") are extensive, sterile, rocky areas present from the shore to the highest elevations, and more generally characteristic of polar regions [START_REF] De La Rüe | Observation sur les caractères et la répartition de la végétation des îles Kerguelen[END_REF]. Vegetation is patchy, mainly composed of lichens and bryophytes [START_REF] Smith | Terrestrial plant biology of the sub-Antarctic and Antarctic[END_REF][START_REF] Block | Strategies of survival and resource exploitation in the Antarctic fellfield ecosystem[END_REF].

Our study was based on four field campaigns in the Kerguelen Islands (48°30' -50°S, 68°27' -70°35'E ; [START_REF] Lebouvier | Conservation and management in the French sub-Antarctic islands and surrounding seas[END_REF]. The first three campaigns were from December to January (2015-2016, 2016-2017 and 2017-2018), the last campaign was from December 2018 to March 2019.

In the current study, we analysed 319 plants from 19 populations (Fig. 2, Online Resource Table 1) in three different regions of Kerguelen Islands: the northern part with Port Christmas (two populations) and Ile MacMurdo (two populations), the south-western part with one population at Péninsule Rallier du Baty, and the south-eastern part including populations in Vallée des Souilles in Presqu'île Ronarch (three), Ile Australia north (three) and south (four), Ile Longue (two) and Plateau du Vent in Presqu'île Jeanne d'Arc (two).

For each population, general environmental characteristics were recorded regarding types of fellfield (slope, scree slope, plateau, summit, outwash plain), topography (flat, terrace, base or mid or top of slope, summit, other); wind exposure (very sheltered, sheltered, exposed, very exposed); slope; distance to sea; slope aspect; visual estimation of cover in percentage of blocks (>20cm), pebbles (2cm to 20 cm), gravel (2mm to 2 cm), bare soil (<2mm), bryophytes and herbs. Soil conductivity, water saturation and temperature were measured with a TRIME (Time Domain Reflectometry with intelligent Micro Elements) probe. Altitude was determined using GPS (eTrex 10, Garmin). For each sampled population, three samples of bare soil, 200 mL in total, were collected at different locations around cushions to provide a composite soil sample. Half of the soil sample was immediately stored at -20°C for subsequent determination of nitrate concentration (Laboratoire d'Analyse des Sols, INRA, Arras, France). A small portion of the remaining fresh soil was mixed with 30 mL of distilled water and then left for 18-24 h to permit the sedimentation of soil particles, and pH was then determined using a pH meter (BASIC 20 PLUS CRISON, resolution 0.01 pH) [START_REF] Hermant | Endemic species have highly integrated phenotypes, environmental distributions and phenotype-environment relationships[END_REF]). The remaining soil was dried at 105°C for 48 h, being weighed before and after drying. The dried soil was used to determine soil texture and composition (Laboratoire d'Analyse des Sols, INRA, Arras, France) (Online Resource Table 2). The studied cushions were selected following categories identified in an earlier study (Dorey 2017, unpublished). Dorey (2017, unpublished) applied a random sampling of 83 L. kerguelensis cushions in 11 populations selected for their contrasted environments. With exploratory analyses they observed that the percentage of explanation of environmental variables was reduced with the increase of necrosis extent. They found that cushions above a threshold of 10% C o r r e c t e d M a n u s c r i p t 6 necrotic surface area have lower relationship with environmental variables than below this threshold (Online Resource Fig. 1). In our study, we sampled cushions below and above this threshold to better characterize the relationship between the necrosis extent and specific environmental variables for each group. Within each population, we selected the same number of cushions from each category, defining these as 'vigorous cushion' (visually less than 10% of necrotic extent), and 'necrotic cushion' (visually 10% to 80% of necrotic extent). The maximum proportion of necrosis retained in the study was arbitrarily set at 80%, above which we considered the cushion as too affected to be reactive.

For each cushion, pictures were taken vertically from above the cushion and with a scale, without shade on the cushion, allowing to see the top of each apex. An extraction protocol to measure morphological traits by photointerpretation (Dorey 2017, unpublished) was then applied. The cushion height was measured in the field for 229 cushions (data not available for AUS23, AUSN1, AUSN2 and AUSN4 populations). Image analyses were performed with Arcgis 10.6.1. We determined the perimeter of the cushion (Online Resource Fig. 2a), the total surface area of the cushion (Online Resources Fig. 2b), the percentage of neighbouring plants (Online Resource Fig. 2c), the necrotic extent (Online Resources Fig. 2e); the shorter, longer radius and the cushion shape (Online Resources Fig. 2f). Finally, we calculated a cushion compactness proxy [START_REF] Cranston | Variation and response to experimental warming in a New Zealand cushion plant species: Cushion Plant Responses to Warming[END_REF] using the following formula:

We calculated also two ratios, height and short radius and height and long radius of the cushion to provide a proxy of the 3D shape of the cushion. A total of 11 morphological and biotic traits were recorded and their mean values and standard deviations per population are provided in Table 1.

All statistical analyses were carried out in the open-source programming language R, version 3.4.2.

Graphs were produced with the ggplot2 package [START_REF] Wilkinson | the Grammar of Graphics[END_REF]. Random distributions of morphological traits were verified within both necrotic categories and were then pooled except for the necrotic extent trait. Spearman correlogram (PerformanceAnalytics package) was used to determine the allometry of the traits of the 319 individuals, n=279 for variables using height. Based on these allometric relationships, only the surface area, compactness and shape traits were used in subsequent analyses. To determine the significance of inter-population variation in morphological traits, a Kruskal-Wallis test was applied. To study the relationships between cushion morphology and environment, the environmental conditions were divided into six categories (topography and habitat, slope aspect, surface texture, soil texture, soil chemical composition and soil physical characteristics) as shown in Manhattan distance for the geographical coordinates, and 999 permutations for robustness of the pvalue were chosen.

Results

Allometry of traits and morphological variability

The surface area, perimeter, and height of the cushions were strongly positively correlated one to each other (Table 3). However, the cushion compactness was correlated only with the cushion height and the cushion shape was correlated to none of the other morphological traits. Mean cushion shape ratio (relation between short and long diameter) was 0.75 (n = 319, sd = 0.01) with low variation indicating that cushions approximate an ovoid cushion shape. Moreover, the ratios between height and short or long radius were also close to 1 (n = 279, mean ± sd; 0.85 ± 0.03 and 1.17 ± 0.04 respectively). For vigorous and non-vigorous cushions the relationships between the necrosis extent and the three morphological traits (cushion surface area, shape, and compactness) were not significant.

Biotic traits relating to the neighbouring plant community (epiphyte proportion and neighbouring plant proportion) were positively correlated one to each other but also with most of the morphological traits, except for cushion height (not correlated with the proportion of epiphytes) and shape (correlated with none of these biotic traits) (Table 3).

To determine inter-population variability in plant morphological traits, Kruskal-Wallis tests were performed. Significant variation between populations was detected for the cushion surface area (Chisquared test, X 318 = 94.011, p < 0.0001), compactness (Chi-squared test, X 318 = 134.47, p < 0.0001) and shape (Chi-squared test, X 318 = 29.292, p = 0.0377). Between populations, for vigorous cushions the necrosis extent varied significantly (Chi-square test, X 18 = 35.943, p = 0.0072). In contrast, for necrotic cushions inter-population variations of necrosis extent were not significant (Chi-square test, X 18 = 16.348, p = 0.5683).

Relationships between cushion morphology and environment or geography

The variability of the cushion surface area across populations was well explained by the topography and habitat, slope aspect and soil texture models, explaining 75%, 54% and 19% of overall variation respectively (Table 4). In the first model, two conditions were significant, topography and wind C o r r e c t e d M a n u s c r i p t 8 exposure, without significant interaction. A post-hoc test showed that larger cushions were present mid-slope rather than on flat areas (Fig. 3a). A non-significant trend of larger surface area with a decrease of wind exposure was found. For the slope aspect model, the post hoc test did not identify any significant slope aspect influencing the cushion surface area. Finally, soil texture was positively related to the cushion surface area, with a larger percentage of coarse sand being correlated to a larger cushion surface area. Inter-population variability in cushion shape was explained (52% of variation) by the univariate model with slope aspect variable (Table 4), with a lower circular shape preferentially observed with south-east slope aspect compared to locations with south-west or north-east slope aspects or flat locations (Fig. 3b). Besides, variability in cushion compactness was significantly related to the surface texture model (18% of variation; Table 4), and was positively correlated to the proportion of bare soil (Pearson correlation, r = 0.478, n = 19, p= 0.039). To determine relationships between population morphology and geography, we applied Mantel tests separately for each morphological trait. For cushion surface area and shape the results of the Mantel tests were insignificant, respectively, r = -0.178, p = 0.933 and r = -0.022, p = 0.441. The results were significant only for the cushion compactness (r = 0.320, p = 0.011), where longer distances across populations were related to higher inter-population variability of cushion compactness but not of cushion shape or surface area.

