Florian Mermillod-Blondin
email: florian.mermillod-blondin@univ-lyon1.fr

Pierre Marmonier

Mélissa Tenaille

Damien G Lemoine

Michel Lafont

Ross Vander Vorste

Laurent Simon

Laurence Volatier

Ross Vander Vorste

Bottom-up processes control benthic macroinvertebrate communities and food web structure of fishless artificial wetlands

Keywords: functional feeding groups, benthic food webs, invertebrates, benthic biofilm, algae, bacteria, trophic relationships

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Introduction

The relative importance of bottom-up and top-down controls on the structure of communities has long been an intense debate [START_REF] Oksanen | Exploitation ecosystems in gradients of primary productivity[END_REF][START_REF] Hunter | Playing chutes and ladders: heterogeneity and the relative roles of bottom-up and top-down forces in natural communities[END_REF][START_REF] Polis | Food web complexity and community dynamics[END_REF][START_REF] Gruner | A cross-system synthesis of consumer and nutrient resource control on producer biomass[END_REF]. The exploitation ecosystem hypothesis (EEH) predicts that resource availability and predation control densities within adjacent trophic levels [START_REF] Oksanen | Exploitation ecosystems in gradients of primary productivity[END_REF].

Accordingly, increasing primary productivity lengthens food webs by boosting the establishment of predators, that will, in turn, have a greater impact on lower trophic levels. In nutrient-poor ecosystems, biomass of basal species is severely constrained by resource availability and is too low to support a significant consumer biomass [START_REF] Menge | Top-down and bottom-up community regulation in marine rocky intertidal habitats[END_REF]. In more nutrient-rich ecosystems, productivity of basal species increases with elevated resources but their biomass is kept constant by predation as consumers become more abundant [START_REF] Steiner | The effects of prey heterogeneity and consumer identity on the limitation of trophic-level biomass[END_REF]. Although it has been shown that the predictions of EEH are modulated by biotic (e.g., diversity of the food webs) and abiotic (e.g., habitat heterogeneity, disturbance) factors [START_REF] Hunter | Playing chutes and ladders: heterogeneity and the relative roles of bottom-up and top-down forces in natural communities[END_REF][START_REF] Leibold | Species turnover and the regulation of trophic structure[END_REF][START_REF] Menge | Environmental stress, bottom-up effects, and community dynamics: integrating molecular-physiological and ecological approaches[END_REF][START_REF] Foulquier | Surface water linkages regulate trophic interactions in a groundwater food web[END_REF], it provides a robust theoretical framework to evaluate the roles of bottom-up and top-down controls on ecosystem structure (e.g., [START_REF] Shurin | A cross-ecosystem comparison of the strength of trophic cascades[END_REF]Aunapuu et al. 2008;[START_REF] Lezama | Variation of grazinginduced vegetation changes across a large-scale productivity gradient[END_REF][START_REF] Du | Analyzing the importance of top-down and bottom-up controls in food webs of Chinese lakes through structural equation modeling[END_REF][START_REF] Hoset | Changes in the spatial configuration and strength of trophic control across a productivity gradient during a massive rodent outbreak[END_REF].

In wetlands, a historical view that bottom-up controls have greater relative importance than topdown controls in regulating ecosystem structure and function (e.g., [START_REF] Odum | Comparison of population energy flow of an herbivorous and a deposit-feeding invertebrate in a saltmarsh ecosystem[END_REF] has been reconsidered more recently [START_REF] Sala | The dynamics of bottom-up and top-down control in a New England salt marsh[END_REF][START_REF] Moore | Context-dependent consumer control in New England tidal wetlands[END_REF]. Indeed, top predators like fish or invertebrates can significantly control the abundance and the assemblage of their prey [START_REF] Diehl | Fish predation and benthic community structure: the role of omnivory and habitat complexity[END_REF][START_REF] Magnusson | Top-down control by insect predators in an intermittent pond-a field experiment[END_REF][START_REF] Cobbaert | Effects of a top invertebrate predator (Dytiscus alaskanus; Coleoptera: Dytiscidae) on fishless pond ecosystems[END_REF][START_REF] Laske | Top-down control of invertebrates by Ninespine Stickleback in Arctic ponds[END_REF]) and top-down control of grazers on primary producers play a key role on wetland functioning and stability (e.g., limitation of algal blooms; Hillebrand 2009; [START_REF] Bertness | Herbivory drives the spread of salt marsh die-off[END_REF]. In this context, artificial wetlands, which are increasingly developed to compensate for the loss of natural wetlands [START_REF] Davidson | How much wetland has the world lost? Long-term and recent trends in global wetland area[END_REF], could be useful tools for determining the respective roles of bottom-up and top-down forces on community structures because they allow for the selection of systems with distinct productivities.

Moreover, these artificial systems may be managed for controlling the water flux of organisms, reducing the heterogeneity in biological colonization among wetlands [START_REF] De Meester | Ponds and pools as model systems in conservation biology, ecology and evolutionary biology[END_REF].

We tested the predictions of EEH in shallow (water depth < 1.5 m) artificial wetlands having area of less than 2 ha and benthic habitat characterized by dense biofilms of phototrophic and heterotrophic micro-organisms [START_REF] Gette-Bouvarot | Coupling hydraulic and biological measurements highlights the key influence of algal biofilm on infiltration basin performance[END_REF]. To test the respective influences of bottom-up and top-down controls on benthic invertebrate communities, we selected three fishless shallow artificial wetlands characterized with contrasting primary productivities (estimated from net photosynthesis of benthic biofilms) associated with differences in sediment composition (proportion of clay and silt) and algal biomass at the water-sediment interface. We expected components of the biofilms to play a key role as food sources for the benthic food web creating a bottom-up effect. However, in accordance with the EEH we hypothesized the relative importance of bottom-up and top-down effects to shift from a dominance of bottom-up controls to a dominance of top-down controls with increasing primary productivity. To evaluate the response of food webs, we classified benthic invertebrate communities into trophic functional groups by including both the vertical (linear trophic chain from basal resources to predators through consumers) and horizontal dimensions of food webs (species diversity at each trophic level, following [START_REF] Srivastava | Reducing horizontal and vertical diversity in a foodweb triggers extinctions and impacts functions[END_REF]. Under a bottom-up control scenario, we predicted a positive relationship between the abundance of grazers and scrapers and primary productivity and algal biomass at the watersediment interface, a positive relationship between deposit-feeders (bacterivores) and bacterial abundance and organic matter (% of total organic carbon and total nitrogen) in sediment, and a positive relationship between macroinvertebrate predator abundance and the abundance of preys at lower trophic levels. Alternatively, under a top-down control scenario, we predicted no positive relationships between consumers and their food sources.