Relationship between cushion necrosis and environment or geography

We investigated whether environmental variables explained necrosis extent across populations. For necrotic cushions models were insignificant (data not shown). For vigorous cushions, only the sodium concentration (ANOVA, F 14 = 0.081, p = 0.0219) was significant in the soil composition model (adjusted R² = 27%, Fig. 4) Mantel tests of the necrosis / geographic distance relationships were significant for neither vigorous nor necrotic cushions (r = -0.05, p = 0.645; and r = -0.05, p = 0.530, respectively).

Discussion

Cushion allometry was observed for most of the morphological traits analysed. Three traits were selected as representative of the allometry: cushion surface area, shape and compactness. For each trait, the variation among the populations could be explained by environmental and geographical variables. Finally, necrosis extent did not vary with morphological traits but in vigorous cushions inter-population variation was related to soil composition.

Allometry of traits and morphological variability

A previous study indicated a strong relationship between height and diameter in L. kerguelensis [START_REF] Hermant | Endemic species have highly integrated phenotypes, environmental distributions and phenotype-environment relationships[END_REF]). Here we determined that along with height and diameter, shape and The degree of necrosis was not related to the morphological traits studied, suggesting that necrosis is not encouraged by a particular morphology, contrary to our initial expectation that necrosis would cause sagging and change in cushion shape leading to lower cushion compactness as observed in Azorella macquariensis [START_REF] Bergstrom | Rapid collapse of a sub-Antarctic alpine ecosystem: the role of climate and pathogens[END_REF]. Vigorous cushions (<10% necrosis) showed interpopulation variability in mean necrosis, varying from 0.70% to 5.18%. This suggests that some populations are less subject to necrosis or that recovery can occur, as has been noted in a previous survey (Dorey, 2017 unpublished). However, for necrotic cushions (>10% necrosis) inter-population variation was not significant, confirming the presence of necrosis in all the populations studied without influence of specific environmental conditions.

Relationships between environment, geography and morphology

The surface area of cushions was related to topography and wind exposure, with larger cushions predominantly developing mid-slope rather than on flat areas, and a trend of larger cushions in less wind-exposed populations. In the fragmented landscape and harsh climate of Kerguelen, topography and wind are two major environmental components. The hilly topography of the island (Fig. 2) generates heterogeneity in rainfall, water retention and wind exposure (Aubert de la Rüe 1964;

Wagstaff and Hennion 2007). Mid-slope areas receive runoff water from higher altitude and at the time drain to lower altitude, unlike flatter areas, while wetlands usually develop at the base of slopes.

The Kerguelen Islands, like other sub-Antarctic islands, are considered to have moist climates (Hennion et al. 2006b) and sub-Antarctic plants are known for their requirement of water for growth [START_REF] Dorne | Physiological adaptation to sub-antarctic climate by the kerguelen cabbage, Pringlea-antiscorbutica R[END_REF][START_REF] Hennion | Ecology and seed morphology of endemic species from Kerguelen Phytogeographic Zone[END_REF]. Thus, mid-slope may provide optimal water supply for the growth of L. kerguelensis. Furthermore, wind exposure induces mechanical stress and can accelerate transpiration of plants [START_REF] Körner | Alpine plant life: functional plant ecology of high mountain ecosystems[END_REF][START_REF] Haussmann | Fine scale variability in soil frost dynamics surrounging cushions of the dominat vascular plant species (Azorella selago) on sub-antarctic Marion island[END_REF][START_REF] Gardiner | Review: Wind impacts on plant growth, mechanics and damage[END_REF].

Plants from L. kerguelensis developing in less windy environments such as mid-slope may be less subject to extreme meteorological events like storms or severe frost. Finally, contents of coarse sand in the underlying soil was positively correlated to cushion surface area. Sandy soils are generally well C o r r e c t e d M a n u s c r i p t 10 draining, with low water storage capacity and a high usable fraction [START_REF] Körner | Alpine plant life: functional plant ecology of high mountain ecosystems[END_REF]. This finding suggests that ease of water uptake favours larger cushions.

Variation in cushion shape between populations was mainly explained by slope aspect. In Kerguelen, north and north-west slope aspects provide the greatest solar exposure but are also the most exposed to wind, and vegetation is more luxurious away from these slope aspects (south or east aspect) [START_REF] Werth | Spatial and temporal variation in damage and dieback in a threatened subantarctic cushion species[END_REF]; Aubert de la Rüe 1964). Our data indicated lower occurrence of circular cushions on the southeast aspect, less exposed to wind, suggesting that the circular shape might be associated with wind exposure in L. kerguelensis. This finding is in agreement with the recent air flow dynamics model on A. selago where cushions with more pronounced crescent shapes were in habitats with less airflow turbulence [START_REF] Combrinck | Wind and seed: a conceptual model of shape-formation in the cushion plant Azorella selago[END_REF].

Variability in cushion compactness was partly explained by the proportion of bare soil and geographic distance between populations. Greater cushion compactness was correlated with a higher proportion of bare soil. This may indicate lower protection against wind than on rocky ground, possibly in relation to greater exposure to cold or desiccation stressed. It is interesting to note that compactness in cushion plants generally co-varies with altitude, that may be correlated to temperature and humidity variation [START_REF] He | Variation of biomass and morphology of the cushion plant Androsace tapete along an elevational gradient in the Tibetan Plateau: Cushion Plant Morphology Along Elevation[END_REF][START_REF] Cranston | Variation and response to experimental warming in a New Zealand cushion plant species: Cushion Plant Responses to Warming[END_REF]Zhao et al. 2018). However, L. kerguelensis did not show compactness variation across the altitudinal gradient examined here. In summary, cushion surface area, shape and compactness were all driven primarily by two major environmental variables: wind intensity and water availability.

Relationships between environment, geography and necrosis

The only environmental variable significantly linked with variation in necrosis was soil sodium content for cushions with low necrosis extent only. The nearest distance to the sea was negatively related to soil sodium concentration (t 17 = 2.27, p = 0.036), and a decrease with altitude was observed.

Salt is carried in wind-blown sea spray from which it enters the soil, and in the sub-Antarctic marine salt may be found far from the coast due to permanent strong winds [START_REF] Smith | Chemical composition of precipitation at Marion Island (sub-antarctic)[END_REF][START_REF] Jenkin | Vegetation of the McDonald Islands, sub-Antarctic[END_REF].

Furthermore, in Marion Island salinity gradient with altitude and distance from the sea is observed [START_REF] Conradie | Spatial Variation in Soil Chemistry on a Sub-Antarctic Island[END_REF]. The mean soil sodium concentration was similar in our sites in Kerguelen to that in Marion Island [START_REF] Conradie | Spatial Variation in Soil Chemistry on a Sub-Antarctic Island[END_REF], supporting the hypothesis of a similar marine influence in both islands. Nevertheless necrosis extent in L. kerguelensis was not directly related to the nearest distance to the sea (t 17 = -1.43, p = 0.170).

From a physiological point of view, sodium induces adjustment of osmotic potential in plants through various mechanisms [START_REF] Navarro | Effects of sodium chloride on water potential components, hydraulic conductivity, gas exchange and leaf ultrastructure of Arbutus unedo plants[END_REF][START_REF] Shabala | Learning from halophytes: physiological basis and strategies to improve abiotic stress tolerance in crops[END_REF]. Ion homeostasis (partly with sodium) is achieved during abiotic stress and allows to keep an osmotic potential sufficient for water absorption and then sustain turgor of the cells [START_REF] Pardo | Plants and sodium ions: keeping company with the enemy[END_REF]. Furthermore, sodium at high concentration can reduce stomata density and then alter photosynthesis [START_REF] Orsini | Beyond the ionic and osmotic response to salinity in Chenopodium quinoa: functional elements of successful halophytism[END_REF]). In the sub-Antarctic region, studies of salt stress have been performed on several plant species some of which are C o r r e c t e d M a n u s c r i p t 11 spread from the coast up to fellfields, concluding that such plants could be defined as glycophytes tolerant of salt stress [START_REF] Smith | Plant responses to osmotic stress in the coastal zone of Marion Island[END_REF][START_REF] Hennion | Accumulation of organic and inorganic solutes in the subantarctic cruciferous species Pringlea antiscorbutica in response to saline and cold stresses[END_REF][START_REF] Hummel | Characterization of environmental stress responses during early development of Pringlea antiscorbutica in the field at Kerguelen[END_REF]). For L.

kerguelensis, higher sodium concentrations in soil may add to other abiotic stresses in their habitats and increase their sensitivity to other osmotic stresses such as drought as was found in other plants [START_REF] Jamshidi Goharrizi | Combined effects of salinity and drought on physiological and biochemical characteristics of pistachio rootstocks[END_REF]. Taken together, our data suggest that necrosis may be related to water availability, although a pathogenic origin cannot be excluded. Our results are consistent with the findings on A. macquariensis where dieback was related to summer water availability [START_REF] Werth | Spatial and temporal variation in damage and dieback in a threatened subantarctic cushion species[END_REF][START_REF] Bergstrom | Rapid collapse of a sub-Antarctic alpine ecosystem: the role of climate and pathogens[END_REF][START_REF] Dickson | Spatial variation in the ongoing and widespread decline of a keystone plant species[END_REF] and do not support a pathogenic origin.

Possible impact of climate change on L. kerguelensis

In summary, cushion surface area, shape and compactness were primarily driven by two environmental variables: wind intensity and water availability. Cushion necrosis also seems to be linked with multiple stresses and mainly osmotic stresses such as drought. However environmental changes expected with the RCP 6.0 model include an overall +1.26 to + 1.50°C temperature increase by 2081-2100, along with 0.1% to 10.0% lower water availability [START_REF] Harter | Impacts of global climate change on the floras of oceanic islands -Projections, implications and current knowledge[END_REF], which will increase stress for plant populations already facing low water availability or unfavorable slope aspect, exposing them to higher wind desiccation. Indeed, change already observed in precipitation in the Kerguelen Islands over the past 60 years [START_REF] Lebouvier | The significance of the sub-Antarctic Kerguelen Islands for the assessment of the vulnerability of native communities to climate change, alien insect invasions and plant viruses[END_REF]) may have triggered necrosis or enhanced the process, as observed on Macquarie Island with increased wind speed, hours of sunshine and evapotranspiration over 17 years [START_REF] Bergstrom | Rapid collapse of a sub-Antarctic alpine ecosystem: the role of climate and pathogens[END_REF][START_REF] Dickson | Spatial variation in the ongoing and widespread decline of a keystone plant species[END_REF]. Under the scenario outlined above cushion necrosis in L. kerguelensis may therefore become increasingly frequent.

Conclusions

Our study confirms that L. kerguelensis cushions tend to form a hemispherical shape. The size and compactness of the plant vary with environmental conditions (topography, slope aspect, wind exposure and soil texture) and geography, all of which are related to wind intensity and/or water Table 1: Morphological traits of L. kerguelensis and biotic traits per population (mean +/-standard deviation): number of individuals, cushion surface area, cushion perimeter, cushion compactness, cushion shape, cushion height, ratio height/short radius, ratio height/long radius, proportion of neighbouring plants in contact with the perimeter, proportion of epiphytes across the cushion surface area, necrosis extent for vigorous cushions (defined with less than 10% necrosis extent) and necrosis extent for necrotic cushions (defined with more than 10% necrosis extent). NA= data not available.

For necrosis extent of vigorous and necrotic cushions mean and standard deviations were calculated per category per population. ]3-10°],]10-30°],]30-80°]); distance to the closest sea shore; slope aspect (N, NW, W, SW, S, SE, E, without dominant slope aspect); visual estimation of blocks (>20cm), pebbles (2cm to 20 cm), gravel (2mm to 2 cm), bare soil (<2mm), bryophyte and herbaceous cover in percentage.

Online Resource

Soil physical characteristics

Conductivity 50 µs cm ^-1 to 996 µs cm -1 pH 5.2 to 6.9. Water saturation 5.77% to 60.79%. Organic matter 7.34 to 72.1 g kg -1

Online Resource Figure 1: Redundancy analysis (RDA) explanation percentage; from Dorey 2017, unpublished; cushions were selected randomly and determination of necrosis extent was done with the same methods as in our study detailed in Online Resource Figure 2; two groups are defined: 0-10% necrosis extent (n=42) and 10-100% necrosis extent (n=41). Black dots represent RDA explanation percentage for environmental variables (altitude, slope, % block cover, % pebbles cover, % gravel cover, % bare soil cover, % bryophyte cover, % herbaceous cover, mean height of neighbouring plants, neighbouring plant proportion) and black triangles represent RDA explanation percentage for soil variables (soil water content, pH, conductivity, soil depth, clay, fine silt, coarse silt, fine sand, coarse sand, C N -1 , organic matter, CEC, NH 4 + , NO 3 -). Other thresholds were tried but were less significant (Dorey 2017, unpublished). Point features were used to determine the number of vigorous (green) and necrotic (red) apices. (e) A circular area surrounding each point entity was created per apex category with a fixed size, then merged to create a unique area entity, avoiding potential superposition of the circular areas; vigorous surface area (light pink) and necrotic surface area (magenta) were determined respectively, then a ratio between the total surface area of the cushion and the necrotic surface area was done and further called percentage of necrosis extent. (f) A cushion shape proxy was generated, the smallest rectangle (white)

Online

where the cushion fit was automatically defined around the perimeter polygon, the middle of the rectangle is defined as the centroid of the cushion (red cross). Cushion diameters (short and long) were assessed by measurement of the two orthogonal diameters at the centroid up to the perimeter polygon (yellow lines) (extension ET GeoWizards), short and long radius are respectively half of the short and long diameter. If necessary another polygonal feature was applied to delimit the surface area of the epiphytes (plants growing on the cushion) which are present on some cushions.

  correlated. Similar allometry has been reported elsewhere in other species of cushion plants[START_REF] Niklas | Plant allometry: the scaling of form and process[END_REF]. Besides, shape ratio showed low variation suggesting a conserved circular shape, and height-radius ratio close to 1 strengthened the hemispherical shape of L. kerguelensis cushions. Our study confirms the position of L. kerguelensis in the class C1 (hemispherical, compact and hard cushion) of[START_REF] Aubert | 1914-2014: A revised worldwide catalogue of cushion plants 100 years after Hauri and Schröter[END_REF]. Positive correlations were also observed between cushion morphology and biotic variables, suggesting that larger cushions of L. kerguelensis (surface area, perimeter and compactness) provide greater opportunity to trap dispersing seeds and are favorable environments for other plant species. Furthermore larger and older cushions might have provided a longer period of colonization opportunities for epiphytes. Similar results were observed for various cushion plants in the high Andes[START_REF] Arredondo-Núñez | How beneficial are nurse plants? A meta-analysis of the effects of cushion plants on high-Andean plant communities[END_REF]).

  Figures and Tables

Figure 1 :

 1 Figure 1: Lyallia kerguelensis cushion. An epiphytic Festuca contracta T. Kirk plant is present in the cushion, the neighbouring plants on the left of the picture are Agrostis magellanica Lam.. Necrotic and vigorous apices are shown (respectively red and white arrows).

Figure 2 : 1 Figure 3 :

 213 Figure 2: Locations of the 19 studied populations of Lyallia kerguelensis (black triangles) in Kerguelen Islands on an altitudinal background (NASA -SRTM 30M, 2005); the black dot indicates the research station, labels indicate important features in the island. Top right map: distribution of L. kerguelensis on Kerguelen Islands (black dot) in April 2018; data from the Réserve Naturelle Terres Australes Françaises and Institut Polaire Français, programs 136 and 1116) map background from IGN 1/200 000 georeferenced with spatial adjustment (D. Fourcy, unpublished). The small globe sets the Kerguelen Islands in the South Indian Ocean. ArcGIS 10.8.1

Figure 4 :

 4 Figure 4: Relationship between soil sodium concentration and mean necrosis extent (%); a panel: linear regression t 14 = 2.922, p = 0.0112, n=16The grey strips represent the 95% confidence interval.

Resource Figure 2 :

 2 Analyses were done with Arcgis 10.6.1, using a WDS_1994_World_Mercator spatial reference. (a) The rule with a 90° angle, next to the cushion is used to calibrate the system as the length between the red crosses is indicated on the rule. (b) Polygonal entity was created to determine the perimeter of the cushion (blue line) and the surface area. (c) The proportion of the perimeter merging with other plants, referred to as the percentage of neighbouring plants (orange line). (d) Each apex is composed of an apical meristem, terminal leaves visible on the pictures and older leaves hidden inside the cushion (thus not visible on the picture).

Table 2 .

 2 In each category, Spearman correlogram was used to determine the covarying variables. A multivariate model was established between each morphological trait and the environmental categories. Normality and heteroscedasticity were checked respectively with the

	𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛
	=	𝐶𝐶𝐶𝐶𝑛𝑛𝑎𝑎 𝑛𝑛𝑛𝑛𝐶𝐶𝑛𝑛𝑛𝑛𝑛𝑛

7

Shapiro test

[START_REF] Royston | Algorithm AS181: The W test for Normality[END_REF]

and the ncvTest (package car, Fox and Weisberg 2019) and, if necessary, a box-cox transformation was applied (package MASS,

[START_REF] Venables | Modern Applied Statistics with S, Four[END_REF]

. Then an AIC backward (package MASS) analysis was applied to obtain a minimum adequate model. For the multivariate cushion surface area and shape model with topography and habitat, a post hoc Emmeans test was applied with the Tukey method (emmeans package). Finally, to study geographical distance between populations a Mantel test (Ade4 package) was performed; Euclidian distance for the trait,

Table 2 :

 2 Environmental characteristics used in the multivariate models of the 19 populations of L.

kerguelensis studied. Topography and type of habitat: altitude, type of fellfield (slope, scree slope, plateau, summit, outwash plain), topography (flat, terrace, base or mid or top of slope, summit, other), wind exposure (very sheltered, sheltered, exposed, very exposed), range of slopes (0°, ]0-3°], ]3-10°],]10-30°],]30-80°]); slope aspect (N, NW, W, SW, S, SE, E, without dominate slope aspect); visual estimation of surface texture: pebbles (2cm to 20 cm), bare soil (<2mm) and herbaceous cover in percentage; soil texture: clay and coarse sand percentage; soil chemical composition: Na, Mn, Al cmol.kg -1 , P and N g.kg -1 , ratio C.N -1 ; soil physical characteristics: pH, organic matter and soil humidity.