Material and Methods

Study site

Our experiment took place in a pumping well field of « Crépieux-Charmy » (3.75 km 2 ) located in the Rhône River floodplain (France), which provides drinking water for the metropolitan area of Lyon (1,280,000 inhabitants supplied). Artificial wetlands (surface area = 0.85-1.5 ha) have been built to recharge aquifer with river water. All wetlands were built by excavation in natural soils in areas with reduced riparian vegetation and were partially filled with a layer of calibrated sand about 20 cm thick and characterized by a low organic matter content (total organic carbon < 0.1% of sediment dry mass). Their main water supply is pumped in a channel (Vieux Rhône) of the Rhône River using centrifugal pumps (rotation: 1000 rounds per minute) that markedly limit the transport of top-predator fishes from Rhône River to wetlands. Despite similar designs, wetlands evolved over time due to the accumulation of fine particles (< 63 µm) at the water-sediment interface, influencing the availability of the benthic habitat for biofilms. Indeed, presence of fine sediments on a coarse and poorly organic sediment ameliorate the physical attachment and the nutrient availability for benthic biofilms [START_REF] Sundbäck | Importance of benthic nutrient regeneration during initiation of macroalgal blooms in shallow bays[END_REF][START_REF] Van De Koppel | Do alternate stable states occur in natural ecosystems? Evidence from a tidal flat[END_REF][START_REF] Watermann | Competition between benthic cyanobacteria and diatoms as influenced by different grain sizes and temperatures[END_REF]. For the present study, we selected three wetlands (called A, B, C) that were characterized by varying amounts of fine particles (proportions determined by laser diffractometry following [START_REF] Estragnat | Does the efficiency of grazer introduction to restore and preserve the hydraulic performance of infiltration basins depend on the physical and biological characteristics of the infiltration media?[END_REF]) at the water-sediment interface (Fig. 1A, one-way ANOVAs, comparison of fine sediment proportion among wetlands, F2,15 = 29.6, p < 0.001), leading to contrasted benthic habitats for benthic biofilms. Following the decreasing gradient of fine particle amount from wetland A to wetland C, algal biomass and net photosynthesis activity (see below for details on measurements) of the biofilm decreased from wetland A with a mean chlorophyll a content of 2.85 µg per cm 2 and positive net production to wetland C with a mean chlorophyll a content of 1.55 µg per cm 2 and negative net production as photosynthesis did not compensate for respiration (Fig. 1BC, one-way ANOVAs, comparisons of algal biomass and net photosynthesis among wetlands, F2,15 = 26.6, p < 0.001 for algal biomass, F2,15 = 23.6, p < 0.001 for net photosynthesis). These conditions allowed us to evaluate the trophic relationships between primary productivity, benthic food sources, and invertebrate communities.

Environmental conditions before sampling date

The three studied wetlands had the same water supply and water column depth during a period of 6 weeks before sampling date (on 22 nd of October 2014), minimizing the potential influence of hydrological conditions on community structure. During this period, we also verified that the three wetlands were characterized by comparable physico-chemical conditions in the water column. In situ monitoring of water temperatures showed similar variations (between 14.7 and 24.6 °C) in the three artificial wetlands. Weekly measurements of specific conductivity, pH, and dissolved oxygen (DO) concentrations performed with a HQ40D multiparameter (HACH) also indicated comparable values among artificial wetlands with mean (± standard deviation) values of 337 (± 13) μS/cm, 8.3 (± 0.3), and 9.4 (± 0.5) mg/L for specific conductivity, pH and DO, respectively. Nutrient concentrations were also measured weekly in the water column of the wetlands following Gette- [START_REF] Gette-Bouvarot | Coupling hydraulic and biological measurements highlights the key influence of algal biofilm on infiltration basin performance[END_REF]. Comparable dissolved nutrient concentrations and fluctuations were found in the three wetlands with concentrations of N-NO3 -, N-NH4 + and P-PO4 3-of 0.99 ± 0.21 (mean ± SD) mg/L, 32.78 ± 20.13 µg/L and 12.29 ± 9.27 µg/L, respectively.

Sampling design

Biofilm and invertebrate samples were collected on the same day (October 22, 2014) to obtain a data set linking benthic fauna with characteristics of the sedimentary biofilm. Analyses on benthic samples were performed to determine biofilm characteristics (fluorescence measurements of benthic algal biomass and composition, bacterial abundance, total organic carbon and total nitrogen of the biofilm) and benthic communities of invertebrates. To facilitate the collection of benthic samples, the water pumping into the study wetlands was stopped 2 days before the sampling date to have between 30 and 40 cm of water level in all wetlands. In each wetland, we randomly selected 6 replicate zones. For biofilm analyses, the top layer (0-1 cm) of sediments was cored in each zone using cut syringes (internal diameter = 15 mm). Fifteen samples were collected, mixed and homogenized per zone to obtain representative sediment samples. For net photosynthetic activity of the biofilm, analyses were performed on one intact core collected per zone. Benthic invertebrates were sampled in each zone using a 200-µm-mesh surber net following core collections for sediment and biofilm analyses. The top sediment layer was collected on a surface of 150 cm 2 per zone and preserved with 96% ethanol. All samples were then stored at ~10°C during transport to the laboratory within 4 hours.

Chemical and biological analyses on sediments

The biomass of green algae, diatoms and cyanobacteria at the water-sediment interface were estimated by in situ measurements of chlorophyll-a concentration and were expressed as chlorophyll-a equivalent per unit of surface. These measurements were performed with a benthic fluorimetric probe (BenthoTorch, bbe Moldaenke GmbH) (see Harris and Graham 2015 for details).

The bacterial cell numbers were evaluated on sediment previously fixed with formaldehyde (final concentration 2 % v/v) by direct microscopy counts after incubation of the sediment with the Syto9 green fluorescent nucleic acid stain (Invitrogen). Details of the protocol were given in Gette- [START_REF] Gette-Bouvarot | Coupling hydraulic and biological measurements highlights the key influence of algal biofilm on infiltration basin performance[END_REF]. The total bacterial abundance (cells/g of dry sediment) was then determined.

Total organic carbon (TOC) and total nitrogen (TN) were measured as estimators of the sedimentary biofilm biomass. Following [START_REF] Mermillod-Blondin | Dynamics of dissolved organic carbon (DOC) through stormwater basins designed for groundwater recharge in urban area: Assessment of retention efficiency[END_REF], TOC and TN were determined by high-temperature combustion of in situ pre-acidified (HCl 2N, to remove inorganic carbon) dry samples (60°C, 48h) and subsequent measurements of CO2 and N2 by thermal conductometry using an elemental analyzer (FlashEA 1112, Thermo Electron Corporation).

Net photosynthetic activity (NP) was determined by measuring oxygen production at 15 °C (close to field temperature at sampling date) on intact biofilms (4 cm 2 collected with a cut syringe, see above) using 4 mL respiration chambers fitted with micro-probes (Microrespiration system, Unisense, see Gette-Bouvarot et al. 2015 for details). Photosynthesis measurements were performed with an irradiance of 150 µmol.m -2 .s -1 , close to maximal photosynthetically active radiations (PAR) measured at midday with a between 20 and 130 µmol m -2 s -1 depending on meteorological conditions). For measurements, total dissolved inorganic carbon concentration was adjusted to 10 mM with KHCO3 to ensure sufficient supply of inorganic carbon during the incubation and pH was maintained at 6.5 by addition of a citrate buffer. Net photosynthesis rate was expressed as nmol of O2.min -1 .cm -2 of sediment surface.

Analyses on benthic invertebrates

Benthic invertebrates were collected by scraping a surface area of 150 cm 2 with a surber net in each sampled zone. Invertebrates were sorted and identified using [START_REF] Tachet | Invertébrés d'eau douce: systématique, biologie, écologie[END_REF] in the laboratory using a stereomicroscope. All taxa were classified into seven trophic functional groups (grazers, scrapers, deposit-feeders, filter-feeders, predators-parasites, detritivores and shredders) according to their trophic affinities [START_REF] Tachet | Invertébrés d'eau douce: systématique, biologie, écologie[END_REF]. When one taxon was affiliated to 2 or more functional groups, we applied a percentage of functional affinity in the abundance table for the given taxon (i.e., 50 % and 50 % for one taxon affiliated to 2 functional groups).

Statistical analyses

One-way ANOVAs were performed to compare biofilm variables and the abundances and richness (numbers of taxa) of each functional group of benthic invertebrates among wetlands. The relationship between the biofilm table and the fauna table was evaluated by a co-inertia analysis (see [START_REF] Dolédec | Co-inertia analysis: an alternative method for studying speciesenvironment relationships[END_REF][START_REF] Dray | Co-inertia analysis and the linking of ecological data tables[END_REF]. This multi-table analysis was performed to examine the links between the two tables, each table using the same 18 samples as rows. The relationship between each pair of data sets was measured using the Rv-coefficient (vectorial correlation coefficient; [START_REF] Robert | A unifying tool for linear multivariate statistical methods: the RVcoefficient[END_REF]. The Rv-coefficient between two tables ranges between 0 and 1 and is a multidimensional equivalent of the ordinary correlation coefficient between two variables. For two given data sets Xk and Xl, the Rv-coefficient writes as follows: Linear regressions were performed to determine the significance of the correlations among biofilm characteristics and the abundance and richness of invertebrate functional groups (e.g., green algae and grazer abundance). For all variables, the normality and the homoscedasticity of the residuals were verified using the Shapiro-Wilk's test and the Levene's test, respectively. ANOVA analyses, co-inertia analysis, linear regressions and graphical displays were performed using functions in the ade4 package [START_REF] Chessel | The ade4 package-I-One-table methods[END_REF][START_REF] Dray | The ade4 package-II: Two-table and K-table methods[END_REF]) in R (R Development Core team 2015).

𝑅𝑣(𝑋 𝑘 , 𝑋 𝑙 ) = 𝐶𝑜𝑣𝑣(
Significance for statistical tests was accepted for a first species risk α < 0.05.

Results

Biofilm characteristics of the three wetlands

Algal colonization of the water-sediment interface was significantly different among wetlands (Fig.

2

) and followed the gradient of net productivity measured (Fig. 1): green algae and diatom biomasses were positively correlated (Pearson's r = 0.829) and exhibited decreasing values from wetland A to wetland C with intermediate values in wetland B (one-way ANOVAs, F2,15 = 58.9, p < 0.001 for green algae and F2,15 = 10.1, p < 0.01 for diatoms). The densities of these two algal groups were positively correlated with the net photosynthetic activity (R 2 = 0.65, p < 0.001, n = 18 with green algae biomass, and R 2 = 0.46, p < 0.01, n = 18 with diatom biomass). Cyanobacteria biomass showed a distinct pattern as it was significantly lower in wetland B than in the two other wetlands (Fig. 2, one-way ANOVA, F2,15 = 79.4, p < 0.001). Percentages of total organic carbon and total nitrogen were also the lowest at the sediment interface of wetland B (one-way ANOVAs, F2,15 = 5.1, p < 0.05 for total organic carbon and F2,15 = 5.9, p < 0.05 for total nitrogen). Hence, we detected a significant and positive correlation between cyanobacteria biomass and total nitrogen (R 2 = 0.37, p < 0.01, n = 18) but not between cyanobacteria biomass and total organic carbon (R 2 = 0.18, p > 0.05, n = 18). In contrast, bacterial abundances were negatively correlated with cyanobacteria biomasses (R 2 = 0.55, p < 0.001, n = 18) and exhibited higher cell numbers in wetland B than in the other wetlands (one-way ANOVA, F2,15 = 14.6, p < 0.001).

Functional composition of benthic invertebrate communities in the three wetlands

Abundances and richness of grazers and scrapers were significantly different among wetlands (oneway ANOVAs, F2,15 = 14.1, p < 0.001 for grazer abundance, F2,15 = 11.8, p < 0.001 for scraper abundance, F2,15 = 5.0, p < 0.05 for grazer richness, and F2,15 = 15.3, p < 0.001 for scraper richness).

Following the decreasing gradient of net productivity, they were the highest in wetland A and the lowest in wetland C with intermediate values in wetland B (Fig. 3). The abundance of filter-feeders tended to follow the same pattern with highest values in wetland A (Fig. 3, one-way ANOVA, F2,15 = 7.8, p < 0.01) although filter-feeder richness did not vary significantly among wetlands (one-way ANOVA, F2,15 = 1.7, p > 0.22). Deposit-feeders exhibited a particular distribution compared with grazers, scrapers and filter-feeders (Fig. 3). They were the most numerous in wetland B (one-way ANOVA, F2,15 = 10.8, p < 0.01) but their richness was low (between 1 and 2 taxa) and not significantly different among wetlands (one-way ANOVA, F2,15 = 2.5, p > 0.1). In contrast with the other functional groups, the abundance and richness of predators were not significantly different among wetlands (Fig. 3, one-way ANOVA, F2,15 = 2.4, p > 0.1 for predator abundance and F2,15 = 2.3, p > 0.15 for predator richness).

Relationships between biofilm characteristics and functional composition of benthic invertebrate communities

The first two axis of co-inertia analysis explained more than 95% of the co-variation between biofilm and fauna datasets and the structure shared by the two datasets was significantly different from a structure expected from random association (Rv = 0.60, p < 0.001). Samples were discriminated among wetlands (Fig. 4A) and the short arrows displayed on Fig. 4A underlined the good fit between biofilm and fauna datasets in the shared structure.