Table 3 :

 3 Spearman correlations and p-values for correlations of each morphological and biotic trait.

Cushion surface area, cushion perimeter, cushion height, cushion shape, cushion compactness, proportion of epiphytes across the cushion surface area and proportion of neighbouring plants in contact with the cushion perimeter. For volume and height n = 229, the other traits n = 319. Asterisks indicate the significance of the correlation * p < 0.05; ** p < 0.01; *** p < 0.001

Table 4 :

 4 Minimum adequate multivariate linear models of morphological traits and environmental conditions; n = 19, adjusted R² (ad. R²), degree of freedom (df), F-value and p-value are presented.

	Cushion surface area model with variables of topography and habitat, of slope aspect and of soil
	texture; cushion shape model with variable of slope aspect; cushion compactness model with variable
	of texture surface; bold numbers indicate significant variables.

Table 4 :

 4

		Cushion surface area
		Ad. R²	df	F-value	p-value
	Topography & habitat	75%	8
	Topography			9.153	0.0037
	Wind exposure			9.403	0.0154
	Type of fellfield			1.913	0.2017
	Slope aspect	64%	10
	Slope aspect			4.978	0.001
	Soil texture	19%	17
	Coarse sand			2.273	0.0363
		Cushion shape
		Ad. R²	df	F-value	p-value
	Slope aspect	52%	10
	Slope aspect			4.455	0.0352
		Cushion compactness
		Ad. R²	df	F-value	p-value
	Surface texture	18%	17
	Bare soil			4.974	0.0395

Table 1 :

 1 environmental characteristics of the 19 populations of L. kerguelensis studied. At population scale: longitude; latitude; altitude; type of fellfield (slope, scree slope, plateau, summit, wall, fault, outwash plain); topography (flat, terrace, base or mid or top of slope, summit,

	other); wind exposure (very sheltered, sheltered, exposed, very exposed); range of slope (0°, ]0-3°],

Table 2 :

 2 soil characteristics of the 19 populations of L. kerguelensis studied; soil

	texture (clay, sand, silt), soil chemical composition (Na, Mn, Al, CEC, P, N, C.N ^-1 , NO 3	-, NH 4	+ ) and
	soil hydric characteristics (pH, conductivity and soil water saturation).
	Online Resource

Table 3 :

 3 quantitative description of physical attributes and environmental parameters of the 19 populations studied. Topography and type of habitat: altitude, type of fellfield (slope, scree slope, plateau, summit, wall, fault, outwash plain), topography (flat, terrace, base or mid

	or top of slope, summit, other), range of slope (0°, ]0-3°], ]3-10°],]10-30°],]30-80°],), slope aspect (N,
	NW, W, SW, S, SE, E, without dominant slope aspect); distance to the sea and wind exposure (very
	sheltered, sheltered, exposed, very exposed); visual estimation of surface texture: mineral block
	(>20cm), pebbles (2cm to 20 cm), gravel (2cm to 2mm), bare soil (<2mm), bryophyte and herbaceous
	cover in percentage; soil texture: clay, silt (fine + coarse) and sand (fine + coarse) percentage; soil
	chemical composition: Ca 2+ , P, N, ratio C N -1 , Mn, Al, Na, CEC, NO 3	-, NH 4	+ , Fe; soil physical
	characteristics: conductivity, pH, soil water saturation and organic matter.

Acknowledgments

The research project No 1116 PlantEvol (resp. F. Hennion) was performed at Kerguelen station and was supported by the French Polar Institute (IPEV). This research was also supported by CNRS IRP grant "AntarctPlantAdapt" (F. Hennion). We thank B. Labarrere (UMR ECOBIO, Université de Rennes 1, Rennes, France), G. Bouger (UMS OSUR, Université de Rennes 1, Rennes, France), F. Lamy, T. Robert, V. Normand (UMR ESE, Université Paris-Saclay, Orsay, France), A. D'Hont (UMR AGAP, CIRAD, Montpellier, France), IPEV logistics and Réserve Naturelle of Terres Australes et Antarctiques Françaises for help in material and data collection during the four summer campaigns (2015-2019). We are grateful to L. Madec (UMR ECOBIO, Université de Rennes 1, Rennes, France) for help in statistical analyses, and P. Convey (BAS, Cambridge, UK) for his comments and corrections which improved a first version of the manuscript. We thank Chief Editor Dieter Piepenburg, P.C. le Roux (University of Pretoria) and two anonymous referees for their thorough comments that helped us improve the manuscript.

This research was supported by the French Polar Institute (IPEV, program 1116 PlantEvol) and CNRS (IRP grant "AntarctPlantAdapt", F. Hennion). L.J.M. was supported by a PhD grant from the Ministry of Research and Education (France).

Images and R-Script are available at first author's convenience. The datasets generated during the current study are available online on Osuris geonetwork. https ://www.osuri s.fr/geone twork /srv/ fre/catal og.searc h#/metad ata/7528f bbb-e1ac-4db2-a179-f20d3 f03ff 83.

Table3

Click here to access/download;Table ;Table_3.docx


Dr Xinliang Xie

Chaoyue Chen

Zhe Chen

Wen Wang

Yan Wang

Hanlin Liao

Achieving simultaneously improved tensile strength and ductility of a nano-TiB 2 /AlSi10Mg composite produced by cold spray additive manufacturing

Keywords: Metal matrix composites, Cold spray, Additive manufacturing, Friction stir processing, Mechanical properties, Metallurgical bonding

The premature failure of components due to poor inter-particle bonding is the most critical issue in cold spray (CS) additive manufacturing. Herein, a hybrid strategy combining gas-atomization (involving in-situ reaction), CS, and post-friction stir processing was proposed to design a nano-TiB2/AlSi10Mg composite. Multiscale characterization in terms of X-ray diffraction and scanning and transmission electron microscopy was conducted to track microstructure evolution for understanding the mechanisms determining mechanical performance of the as-produced composites. The results showed simultaneous improvement in both ultimate tensile strength (365 ± 35 MPa) and ductility (16.0 ± 1.2 %), which represents a breakthrough. The strengthening and toughing mechanisms were attributed to the fine matrix grains with the significantly improved metallurgical inter-particle bonding, and the uniformly distributed TiB2 nanoparticles as reinforcement that was strongly bonded with the matrix (i.e. the formation of semi-coherent TiB2/Al interface). This study provides new guidance for hybrid additive manufacturing of metal matrix composites with high performance.

Introduction

Cold spray (CS), as a solid-state deposition technique for surface coating, has recently been defined as an unconventional additive manufacturing (AM) process to fabricate net-shaped components and to repair damaged parts [START_REF] Shuo Yin | Cold spray additive manufacturing and repair: Fundamentals and applications[END_REF][START_REF] Li | Solid-state additive manufacturing and repairing by cold spraying: A review[END_REF][START_REF] Shuo Yin | Advanced diamond-reinforced metal matrix composites via cold spray: properties and deposition mechanism[END_REF]. Unlike thermal spray and laser-based AM processes, CS uniquely relies on plastic deformation mechanisms of the highly accelerated micron-sized particles. Generally, the CS component is built layer-by-layer primarily through localized metallurgical bonding and mechanical interlocking, which corresponds to a relatively weak inter-particle (also called inter-splat) bonding [START_REF] Xie | New insights into the coating/substrate interfacial bonding mechanism in cold spray[END_REF][START_REF] Shuo Yin | Deposition behavior of thermally softened copper particles in cold spraying[END_REF][START_REF] Borchers | Microstructural bonding features of cold sprayed face centered cubic metals[END_REF]. Moreover, the localized work-hardening effect induced by the intensive plastic deformation of the particles significantly lowers its ductility, leading to a brittle feature of the CS deposit. Thus, very limited applications of CSAM have been exploited to structural load-bearing parts due to the unexpected premature failure.

So far, Al matrix composite (AMC) composites fabricated by CSAM have received increased attentions due to their high potentials for aircraft and marine applications. It has been widely accepted that certain mechanical properties (e.g. microhardness, wear performance) of the CS AMCs can be largely improved by properly integrating ceramic reinforcement particles into the Al matrix [START_REF] Meydanoglu | Microstructure, mechanical properties and corrosion performance of 7075 Al matrix ceramic particle reinforced composite coatings produced by the cold gas dynamic spraying process[END_REF][START_REF] Yu | Microstructure, mechanical property and wear performance of cold sprayed Al5056/SiCp composite coatings: effect of reinforcement content[END_REF][START_REF] Qiu | Microstructure, microhardness and tribological behavior of Al2O3 reinforced A380 aluminum alloy composite coatings prepared by cold spray technique[END_REF][START_REF] Yang | Electrochemical characterization and microstructure of cold sprayed AA5083/Al2O3 composite coatings[END_REF][START_REF] Li | A review of advanced composite and nanostructured coatings by solid-state cold spraying process[END_REF]. However, it is still challenging to obtain AMCs with high tensile performance owing to the poor bonding of the interfaces between Al matrix and ceramic particles and/or between inter-splats as well as some defaults (e.g. residual pores) within the composite deposits [START_REF] Tariq | Cold spray additive manufacturing: A viable strategy to fabricate thick B4C/Al composite coatings for neutron shielding applications[END_REF]. It was reported by Kumar et al. [13] that the CS SiC/Al composites were fractured at a very early stage with very low tensile strength and almost no ductility. The observation of fracture surface revealed a completely brittle feature with the fracture primarily occurred through poorly bonded interfaces of inter-splats and SiC/Al matrix.

Post-heat treatment (HT) and friction stir processing (FSP) are the most commonly used techniques to enhance mechanical properties of the CS components by improving the inter-particle bonding and reducing porosity and residual stress [START_REF] Shuo Yin | Cold spray additive manufacturing and repair: Fundamentals and applications[END_REF][START_REF] Shuo Yin | Annealing strategies for enhancing mechanical properties of additively manufactured 316L stainless steel deposited by cold spray[END_REF][START_REF] Coddet | On the mechanical and electrical properties of copper-silver and copper-silver-zirconium alloys deposits manufactured by cold spray[END_REF]. HT can partially heal the CS-induced defects through temperature-dependent atomic diffusion, while structure coarsening caused by high temperature often degrades the strength [START_REF] Coddet | On the mechanical and electrical properties of copper-silver and copper-silver-zirconium alloys deposits manufactured by cold spray[END_REF][START_REF] Huang | The effects of heat treatment on the mechanical properties of cold-sprayed coatings[END_REF][START_REF] Shuo Yin | Microstructure and mechanical anisotropy of additively manufactured cold spray copper deposits[END_REF]. Furthermore, the interfacial bonding between metallic particle and ceramic particle can hardly be improved via atomic diffusion by HT, resulting in very limited improvement in tensile properties of the CS AMCs [START_REF] Yang | Optimization of cold-sprayed AA2024/Al2O3 metal matrix composites via friction stir processing: Effect of rotation speeds[END_REF][START_REF] Yang | Cold sprayed AA2024/Al2O3 metal matrix composites improved by friction stir processing: Microstructure characterization, mechanical performance and strengthening mechanisms[END_REF]. Comparatively, severe plastic deformation involved in the FSP process can further modify the microstructure in the nugget zone, which not only produces fine and equiaxed grains but also increases the metallurgical inter-particle bonding [START_REF] Huang | Microstructural evolution and mechanical properties enhancement of a cold-sprayed CuZn alloy coating with friction stir processing[END_REF] [START_REF] Wang | High-performance bulk pure Al prepared through cold spray-friction stir processing composite additive manufacturing[END_REF]. (should be [START_REF] Huang | Microstructural evolution and mechanical properties enhancement of a cold-sprayed CuZn alloy coating with friction stir processing[END_REF][START_REF] Wang | High-performance bulk pure Al prepared through cold spray-friction stir processing composite additive manufacturing[END_REF]) For instance, post-FSP has significantly increased the ultimate tensile strength (UTS) and elongation of CS pure Al by around 45% and 1336%, respectively [START_REF] Wang | High-performance bulk pure Al prepared through cold spray-friction stir processing composite additive manufacturing[END_REF]. Another advantage of post-FSP is to break down and then more homogenously redistribute the reinforcement particles in some AMCs. As a result, the strengthening effect of the second-phase particles is enhanced. For instance, it was found, in the CS SiC/5056Al composites, that the post-FSP resulted in the fine microstructure containing the redistributed and refined reinforcement particles, which was accompanied with improved hardness and wear performance [START_REF] Hodder | Fabrication of aluminum-alumina metal matrix composites via cold gas dynamic spraying at low pressure followed by friction stir processing[END_REF][START_REF] Huang | Modification of a cold sprayed SiCp/Al5056 composite coating by friction stir processing[END_REF][START_REF] Huang | Effect of tool rotation speed on microstructure and microhardness of friction-stir-processed cold-sprayed SiCp/Al5056 composite coating[END_REF]. However, to the authors' best knowledge, unlike the CS pure Al [START_REF] Wang | High-performance bulk pure Al prepared through cold spray-friction stir processing composite additive manufacturing[END_REF], the currently available results only showed limited improvement in strength-ductility trade-off for AMCs by the post-FSP. For example, post-FSP conditions were tentatively optimized to further improve tensile properties of the CS Al2O3/2024Al composite using mechanically mixed composite powder [START_REF] Yang | Optimization of cold-sprayed AA2024/Al2O3 metal matrix composites via friction stir processing: Effect of rotation speeds[END_REF][START_REF] Yang | Cold sprayed AA2024/Al2O3 metal matrix composites improved by friction stir processing: Microstructure characterization, mechanical performance and strengthening mechanisms[END_REF]. However, the strength-ductility tradeoff issue has not been solved mainly due to the presence of large Al2O3 particles and poor Al2O3/Al interfacial bonding induced by mechanical blending. Hence, it is considered that the selection of proper size and type of reinforcement in powder feedstock for CS is also critical in determining the final microstructure and properties tailored by FSP.

In the present work, a hybrid AM strategy combining gas-atomization (involving in-situ reaction), CS and post-FSP was proposed and validated and was used to design new AMCs with concurrently improved tensile strength and ductility. A specially designed Al matrix composite powder reinforced with in-situ formed TiB2 particles was used as the feedstock to fabricate AMCs components by CSAM. By using such a specially designed composite powder, it is expected to solve the abovementioned major problems, like uneven distribution of reinforcements and poor interfacial bonding, involved in traditional CS process. Followed by CS, FSP treatment was applied to modify the microstructure, reduce the defects, and improve the mechanical property of the CS TiB2/AlSi10Mg composites. Balanced UTS (365 MPa) and ductility (elongation of 16.0 %) were simultaneously achieved in the final composite. The strengthening mechanisms of the AMCs related to the microstructure evolution were discussed in detail.

Experimental details 2.1 CS deposition and FSP treatment

A specially designed 7.0 wt. % TiB2/AlSi10Mg composite powder was produced by in-situ chemical reaction using the mixed K2TiF6 and KBF4 salts in molten Al followed by gas atomization. Details about the processing parameters and conditions were reported elsewhere [START_REF] Peng | Selective laser melting of nano-TiB2 decorated AlSi10Mg alloy with high fracture strength and ductility[END_REF][START_REF] Xie | Cold spray additive manufacturing of metal matrix composites (MMCs) using a novel nano-TiB2reinforced 7075Al powder[END_REF]. The chemical composition of the powder is 9.81 wt.% Si, 0.32 wt.% Mg, 7.0 wt.% TiB2 (∼4.3 vol.%) with Al balance, measured through inductively coupled plasma atomic emission analysis (ICP-AES). The composite powder as well as the pure AlSi10Mg powder after sieving were used as the feedstocks for CS.

A homemade helium circulation CS system (LERMPS, UTBM, France) was used to build a component with over 2.5 mm thick onto grit-blasted Al substrates (140×80×4 mm). Helium was used as the propellant and carrier gases with an inlet pressure and temperature of 1.8 MPa and 320 °C, respectively. Nozzle standoff distance and traverse speed were set to 30 mm and 100 mm/s, respectively. Post-FSP was done using a commercial friction stir welding machine (FSW-RL31-010, Beijing FSW Technology Co., Ltd, China), equipped with an H13 steel stir-tool with a threaded pin of 3.5 mm in root diameter and 2.0 mm in length, and a concave shoulder of 10 mm in diameter.

Rotation and traverse speeds were set to 1500 rpm and 500 mm/min, respectively. The tool was set to a 2.5° tilt angle to avoid surface defects. The as-acquired samples are shown in Fig. 1a. The same CS and FSP processes were applied to the gas-atomized AlSi10Mg powder (without TiB2 particles) for comparison.

Materials characterization

The as-sprayed deposits before and after FSP treatment as well as powders were examined by an X-ray diffractometer (Siemens D5000, Germany) with the Co (λ =1.78897 Å) source at a current of 40 mA, a voltage of 35 kV and scan step of 0.02°.