On the factorial map of variables (Fig. 4BC), the horizontal axis indicated that samples characterized by high net photosynthetic activity and high diatom and green algae biomasses were also dense and rich in herbivores (grazers and scrapers). There were significant positive linear correlations between these biofilm characteristics (net photosynthetic activity, diatom or green algae biomass) and the densities of herbivores (grazers and scrapers) (Fig. 5). The richness of grazers and scrapers were also linearly and positively correlated with these biofilm characteristics and more especially with the biomass of green algae (relationships illustrated on Fig. 6). The vertical axis of the co-inertia analysis discriminated the samples with high bacterial densities and high abundances of deposit feeders from those of high cyanobacteria biomass and nitrogen content and low deposit-feeder abundances (Fig. 4BC). The abundance of deposit feeders was significantly and positively correlated with bacterial abundance (Fig. 7A) but not with total nitrogen and total organic carbon (R 2 < 0.1, p > 0.05, n = 18). The abundance of deposit feeders was negatively correlated with cyanobacteria biomass (Fig. 7B). We did not detect clear correlations between biofilm characteristics and the functional groups of filter feeders and predators. The abundance of predators was significantly and positively correlated with the abundance of other invertebrates (Fig. 8). No significant correlations were obtained between predator richness and the abundance and richness of other invertebrates.

Discussion

Although the literature highlighted the occurrence of significant top-down control of communities in wetlands [START_REF] Magnusson | Top-down control by insect predators in an intermittent pond-a field experiment[END_REF][START_REF] Hillebrand | Meta-analysis of grazer control of periphyton biomass across aquatic ecosystems[END_REF][START_REF] Moore | Context-dependent consumer control in New England tidal wetlands[END_REF], top-down forces were apparently weak in our systems. Then, our dataset did not corroborate the prediction of EEH [START_REF] Oksanen | Exploitation ecosystems in gradients of primary productivity[END_REF]) suggesting a significant influence of top-down forces in our most productive wetland. The analysis of the links between biofilms and functional trophic groups of invertebrates clearly showed a dominant bottom-up effect of primary productivity (net photosynthesis) and algal resources on herbivores in artificial wetlands. Indeed, the abundances of both grazers and scrapers were positively and linearly correlated with the biomass of green algae and diatoms in wetlands.

These correlations indicate that the bottom-up control of green algae and diatoms swamped out the top-down influence of herbivores on algal resources. As previous enclosure experiments showed that the introduction of a grazer (the gastropod Viviparus viviparus , a species which is not naturally present in our artificial wetlands) had a significant top-down effect on algae in these artificial wetlands [START_REF] Gette-Bouvarot | Ecological engineering approaches to improve hydraulic properties of infiltration basins designed for groundwater recharge[END_REF][START_REF] Estragnat | Does the efficiency of grazer introduction to restore and preserve the hydraulic performance of infiltration basins depend on the physical and biological characteristics of the infiltration media?[END_REF], we can conclude that pressure by grazers and scrapers naturally present in these systems was not sufficient enough to reduce algal biomass.

Positive relationships between herbivore abundance and biomass of green algae and diatoms contrasted with the lack of relationship detected between herbivores and cyanobacteria biomass.

This result may suggest that herbivores consumed green algae and diatoms rather than cyanobacteria in wetlands, resulting in no relationship between cyanobacteria biomass and herbivore abundance. This suggestion of a poor trophic transfer from cyanobacteria to herbivores is consistent with several studies indicating that cyanobacteria are generally non-attractive trophic sources for invertebrates due to their poor nutritional quality (low essential fatty acids and P contents, [START_REF] Gulati | The role of food quality for zooplankton: remarks on the state-of-theart, perspectives and priorities[END_REF][START_REF] Paerl | Ecology of harmful cyanobacteria[END_REF] and their chemical and structural defense against grazing [START_REF] Demott | Effects of toxic cyanobacteria and purified toxins on the survival and feeding of a copepod and three species of Daphnia[END_REF][START_REF] Soares | Responses of the rotifer Brachionus calyciflorus to two tropical toxic cyanobacteria (Cylindrospermopsis raciborskii and Microcystis aeruginosa) in pure and mixed diets with green algae[END_REF][START_REF] Bownik | Harmful algae: Effects of cyanobacterial cyclic peptides on aquatic invertebratesa short review[END_REF]. Our correlation analyses also suggest that cyanobacteria had a negative influence on deposit-feeders. However, a toxic impact of cyanobacteria on this functional group is highly improbable because examples of negative interactions between cyanobacteria and deposit feeders in the field are lacking in the literature and, in contrast, a study reported a positive effect of the fixation of N2 by cyanobacteria on deposit feeders [START_REF] Karlson | Nitrogen fixed by cyanobacteria is utilized by deposit-feeders[END_REF]. Moreover, we did not detect any negative influence of cyanobacteria biomass on grazers and scrapers, two trophic functional groups that would be the most affected by toxin produced by cyanobacteria [START_REF] Bownik | Harmful algae: Effects of cyanobacterial cyclic peptides on aquatic invertebratesa short review[END_REF]). Together, these results suggest that cyanobacteria were probably not toxic for invertebrates in our artificial wetlands.

In addition to the positive relationship between algal (green algae and diatoms) biomass and herbivore abundances, we also showed that grazer and scraper richness were positively correlated with the biomasses of green algae, diatoms and photosynthetic activity. These results indicate that basal resources and net primary productivity may positively influence the horizontal diversity of the food web (sensu [START_REF] Srivastava | Reducing horizontal and vertical diversity in a foodweb triggers extinctions and impacts functions[END_REF]. This positive relationship between primary productivity (and energy associated with trophic resources) and herbivore taxa richness can be related to the "species-energy theory" [START_REF] Wright | Species-energy theory: an extension of species-area theory[END_REF][START_REF] Srivastava | Why more productive sites have more species: an experimental test of theory using tree-hole communities[END_REF][START_REF] Evans | Species-energy relationships at the macroecological scale: a review of the mechanisms[END_REF]. Indeed, several studies support the hypothesis that energy correlates positively with species richness (e.g, [START_REF] Huston | Biological diversity: the coexistence of species[END_REF][START_REF] Evans | Species-energy relationships at the macroecological scale: a review of the mechanisms[END_REF]. In our field conditions, we can expect that an increase in benthic primary productivity could have elevated the amount of certain individual resource types (i.e., algal species), enabling specialist herbivore species to feed on them.