Polished cross-sectional microstructures of the samples were observed using an optical microscope (Nikon, Japan) and a scanning electron microscope (SEM) equipped with an energy-dispersive spectroscopy unit (JSM5800LV, JEOL, Japan). Average porosities of samples were determined by analyzing five cross-sectional images using ImageJ software (ImageJ, NIH, Bethesda, Md.). To clearly observe the microstructure, specimens were polished and etched at room temperature using Kroll's reagent (3ml HF + 6ml HNO3 + 100ml H2O). Another SEM instrument (JSM7800F, JEOL, Japan) equipped with an EBSD detector from Oxford instruments was employed to reveal information about grain structure. Samples for SEM/EBSD characterization were prepared by mechanical polishing and final surface iron milling using a Fischione instruments model 1060 miller. EBSD measurements were conducted on the crosssections of deposits at an electron energy of 25 KeV and with a step size of 100 nm. An FEI Tecnai G2 microscope, operating at 200 kV, was used for transmission electron microscopy (TEM) characterization. Cross-sectional TEM samples were prepared by mechanical polishing followed by final ion milling using a Gatan Model 691 precision ion polishing system. Microhardness measurements were performed on the cross-section of the deposits using a Vickers hardness indenter (Leitz, Germany) with a load of 100g for 15s. Ten measurements for each sample were conducted to have an average value. A mediumsized tensile specimen, with the width in the gauge section of 2 mm, total length of 37 mm and thickness of 2 mm, was tested using a Zwick/Roell Z100 machine at a strain rate of 1 mm/min. The location and the dog-bone specimen size are shown in Fig. 1b.

Results

Composite powder characterization

As shown in Fig. 2a, the gas-atomized composite powder exhibits a near-spherical shape having an average particle size of 37 μm (see Fig. 2c). Apart from a few residual particle clusters as marked by red circles in Fig. 2b, the fine TiB2 reinforcement particles are homogeneously distributed within the composite particle. A close look (Fig. 2d) reveals that the TiB2 nanoparticles tend to be segregated at the α-Al grain boundaries along with the well-connected network of eutectic Si particles (Fig. 2b andd). The equiaxed Al grains in the matrix are in the size ranging from 1 to 5 μm with an average of around 2.6 μm (Fig. 2e andf).

Phase composition

Fig. 3a andb display the XRD patterns of the pure AlSi10Mg and TiB2/AlSi10Mg composite powders, deposits before and after FSP treatment. The composition phases, including α-Al, Si, and Mg2Si, in the as-sprayed AlSi10Mg powder, were well preserved in the CS and FSP treated samples. In addition to these three phases, peaks corresponding to the TiB2 phase can be observed on the composite patterns. Moreover, XRD patterns of the pure AlSi10Mg and TiB2/AlSi10Mg composite samples show that Si peaks of FSP treated samples have higher intensities than those in the as-sprayed state. This indicates a significant decrease of Si solid solubility in Al caused by the thermal effect during FSP, as the temperature in the stir zone can be of the order of 80 % of the melting temperature of the Al alloys [START_REF] Rajiv S Mishra | Friction stir welding and processing[END_REF].

Microstructure evolution

Fig. 4a andb show the microstructure with visibly deformed splat morphology of the CS pure AlSi10Mg and TiB2/AlSi10Mg composite deposits, respectively. The evidence of inter-splat boundaries as well as some small pores located in the junction areas clearly indicate the weak metallurgical inter-particle bonding due to insufficient plastic deformation, the fundamental weakness of CS even though helium was used as the propellant gas. The porosity measurements yield around 0.35% and 0.24% for the pure AlSi10Mg and TiB2/AlSi10Mg composite deposits, respectively. The magnified images in Fig. 5a and b demonstrate the eutectic Si particles around α-Al grains, which are well preserved from the initial powders. In addition to the eutectic Si particles, finer dispersed Si nanoparticles precipitated from the supersaturated Al matrix are also presented due to the thermal softening effect during CS. The clusters and uneven distribution of the TiB2 nanoparticles in the initial powder are retained in the CS composite component (Fig. 4b and Fig. 5b). The TEM image in Fig. 8a (description of Fig. 8 is before figures 6 and 7 ???) confirms that TiB2 clusters and nanoparticles are primarily dispersed at grain boundaries, while the Si nanoparticles are located not only at grain boundaries but also inside the α-Al cells. Based on the EBSD orientation maps (Fig. 6a andb), both the CS pure AlSi10Mg and TiB2/AlSi10Mg composite samples display bimodal grain size distributions containing a large number of ultrafine gains at the inter-particle boundaries and deformed micron-sized grains within the particles (see Fig. 7a andb). Formation of these ultrafine grains can be attributed to the dynamic recrystallization via dislocation accumulation and sub-grain rotation at a high strain-rate during high-velocity particle impact [START_REF] Liu | Formation of refined grains below 10 nm in size and nanoscale interlocking in the particle-particle interfacial regions of cold sprayed pure aluminum[END_REF][START_REF] Zou | Dynamic recrystallization in the particle/particle interfacial region of coldsprayed nickel coating: electron backscatter diffraction characterization[END_REF][START_REF] Cavaliere | Mechanical and microstructural behavior of nanocomposites produced via cold spray[END_REF]. Nevertheless, both the CS pure AlSi10Mg sample and TiB2/AlSi10Mg composite sample have average grain sizes of about 1.5 µm.

Comparatively, Fig. 4c andd show that the FSP treated samples in the nugget zone are fully dense without visible pores (porosity less than 0.2%) and inter-splat boundaries, indicating the enhanced metallurgical bonding. Eutectic Si structure in the as-sprayed state disappeared and presented as nanosized Si individual particles after FSP treatment. According to EBSD maps shown in Fig. 6c andd, both the deposits display a homogenous structure containing equiaxed fine grains. A homogeneous structure is commonly observed in the FSP treated samples, which can be explained by the continuous dynamic recrystallization, in which a continuous introduction of strain is coupled with rapid recovery and the migration of sub-grain/boundaries [START_REF] Darzi Bourkhani | Through-thickness inhomogeneity in microstructure and tensile properties and tribological performance of friction stir processed AA1050-Al2O3 nanocomposite[END_REF][START_REF] Gopalakrishnan | Production and wear characterisation of AA 6061 matrix titanium carbide particulate reinforced composite by enhanced stir casting method[END_REF].

During FSP, the material in the stir zone experienced severe deformation, which continuously induced a very high density of dislocations. With the continued deformation, the stored energy contained in these dislocations drove the dynamic recovery and recrystallisation processes. The composite sample has an average size of about 1.4 μm, which is slightly smaller than that of the pure AlSi10Mg sample (~1.6 μm). The more refined grains of the composite sample may be due to the presence of TiB2 nanoparticles, which can prevent the migration of dislocations and recovery process.

As shown in Fig. 4d andFig 9b andc). This OR is consistent with previous literature [START_REF] Paul | Crystallography of engulfed and pushed TiB2 particles in aluminium[END_REF][START_REF] Ma | Atomic-scale investigation of the interface precipitation in a TiB2 nanoparticles reinforced Al-Zn-Mg-Cu matrix composite[END_REF]. The overall semi-coherent TiB2/Al interface due to the in-situ chemical reaction in the melt contributed to the formation of the high interfacial bonding [START_REF] Ma | Atomic-scale investigation of the interface precipitation in a TiB2 nanoparticles reinforced Al-Zn-Mg-Cu matrix composite[END_REF].

Mechanical properties

Microhardness

Fig. 10 illustrates the microhardness values measured on the cross-section of the CS and CS+FSP treated samples. The microhardness of the as-sprayed AlSi10Mg deposit is about 139±4HV0.1, which is two times that of the as-casted (67 HV0.1) [START_REF] Zyguła | Mechanical properties and microstructure of AlSi10Mg alloy obtained by casting and SLM technique[END_REF]. This significant increment in microhardness can be explained by the enhanced work hardening effect as a result of severe plastic deformation of the particles during CS deposition. Since He was used in this study to accelerate the particles to a very high velocity, the particles would be largely deformed and lead to a large amount of residual stress and dislocations within the matrix, which contribute significantly to increase the microhardness in the as-sprayed state. The as-sprayed composite deposit reaches a higher value of 179±5 HV0.1, showing about 22% increment compared to that of the pure AlSi10Mg deposit. The increased microhardness in composite deposit should be attributed to the uniformly distributed TiB2 particles in the Al matrix. The microhardness of the composite deposits in the stir zone is about 110±3 HV0.1, which is much higher than that of the pure AlSi10Mg sample (82 HV0.1) due to the reinforcement of nano/micro TiB2 particles. Nevertheless, the microhardness of the FSP treated samples show a significant decrement compared to the as-sprayed states. Since the grain size changed a little after FSP treatment, residual stress relaxation induced by the thermal softening effect and recovery process should be mainly responsible for the decreased microhardness.

Tensile properties

As shown in Fig. 11a, CS samples (the AlSi10Mg alloy and the TiB2/AlSi10Mg composite) prematurely failed (elongation < 1.0%) due to the poor inter-splat bonding and enhanced work hardening effect, which is commonly observed in most CS samples [START_REF] Shuo Yin | Microstructure and mechanical anisotropy of additively manufactured cold spray copper deposits[END_REF][START_REF] Wei | Solidstate additive manufacturing high performance aluminum alloy 6061 enabled by an insitu micro-forging assisted cold spray[END_REF]. The post-FSP led to a dramatic improvement in UTS and elongation of both materials to 295 ± 30 MPa and 21 ± 0.8 % for the AlSi10Mg alloy, and 365 ± 35 MPa and 16 ± 1.2 % for the TiB2/AlSi10Mg composite. The composite also exhibited a slightly higher work hardening capacity than the AlSi10Mg sample during plastic deformation before yielding point (Fig. 11b). Compared with many other CS and posttreated AMCs reported in the literatures displaying the strength-ductility trade-off, the present result shows an outstanding balance of strength and ductility, representing a breakthrough for this type of composite materials as shown in Fig. 11c.

The typical fracture surfaces of both CS samples show primary brittle fracture along with inter-splat interfaces and the intergranular fracture (as exemplified in Fig. 12a andd), while those of both post-FSP samples show the highly ductile rupture mode dominated by fine dimples (Fig. 12b andc). The dimple size (a few microns) is related to the grain size as Grain boundaries act as sites for crack nucleation to control the size of the dimples. The dimple size of the composite is slightly smaller than that of the AlSi10Mg alloy, agreeing with the statistical distributions of grain size (Fig. 7c andd).

The dispersed TiB2 and Si nanoparticles refined the recrystallized grains by promoting grain nucleation and hindering grain coarsening, while the additional effect of TiB2 nanoparticle is limited in the composite. The fact that the nanoparticles are visible at bottoms of the dimples suggests the ductile rupture by decohesion of the Si/Al and/or TiB2/Al interfaces.

AMCs

For a certain MMC, it is well established that the final mechanical properties depend largely on the following four factors [START_REF] Peng | Selective laser melting of nano-TiB2 decorated AlSi10Mg alloy with high fracture strength and ductility[END_REF][START_REF] Mehdi Rahimian | Investigation of particle size and amount of alumina on microstructure and mechanical properties of Al matrix composite made by powder metallurgy[END_REF][START_REF] Saravanan | Effect of particulate reinforced aluminium metal matrix composite-a review[END_REF]: i) interior defect level; ii) matrix texture and grain size; ii) reinforcement particle size and distribution; and iv) interfacial bonding state between the reinforcement particle and the matrix. Here, as illustrated in Fig. 13, we are trying to establish the relationship between these factors and mechanical properties of CS AMCs prepared by different methods. Importantly, free of any processing defects is the starting point before the correlation between microstructure (e.g. grain structure and TiB2 particles) and properties can be properly done. Generally, a higher particle impact velocity leads to enhanced plastic deformation and metallic bonding of the deformed splats in CS process. However, there are still some interior defects, such as pores and poor intersplat boundaries, presented in the as-sprayed state even though helium was used as the propellent gas. Considering that a fracture could primarily take place around these defects, it is vitally important to obtain a fully dense and defect-free structure. HT can partially heal micro-pores and tightly bonded inter-splat interfaces though atomic diffusion. It should be emphasized that the post-FSP treatment effectively altered the prominent mechanical interlocking in the CS state to the full metallurgical inter-splat bonding. Significant improvement of tensile properties, particularly the ductility, of both the AlSi10Mg alloy and the TiB2/AlSi10Mg composite after post-FSP (Fig. 11) witnesses the elimination of such defects, introduced by CS. Accordingly, it promoted the transition from the brittle to ductile fracture mode in the post-FSP samples (Fig. 12), whatever the TiB2 reinforcement particles get involved or not.

The microstructure of the initial composite powder can be well preserved in the CS deposit except that high strain stress and numbers of fine grains were generated near highly deformed inter-splat boundaries. These ultrafine grains and the localized work hardening effect could increase the microhardness and strength of the CS deposit, but remarkably reduce the ductility. During FSP, the material in the SZ (did you define SZ before?) experienced severe deformation, which continuously drove the dynamic recovery and recrystallisation processes. Compared to the as-sprayed state, a homogenous structure with fine grains (0.5-3 µm) and more uniformly distributed residual stress were obtained for both the pure AlSi10Mg and TiB2/AlSi10Mg composite samples after FSP treatment (Fig. 6 c andd). More importantly, the vigorous stirring action of the rotating tool fractured the clusters (agglomerated TiB2 nanoparticles) into homogeneously distributed individual TiB2 nanoparticles in Al matrix [START_REF] Hb Michael Rajan | Influence of friction stir processing on microstructure and properties of AA7075/TiB2 in situ composite[END_REF]. It should be highlighted that post-FSP treatments of CS AMCs in the previous studies failed to fracture the micron-scaled reinforcement particles into nanoscales using the mechanical blended mixtures [START_REF] Yang | Optimization of cold-sprayed AA2024/Al2O3 metal matrix composites via friction stir processing: Effect of rotation speeds[END_REF][START_REF] Yang | Cold sprayed AA2024/Al2O3 metal matrix composites improved by friction stir processing: Microstructure characterization, mechanical performance and strengthening mechanisms[END_REF].

As revealed by TEM images (Fig. 9), the robust interfacial bonding between the reinforcement particle and Al matrix was successfully induced by using the gas atomized in-situ TiB2/AlSi10Mg composite powder. Such a semi-coherent interface cannot be achieved by CS of mechanical blended powders even after FSP treatment.

That is to say the initial state of the composite powder is of great necessary to achieve the designed mechanical performance. Therefore, the abovementioned four critical issues for producing AMCs were successfully overcome by CS of gas-atomized composite powder followed by post-FSP.

Possible strengthening and toughening mechanisms

To understand the strengthening behavior of TiB2 nanoparticles in the composites, several aspects should be considered, e.g. the dispersion of TiB2 nanoparticles, grain refinement induced by TiB2 particles, load transfer between TiB2 nanoparticles and Al matrix. The homogenous fine-grained structure decorated by the uniformly distributed TiB2 nanoparticles is highly beneficial for the enhancement of both tensile strength and ductility. This is because the TiB2 clusters formed in the initial powder (Fig. 1) and almost remained in the CS component (Fig. 4) can result in stress concentration that significantly degrades ductility. The TiB2 nanoparticles homogeneously redistributed inside the micron-sized grains in the matrix at the post-FSP state increased the effectiveness of the Orowan strengthening ( Orowan σ ∆

). The contribution from the dispersed TiB2 nanoparticles is estimated by the following equation [START_REF] Liu | Nanostructured highstrength molybdenum alloys with unprecedented tensile ductility[END_REF]:

2 2 , , 0.13 ln 2 σ σ λ   ∆ ≈ ∆ =     p TiB m Orowan Orowan TiB d G b b (1) 2 2 1/3 , , 1 1 2V λ
      = -           p TiB p TiB d (2)
Where m G and b are the shear modulus of the matrix (~26.5 GPa) and the Burgers vector (~0.286 nm) [START_REF] Peng | Selective laser melting of nano-TiB2 decorated AlSi10Mg alloy with high fracture strength and ductility[END_REF] [45], respectively. The contribution by grain refinement (Hall-Petch equation) can be expressed as follows [START_REF] Gutierrez-Urrutia | Contribution of microstructural parameters to strengthening in an ultrafine-grained Al-7% Si alloy processed by severe deformation[END_REF]:

1 1 2 2 Hall-Petch 0 σ - -   ∆ = -     k d d ( 3
)
where k is a constant ( ~50 MPa/√(µm)) [START_REF] Gutierrez-Urrutia | Contribution of microstructural parameters to strengthening in an ultrafine-grained Al-7% Si alloy processed by severe deformation[END_REF], d and 0 d are the average grain size of the composite and unreinforced AlSi10Mg sample, respectively. Using the average grain sizes obtained from the SEM/EBSD analyses (Fig. 7c andd), the strengthening effect due to further grain refinement is estimated to be only 2.73 MPa in the composite sample.

Moreover, our TEM results (Fig. 9b andc) show the OR between the TiB2 nanoparticle and Al matrix, suggesting the strong TiB2/Al interfacial bonding, which ensures an effective load transfer from matrix to nanoparticles [START_REF] Peng | Selective laser melting of nano-TiB2 decorated AlSi10Mg alloy with high fracture strength and ductility[END_REF][START_REF] Hb Michael Rajan | Influence of friction stir processing on microstructure and properties of AA7075/TiB2 in situ composite[END_REF]. The loadbearing mechanism is in this case responsible for the improved tensile strength of the composite. It can be described as below [START_REF] Vc Nardone | On the strength of discontinuous silicon carbide reinforced aluminum composites[END_REF]:

2 , 1.5 σ σ ∆ = load p TiB i V (4)
According to the previous study [START_REF] Peng | Selective laser melting of nano-TiB2 decorated AlSi10Mg alloy with high fracture strength and ductility[END_REF], the bonding strength between the nano-particles and the Al-Si matrix σ i can be estimated to be around 676 MPa. Thus, the contribution due to the load-bearing mechanism leads to an increment in the strength of around 36.5 MPa in the composite sample. Compared with the post-FSP AlSi10Mg sample, the total increment of tensile strength is around 70 MPa in the post-FSP composite (Fig. 11a). This was attributed to the Orowan strengthening and load-bearing mechanisms promoted by the dispersed TiB2 nanoparticles.