Consequently, this trophic specialization would have led to reduced niche overlap among herbivores, reducing rates of competitive exclusion and elevating species richness in more productive wetlands. We also suspect that the diversity of individual resource types (i.e., algal species) increased with benthic primary productivity, enabling the highest herbivore diversity in the wetland characterized by the highest algal richness. Nevertheless, we cannot confirm this potential link between the trophic specialization of herbivore taxa and the abundances of individual resource types because green algae and diatom groups contained several species with contrasted trophic preferences for invertebrates (e.g., [START_REF] Tall | Resource partitioning in a grazer guild feeding on a multilayer diatom mat[END_REF][START_REF] Alfaro | Diet of the pulmonate gastropod Onchidella nigricans in the intertidal rocky shore, New Zealand[END_REF]). More precise analyses such as invertebrate gut analyses using metabarcoding approaches [START_REF] Pompanon | Who is eating what: diet assessment using next generation sequencing[END_REF]) would be necessary to determine the trophic niche of each herbivore taxa in association with the availability of algal species in artificial wetlands. This perspective is very promising to evaluate whether the trophic specialization of herbivores in the most productive wetlands can explain the positive relationship between net benthic primary productivity (energy) and herbivore taxa richness in our artificial systems.

The positive and significant linear correlation between the abundances of predators and other aquatic invertebrates also suggest that bottom-up control of predators by their prey blurred the topdown control exerted by predators on their prey in our artificial wetlands. The lack of a strong topdown control of invertebrate predators on other invertebrates in the most productive wetland (wetland A) would be associated with the increased horizontal diversity in food web observed for grazers and scrapers in this wetland. Indeed, increased diversity of a trophic level would mitigate top-down control by predators on this trophic level due to differences in predation pressure among the species of this trophic level. More precisely, a resource base with more species is more likely to contain at least one species that is relatively resistant to consumption and can compensate for the decrease of more consumable species in the presence of consumers [START_REF] Leibold | Resource edibility and the effects of predators and productivity on the outcome of trophic interactions[END_REF][START_REF] Duffy | The functional role of biodiversity in ecosystems: incorporating trophic complexity[END_REF]). However, the mechanism responsible of the lack of top-down cascade in our artificial wetlands is out of the scope of the present study. This further step would require more complete description and quantification of species-species interactions within the food webs.

Considering the detritus-based food web, the positive linear relationship between the bacterial abundance and the abundance of deposit-feeders suggests that the functional group of depositfeeders was also bottom-up controlled by their trophic resources. Nevertheless, the abundances of bacteria and deposit-feeders were not correlated with the quantity of total organic carbon and total nitrogen in the sediment. These results are not consistent with the positive relationship between organic matter quantity and decomposer (micro-and macro-organisms) abundance classically supporting the bottom-up theory in sedimentary environments (Dannovaro et al. 1995;[START_REF] Lin | The effect of organic pollution on the abundance and distribution of aquatic oligochaetes in an urban water basin, Taiwan[END_REF][START_REF] Ramalho | Temporal and spatial variation in the Nazaré Canyon (Western Iberian margin): Inter-annual and canyon heterogeneity effects on meiofauna biomass and diversity[END_REF]. The positive link between bacteria and deposit-feeder abundance observed in the present study was likely due to a positive influence of deposit-feeding activities on bacteria, a process largely demonstrated in a wide range of freshwater sediments (van de [START_REF] Van De Bund | Effects of deposit-feeder activity on bacterial production and abundance in profundal Lake sediment[END_REF][START_REF] Griebler | Some applications for the DMSO-reduction method as a new tool to determine the microbial activity in water-saturated sediments[END_REF][START_REF] Traunspurger | The effects of nematodes on bacterial activity and abundance in a freshwater sediment[END_REF][START_REF] Wieltschnig | Effects of deposit-feeding macrofauna on benthic bacteria, viruses, and protozoa in a silty freshwater sediment[END_REF][START_REF] Nogaro | Ecosystem engineering at the sediment-water interface: Bioturbation and consumersubstrate interaction[END_REF][START_REF] Pigneret | Urban pollution of sediments: Impact on the physiology and burrowing activity of tubificid worms and consequences on biogeochemical processes[END_REF]. A recent experiment performed in one of the artificial infiltration wetlands studied here demonstrated that aquatic oligochaetes, which are the main representatives of depositfeeders in the present study, had a strongly positive and significant influence on the abundance and activity of bacteria in sediments [START_REF] Mermillod-Blondin | Influence of tubificid worms on sediment structure, benthic biofilm and fauna in wetlands: A field enclosure experiment[END_REF]. More precisely, the bioturbation associated with the addition of the tubificid worm Limnodrilus hoffmeisteri increased the bacterial abundance by 2-fold and the microbial hydrolytic activity by 35% without influencing the quantity of total nitrogen and organic carbon in sediments (Table 1). Thus, the top-down and positive effect of deposit-feeders on micro-organisms was the most probable mechanism explaining the positive relationship between bacteria and deposit-feeder abundances in the present experiment.

Conclusions

Our results did not corroborate the predictions of EEH suggesting that top-down controls would have a significant influence on community structure in the most productive ecosystems. Indeed, a dominance of bottom-up processes in the phototroph-based food web occurred in our artificial wetlands whatever their productivity. Our strong linear correlations between the biomasses and productivity of the algal component of the biofilm (especially green algae and diatoms) and the abundances of grazers and scrapers suggest that algal biomass and benthic primary productivity supported the herbivore trophic level. In contrast with the phototroph-based food web, results obtained for the detritus-based food web illustrate a positive effect of deposit-feeding invertebrates on bacteria. As observed in sedimentary environments with oligochaetes [START_REF] Brinkhurst | Preliminary investigation of the exploitation of some potential nutritional resources by three sympatric tubificid Oligochaetes[END_REF][START_REF] Wavre | Interactions between some tubificid oligochaetes and bacteria found in the sediments of Toronto Harbour, Ontario[END_REF][START_REF] Fukuhara | The effects of Branchiura sowerbyi (Tubificidae) on bacterial populations in submerged ricefield soil[END_REF] and polychaetes (gardening effect, Hylleberg 1975), the feeding activities of deposit-feeder such as aquatic oligochaetes might have stimulated organic matter decomposition and bacterial growth in sediments. Thus, the present experiment showed that phototroph-based and detritus-based food webs were not dominated by the same processes in our artificial wetlands as herbivore abundance was likely controlled by the biomass of basal resources (algae) whereas deposit-feeder abundance was not linked to the amount of organic matter in sediments. Different feeding rates of invertebrates and contrasted renewal rate of basal resources (more rapid for algae than for detritic organic matter) between the two food webs may explain the different links between invertebrates and basal food sources reported for both food webs. These links are expected to have implications on the whole ecosystem functioning (nutrient cycling, energy fluxes) and need to be considered in future studies. With this aim, development of labelling approaches using stable isotopes -13 C and 15 N- [START_REF] Christianen | Benthic primary producers are key to sustain the Wadden Sea food web: stable carbon isotope analysis at landscape scale[END_REF]

  would be very promising to precisely quantify the transfer of energy within the phototrophic-based and detritusbased food webs and their consequences on ecosystem functioning. More research on artificial wetlands is needed because these artificial systems have been increasingly developed to compensate for the loss of natural wetlands, particularly in urbanized landscapes[START_REF] Davidson | How much wetland has the world lost? Long-term and recent trends in global wetland area[END_REF]). This is a major challenge for the future, considering the dramatic increase of the urbanization and the role played by wetland ecosystems for biodiversity preservation and nutrient retention(Hanson et al. 2005).

Figure legends : Fig 1 :

 :1 Figure legends:

Fig 2 :

 2 Fig 2: Characteristics of the biofilms in the three wetlands (n = 6 measures per wetland for each

Fig 3 :

 3 Fig 3: Abundances (ind/m 2 ) and richness (taxa/sample) of the five major trophic functional groups

Fig 4 :

 4 Fig 4: Co-inertia analysis showing a) the shared structure obtained from the biofilm and fauna

Fig 6 :

 6 Fig 6: Correlations a) between green algae biomass and grazer richness and b) between green algae

Fig 7 :

 7 Fig 7: Correlations a) between bacterial abundance and deposit-feeder abundance, and b) between

Fig 8 :

 8 Fig 8: Correlation between predator abundance and the abundance of other invertebrates.

  𝑋 𝑘 , 𝑋 𝑙 ) √ 𝑉𝑎𝑣(𝑋 𝑘 ) √𝑉𝑎𝑣(𝑋 𝑙 ) with 𝐶𝑜𝑣𝑣(𝑋 𝑘 , 𝑋 𝑙 ) = Trace (𝑋 𝑘 𝑡 𝐷 𝑛 𝑋 𝑙 𝐷 𝑝 ) as the vectorial covariance and Vav (𝑋 𝑘 ) = Trace (𝑋 𝑘 𝑡 𝐷 𝑛 𝑋 𝑘 𝐷 𝑝 ) and Vav (𝑋 𝑙 ) = Trace (𝑋 𝑙 𝑡 𝐷 𝑛 𝑋 𝑙 𝐷 𝑞 ) as the vectorial variance for Xk and Xl, respectively; Matrix Dn contains row weights (common to both data sets) and matrices Dp and Dq contain the column weights of Xk and Xl, respectively (see[START_REF] Blanc | Etude de la stabilité temporelle des structures spatiales par analyses d'une série de tableaux de relevés faunistiques totalement appariés[END_REF]. To test the significance of Rv-coefficients, rows were randomly permutated within tables. Permutations were repeated 1,000 times to obtain a null distribution of Rv-coefficients. We assessed the statistical significance by determining the proportion of null values that were greater than the observed Rv-

	coefficients.

Table 1 .

 1 Mean values (± SD) of the biofilm variables measured in 6 enclosures without worms and 6 enclosures in which a density of 20,000 individuals.m -2 of the tubificid worm L. hoffmeisteri has been added. Student-t tests were run to evaluate the influence of tubificid worm addition on biofilm

	variables. Modified from Mermillod-Blondin et al. (2018)
		Control enclosures	Worm enclosures
	Measured variables	averaged values ±	averaged values ±	t (10, 0.05)	p-values
		standard deviation	standard deviation
	Bacterial cells	3.20 ± 1.05	6.48 ± 1.90	4.04	0.002
	(10 9 /g of dry sediment)
	Total organic carbon	0.788 ± 0.424	0.704 ± 0.268	0.41	0.689
	(% of dry sediment)
	Total nitrogen	0.117 ± 0.056	0.112 ± 0.034	0.19	0.857
	(% of dry sediment)
	Hydrolytic activity	4.57 ± 1.00	6.16 ± 0.84	2.99	0.013
	(10 -2 µmol of hydrolyzed FDA
	.h -1 .g -1 of dry sediment)

Acknowledgments

We would like to thank Félix Vallier for his help during laboratory analyses. This research was done on the Research Platform of Crépieux-Charmy (Plate-forme de recherche de Crépieux-Charmy) and received financial and technical support from the Urban Community of Lyon (Grand Lyon La Métropole) and Veolia Water (Eau du Grand Lyon). This work was performed within the framework of the EUR H2O'Lyon (ANR-17-EURE-0018) of Université de Lyon (UdL), within the program "Investissements d'Avenir" operated by the French National Research Agency (ANR).

We also would like to thank the two anonymous referees for their pertinent comments that significantly improved an earlier version of our manuscript.

Supplementary Material 1: Table of taxa abundances for each sample (sampled surface: 150 cm 2 ).

638

The last column indicates the affiliation of taxa to functional groups. G: Grazers, S: Scrapers, DF: 639 Deposit-feeders, F: filter-feeders, P: predators and parasites, D: Detritivores, and Shr: Shredders.


T Khelfa
email: tarekkhelfa@nwpu.edu.cn

R Lachhab

H Azzeddine

F Li

J A Muñoz-Bolaños

J M Cabrera-Marrero

T Baudin

A L Helbert

F Brisset

M Khitouni

Thermal Stability of an Al-Mg-Si alloy after Equal Channel Angular Pressing

Keywords: Aluminum alloy, ECAP, Microstructure, Texture, Hardness, Activation energy

AA6060 aluminum alloy was subjected to severe plastic deformation (SPD) through Equal Channel Angular Pressing (ECAP) up to 8 passes via route BC. ECAPed samples isochronally annealed for 1 hour at a temperature range of 150-450°C. The microstructure and texture of the studied material were evaluated by Electron Back scatter Diffraction (EBSD), and the microhardness was characterized by Vickers microhardness testing. It was found that shearing texture is typically enhanced after ECAP processing. Grain size and grain growth kinetics were also studied. ECAP led to a substantial rise in hardness, with stability following 4 passes. Microstructures and material properties were relatively stable up to annealing temperatures of 150 °C. Some sub-micrometer grains were kept in the 8 passes sample to annealing temperatures of 300 °C. Annealing at elevated temperature resulted in a reduction in hardness leading to a rise in grain size and a decrease in dislocation density. After annealing temperature up to 450 °C, the texture index reveals a tendency to the texture weakening and 2 randomization. The activation energy required for the grain growth of the AA6060 alloy was exceptionally low above 300°C.