The enhanced ductility in the post-FSP composite sample may be explained as follows: i) the presence of a large number of Grain boundaries in the fine-grained matrix (0.5-3 µm) contributed to the enhancement of plastic flow and ductility by providing a tortuous path for the crack propagation during yielding and necking processes [START_REF] Bauri | Tungsten particle reinforced Al 5083 composite with high strength and ductility[END_REF]. ii)

Both the dispersed TiB2 and Si nanoparticles promote dislocation plasticity and storage during deformation increasing the work hardening rate of the composite (Fig. 11b). The increased work hardening rate offers a uniform deformation capability equivalent to that of the matrix, thereby enhancing the ductility of the composite [START_REF] Peng | Selective laser melting of nano-TiB2 decorated AlSi10Mg alloy with high fracture strength and ductility[END_REF][START_REF] Liu | Nanostructured highstrength molybdenum alloys with unprecedented tensile ductility[END_REF]. iii) The presence of an OR between intragranular TiB2 nanoparticles and the Al matrix (Fig. 9c) may also help diminish stress localization and early fracture [START_REF] Liu | Nanostructured highstrength molybdenum alloys with unprecedented tensile ductility[END_REF]. Therefore, introducing the uniform dispersion of TiB2 nanoparticles in the grain interior is an effective approach to increase the strength and simultaneously improve the ductility of the composites, because they can generate, pin down, and thus accumulate dislocations within the grains [START_REF] Zhou | The nano-sized TiC particle reinforced Al-Cu matrix composite with superior tensile ductility[END_REF][START_REF] Wu | A nanostructural design to produce high ductility of high volume fraction SiCp/Al composites with enhanced strength[END_REF][START_REF] Zhou | Simultaneously increasing the strength and ductility of nano-sized TiN particle reinforced Al-Cu matrix composites[END_REF].

Conclusions

In this study, a novel gas-atomized composite powder reinforced with in-situ TiB2 particles was used as the feedstock to fabricate TiB2/AlSi10Mg composite using CS followed by post-FSP treatment. The following conclusions can be drawn:

(1) The CS TiB2/AlSi10Mg composite deposits displayed a dense structure with some TiB2 clusters distributed in the Al matrix.

(2) Both the as-sprayed pure AlSi10Mg and TiB2/AlSi10Mg composite deposits possessed high tensile strength but almost no ductility due to the limited metallurgical bonding and work hardening effect.

(3) FSP treatment altered the inter-splat bonding conditions from interlocking or partially metallurgical bonding in the as-sprayed state to a fully metallurgical bonding state.

(4) Simultaneous enhancement in tensile strength and ductility of the CS TiB2/AlSi10Mg composite has been achieved by FSP treatment via redistributing and refining of TiB2 nanoparticles and enhancing interfacial bonding due to the in-situ chemical reaction during gas atomization.

Fig. 1 (

 1 Fig. 1 (a) Photograph of the as-sprayed TiB2/AlSi10Mg composite parts before and after FSP treatment. (b) Schematic diagram of FSP and dimension size of the samples used for tensile testing.

Fig. 2

 2 Fig. 2 Microstructure of the gas-atomized TiB2/AlSi10Mg composite powder: SEM images showing (a) particles and (b) cross-sectional morphology of a typical particle; (c) particle size distribution measured by laser diffraction; (d) TEM bright-field (BF) image showing the distributions of TiB2 nanoparticles (marked by red arrow) and Si particles (marked by yellow allow); (e) cross-sectional SEM/EBSD orientation map of the composite particle (dark contrasts are indexed as TiB2 particle clusters) and (d) corresponding size distribution of Al grains.

Fig. 3

 3 Fig. 3 XRD patterns of the CS and FSP treated deposits: (a) AlSi10Mg; (b) TiB2/AlSi10Mg composite.

Fig. 4

 4 Fig. 4 SEM images showing the cross-sectional morphologies of the (a, b) CS and (c, d) CS+FSP samples in low magnification: (a) and (c) AlSi10Mg; (b) and (d) TiB2/AlSi10Mg composite.

  . 5d, the TiB2 clusters presented in the CS state were broken into fragments, resulting in more homogenous particle distribution. Close observation by TEM shows that the majority of TiB2 and precipitated Si nanoparticles are redistributed homogenously inside the Al grains (Fig. 8b). The well-defined ring patterns in Fig. 8c confirm the formation of Si nanoparticles in the ultrafine Al grain matrix. However, it is difficult to identify the TiB2 and possible Mg2Si particles due to their low quantity only contributing to satellite spots in the SAED pattern. The TEM image in Fig. 9a highlights that the TiB2 nanoparticles are tightly bonded with the Al matrix, showing one of the principal orientation relationships (ORs): [21 � 1 � 0] TiB 2 // [101]Al and (0001) TiB 2 //(1 � 11)Al (Fig.

Fig. 6

 6 Fig. 6 SEM/EBSD orientation maps of the (a, b) CS and (c, d) CS+FSP samples in high magnification: (a) and (c) AlSi10Mg; (b) and (d) TiB2/AlSi10Mg composite.

Fig. 7

 7 Fig. 7 Grain size distributions of the (a, b) CS and (c, d) CS+FSP samples according to EBSD analysis: (a) and (c) AlSi10Mg; (b) and (d) TiB2/AlSi10Mg composite.

Fig. 8 TEM

 8 Fig. 8 TEM BF images showing the TiB2 particle distributions of the TiB2/AlSi10Mg composite deposits (a) before and (b) after FSP treatment. (c) selected area electron diffraction (SAED) pattern of the post FSP sample where only Al and Si phases can be determined without ambiguity from the overlapped ring patterns

Fig. 10

 10 Fig. 10 Microhardness values of the CS and CS+FSP treated samples.

Fig. 11 (

 11 Fig. 11 (a) Tensile stress-strain curves for the AlSi10Mg alloy and the TiB2/AlSi10Mg composite at CS and after post-FSP. (b) Strain hardening rate as a function of true strain obtained from the tensile -strain curves. (c) Comparison of the ultimate tensile strength and elongation values of our samples with other AMCs processed by CS, CS + HT, CS + hot rolling (HR), and CS+FSP treatments[START_REF] Tariq | Cold spray additive manufacturing: A viable strategy to fabricate thick B4C/Al composite coatings for neutron shielding applications[END_REF][START_REF] Kumar | Microstructure and performance of cold sprayed Al-SiC composite coatings with high fraction of particulates[END_REF][START_REF] Yang | Optimization of cold-sprayed AA2024/Al2O3 metal matrix composites via friction stir processing: Effect of rotation speeds[END_REF][START_REF] Yang | Cold sprayed AA2024/Al2O3 metal matrix composites improved by friction stir processing: Microstructure characterization, mechanical performance and strengthening mechanisms[END_REF][START_REF] Xie | Cold spray additive manufacturing of metal matrix composites (MMCs) using a novel nano-TiB2reinforced 7075Al powder[END_REF][START_REF] Qiu | A hybrid approach to improve microstructure and mechanical properties of cold spray additively manufactured A380 aluminum composites[END_REF][START_REF] Gyansah | Materials Science Forum[END_REF][START_REF] Tariq | Thermo-mechanical post-treatment: A strategic approach to improve microstructure and mechanical properties of cold spray additively manufactured composites[END_REF][START_REF] Qiu | In-situ Sip/A380 alloy nano/micro composite formation through cold spray additive manufacturing and subsequent hot rolling treatment: Microstructure and mechanical properties[END_REF].

Fig. 12

 12 Fig. 12 SEM images showing the fracture surfaces of the CS TiB2/AlSi10Mg composite

1

 1 Insight into the key factors determining mechanical performance of CS

Fig. 13

 13 Fig. 13 Radar chart of defect level, reinforcement dispersion homogeneity, grain refinement, and interfacial bonding versus mechanical properties of CS AMCs prepared

  6 vol.%) and the average size of the TiB2 (~100 nm), respectively. λ is the interparticle spacing. As such, the calculated

Acknowledgments

Financial support of X.L. XIE's Ph.D. thesis was from the China Scholarship Council (No. 201604490100), Université Bourgogne -Franche-Comté, and Centre National de la Recherche Scientifique (CNRS). The microscopy facilities in Lille (France) are supported by the Conseil Régional du Nord-Pas de Calais, and the European Regional Development Fund (ERDF). G. JI thanks Campus France for providing travel funding through the PHC Xu Guangqi program (No. 45597XK). Prof. M.X. ZHANG (University of Queensland, Australia) was thanked for paper revision.