Introduction

Severe plastic deformation (SPD) is perhaps one of the most promising techniques to create ultra-fine grained (UFG) materials [START_REF] Valiev | Bulk nanostructured materials from severe plastic deformation[END_REF][START_REF] Valiev | Plastic deformation of alloys with submicron-grained structure[END_REF][START_REF] Valiev | Structure and properties of ultrafine-grained materials produced by severe plastic deformation[END_REF]. Equal-channel angular pressing (ECAP) as among the most efficient and attainable processes of severe plastic deformation (SPD) has extensive applications thanks to the manufacture of nanostructured (NS) and UFG metals and alloys [START_REF] Valiev | Principles of equal-channel angular pressing as a processing tool for grain refinement[END_REF][START_REF] Segal | Plastic working of metals by simple shear[END_REF]. On the other hand, it is well established that using SPD processes, the physical and mechanical properties of metallic materials can be significantly improved [START_REF] Inoue | Improvement of strength, toughness and ductility in ultrafine-grained low-carbon steel processed by warm bi-axial rolling[END_REF]. Additionally, SPD processes may also cause important changes in the evolution of microstructure and crystallographic texture relative to traditional processing methods [START_REF] Naseri | Effect of strain path on microstructure, deformation texture and mechanical properties of nano/ultrafine grained AA1050 processed by accumulative roll bonding (ARB)[END_REF]. AA6000 series aluminum alloys have drawn interest in the aviation, automotive, electrical engineering, and building industries thanks to their lightweight, excellent strength, high conductivity, and reasonable corrosion properties [START_REF] Bobruk | Enhanced strengthening in ultrafine-grained Al-Mg-Si alloys produced via ECAP with parallel channels[END_REF][START_REF] Yuan | Effects of La addition on the mechanical properties and thermal-resistant properties of Al-Mg-Si-Zr alloys based on AA 6201[END_REF][START_REF] Ji | Microstructures and properties of Al-Mg-SI alloy overhead conductor by horizontal continuous casting and continuous extrusion forming process[END_REF][START_REF] Murashkin | enhanced mechanical properties and electrical conductivity in ultrafine-grained Al alloy processed via ECAP-PC[END_REF]. Several studies have concentrated on the evaluation of AA6000 series alloys after ECAP processing [START_REF] Kim | Optimization of strength and ductility of 2024 Al by equal channel angular pressing (ECAP) and post-ECAP aging[END_REF][START_REF] Horita | Equal channel angular pressing of commercial aluminum alloys: grain refinement, thermal stability and tensile properties[END_REF][START_REF] Khelfa | Structure and microstructure evolution of Al-Mg-Si alloy processed by equal-channel angular pressing[END_REF][START_REF] Khelfa | Microstructure and strengthening mechanisms in an Al-Mg-Si alloy processed by equal channel angular pressing (ECAP)[END_REF]. For example in [START_REF] Khelfa | Microstructure and strengthening mechanisms in an Al-Mg-Si alloy processed by equal channel angular pressing (ECAP)[END_REF], AA6060 samples were severely ECAPed at ambient temperature up to 12 ECAP passes via route BC. In overall behavior, samples exhibited an equiaxed array of UFG microstructure and a rapid increase in high-angle grain boundaries (HAGB) fraction after 5 passes while a greater number of passes demonstrate a limited effect on strength and no significant change in the average grain size. In spite of the fact that UFG alloys can have remarkable mechanical properties, they can not be utilized at high temperatures except the microstructures are steady. In fact, the annealing temperature can strongly affect the microstructure evolution which has a significant impact on the physical, chemical, and mechanical characteristics.

Then, developing a better comprehension of the thermal stability behavior of UFG structures produced by ECAP claims interest from the scientific community. Recently, extensive research work has been performed to examine the impact of annealing on microstructure development of highly deformed materials (Cu, Al, Fe, Mg) [START_REF] Liang | Influence of microstructure on thermal stability of ultrafine-grained Cu processed by equal channel angular pressing[END_REF][START_REF] Rangaraju | Effect of cryo-rolling and annealing on microstructure and properties of commercially pure aluminium[END_REF][START_REF] Muñoz | Thermal stability of ARMCO iron processed by ECAP[END_REF][START_REF] Hanna | An investigation of the thermal stability of a Mg-Dy alloy after processing by high-pressure torsion[END_REF][START_REF] Lugo | Thermal stability of ultrafine grains size of pure copper obtained by equal-channel angular pressing[END_REF] and Al alloys [START_REF] Mao | Grain refinement, thermal stability and tensile properties of 2024 aluminum alloy after equal-channel angular pressing[END_REF][START_REF] Nikulin | The Effect of Temperature on Microstructure Evolution in a 7055 Aluminum Alloy Subjected to ECAP[END_REF][START_REF] Dhal | Precipitation phenomena, thermal stability and grain growth kinetics in an ultra-fine grained Al 2014 alloy after annealing treatment[END_REF][START_REF] Prangnell | Continuous recrystallisation of lamellar deformation structures produced by severe deformation[END_REF]. It was reported by Liang et al. [START_REF] Liang | Influence of microstructure on thermal stability of ultrafine-grained Cu processed by equal channel angular pressing[END_REF] that a lower number of ECAP passes results in retarded recrystallization process to longer annealing duration or elevated annealing temperature. With approximately the same strength properties and improved thermal stability, the microstructure got after 2 ECAP passes has some benefit in terms of the cost of production as opposed to 16 ECAP passes. Studying the annealing behavior of Al-0.13% and Mg Al-3%Mg treated by the ECAP process, Prangnell et al. [START_REF] Prangnell | Continuous recrystallisation of lamellar deformation structures produced by severe deformation[END_REF] mentioned that the early stages of annealing were considered to be particularly susceptible to the misorientation and distribution of the low angle boundaries maintained inside the structure of deformation. According to Dhal et al. [START_REF] Dhal | Precipitation phenomena, thermal stability and grain growth kinetics in an ultra-fine grained Al 2014 alloy after annealing treatment[END_REF], after annealing treatment, the precipitation of Cu2Al and AlCuMgSi particles in UFG Al 2014 alloy resulted in better thermal stability. In contrast, Han and Langdon [START_REF] Han | Significance of microstructural thermal stability in an Al-2219 alloy processed by severe plastic deformation, Ultrafine Grained Materials II[END_REF] gave proof that despite the fact that precipitates of θ' or θ are present in the AA 2219 aluminum alloy microstructure, they were not deemed powerful enough to inhibit the substantial growth of grain at high temperatures.

Nevertheless, to the best of our knowledge, no data dealing with the annealing behavior of the ECAPed AA6060 aluminum alloy have been reported until today. Accordingly, the current research aims to investigate the impact of annealing temperature on the microstructure morphology, texture and microhardness evolution of AA6060 aluminum alloy ECAPed up to eight ECAP passes.

Experimental procedure

The material studied is AA6060 aluminum alloy in the aged condition T6 as an extruded rod with a 10 mm diameter. Its detailed chemical composition has been reported elsewhere [START_REF] Khelfa | Structure and microstructure evolution of Al-Mg-Si alloy processed by equal-channel angular pressing[END_REF]. Rods were machined in about 60 mm long cylinders. The initial grain size was ~52 μm. ECAP was done using die with two channels that intersect at an angle of = 90° and an outer curvature angle of Ψ= 37°. With this configuration, a total strain of 1 is impaired in each pass [START_REF] Iwahashi | Principle of equal-channel angular pressing for the processing of ultra-fine-grained materials[END_REF]. The ECAP processing was done with a 0.02 m/s pressing velocity at room temperature. The lubricant used in the experiments was molybdenum disulfide (MoS2). Specimens were deformed up to eight ECAP passes through route BC. Annealing treatments were conducted at various temperatures after ECAP: 150, 200, 250, 300, 350, 400 and 450 ° C for 1 hour under high vacuum, with a heating rate of 20 ° C min -1 .The microstructures of the specimens were examined through electron backscatter diffraction (EBSD) observations in the Scanning Electron Microscope (SEM) with a Field-Emission Gun (FEG) SUPRA 55 VP operating at 20 kV. For EBSD observations, all specimens were cut in the transverse direction (TD) plane from the core area and then mechanically polished from 1200 grit silicon carbide paper till 0.02µm colloidal silica, followed by electro-polishing utilizing Struers A2 electrolyte at 22 V. The examined regions were 100 x100 µm 2 with a step size of 0.1 µm for specimens annealed at 150 °C, and 200 x 200 μm 2 with a step size of 0.2 μm for specimens annealed at 250 °C, 300 °C, 350 °C and 450 °C. TSL Orientation Imaging Microscopy (OIM™) software was used to perform the EBSD data collection. Quantitative texture analysis was performed using the MTex software to calculate the pole figure (111) and the orientation distribution function (ODF). The ODF was measured using the harmonic approach (L equal to 22) with every orientation modeled with a Gaussian function possessing a 5 ° half-width. [START_REF] Bachmann | Schaeben Texture analysis with MTEX[END_REF]. The linear intercept approach was used to calculate the mean grain size. The grain size values were determined with a 5 ° grain tolerance angle.

Vickers micro hardness of the specimens was characterized by SHIMADZU type HMV-2 tester using 100 g load (penetration time of 10 s) in the Extrusion Direction (ED)-Transversal Direction (TD) plane. At least 5 hardness indentations in the central regions of each specimen performed randomly. As reported earlier [START_REF] Khelfa | Microstructure and strengthening mechanisms in an Al-Mg-Si alloy processed by equal channel angular pressing (ECAP)[END_REF], with increasing ECAP passes, the microstructure evolved from a banded structure to a UFG one with elongated and smaller equiaxed grains mostly oriented along the shear direction. At lower recrystallization temperatures and strain (Figures 1c, 2c, and3c), OIM maps display duplex microstructures characteristic of discontinuous recrystallization mechanism, clearly distinguishing deformed areas from recrystallized grains. At more advanced stages of recrystallization, i.e. larger ECAP passes (Figures 4d and4e), the deformed structure's directionality is gradually lost.

Results and discussion

Microstructural evolution during annealing

The microstructures arising from annealing 1 ECAP specimen show that the total recrystallization mechanism begins at a later stage compared to higher ECAP passes. Indeed, annealed at 150 and 250°C, the one and two pass samples (Figures 1a-1b and Figures 2a-2b) have retained almost the same microstructure as the deformed state. However, small recrystallized grains can be noticed in the large deformed grains and gathered in islands at grain boundaries (see the yellow square in Figure 2b) after annealing at 250°C. A similar observation can be found in the sample ECAPed four times at 250°C in a more advanced stage. According to Humphreys et al. [START_REF] Humphreys | A Rollett, chapter 14 -continuous recrystallization during and after large strain deformation[END_REF], discontinuous recrystallization usually takes place on subsequent annealing for the greater original grain sizes and lower strains. Comparable retard in the recrystallization temperature has also been found by several authors [START_REF] Ferrasse | Texture evolution during equal channel angular extrusion (ECAE): part II. An effect of post-deformation annealing[END_REF][START_REF] Jazaeri | The transition from discontinuous to continuous recrystallization in some aluminium alloys: I -the deformed state[END_REF].

As previously stated [START_REF] Khelfa | Microstructure and strengthening mechanisms in an Al-Mg-Si alloy processed by equal channel angular pressing (ECAP)[END_REF], annealing in the range of 100-500°C of AA6060 alloy processed by ECAP leads to the formation of solute rich clusters/Guinier-Preston (GP) zones and to an important reduction in the peak temperature correlated with the 50% of recrystallization with increasing ECAP passes. It was suggested that can impose a pinning impact (Smith-Zener pinning) on grain boundary in the annealed specimen after one ECAP pass, which prevents or retards the mechanism of recrystallization. Studying the recrystallization mechanism at particles in rolled aluminum, Humphreys [START_REF] Humphreys | The nucleation of recrystallization at second phase particles in deformed aluminium[END_REF] has deduced that the recrystallization arises with a random orientation at a pre-existing sub-grain inside the deformation region. Then, the nucleation takes place by rapid sub-boundary migration.

Finally, once the deformation zone is consumed, the grain growth may stop.

Complete recrystallization seems to occur after annealing at temperatures of 450, 300, 300, and 350°C for samples ECAPed once, twice, four and eight times, respectively as displayed at Figures 1e, 2c, 3c, and4d. These results indicate that the kinetics of recrystallization significantly based on the strain imposed. It is well established that a rise in strain contributes to a rise in stored energy, which is the major driving energy for recrystallization. Consequently, the mechanism of recrystallization is accelerated. Inhomogeneous grain growth is obtained and some coarse grains can be observed in the EBSD maps at higher annealing temperatures (see Figures 1e, 2e, 3e, and4e) due to the intense grain growth. However, some small regions with fine grains are still noted. As reported in the literature [START_REF] Humphreys | chapter 1-Introduction[END_REF], the smaller grains are removed during annealing, while the larger grains grow, and the grain boundaries adopt a lower energy configuration. Under some conditions, through the mechanism of abnormal grain growth (secondary recrystallization), this normal grain growth can give way to the selective growth of a few large grains (Figure 3e, for example).

During annealing, changes in the relative proportions of HAGB were found to provide a valid way to determine the nature of the recrystallization mechanism [START_REF] Jazaeri | Quantifying recrystallization by electron backscatter diffraction[END_REF]. As discontinuous recrystallization occurs, the HAGB fraction rises fairly rapidly, while the recrystallizing grains consume the deformed substructure. On the other hand, there is a slight shift in the HAGB fraction during continuous recrystallization [START_REF] Jazaeri | Quantifying recrystallization by electron backscatter diffraction[END_REF]. The evolution of HAGB fraction during isochronal annealing is illustrated in Nevertheless, despite a rise in the annealing temperature, it can be found that for the 4 ECAP specimen, the HAGB fraction remains nearly stable in the 300-450 °C temperature range and above 350 °C for the 2 and 8 ECAP passes specimens. This observation cannot indicate the occurrence of continuous recrystallization since larger grains have been developed during recrystallization. Earlier observations [START_REF] Humphreys | A Rollett, chapter 14 -continuous recrystallization during and after large strain deformation[END_REF][START_REF] Jazaeri | The transition from discontinuous to continuous recrystallization in some aluminium alloys: I -the deformed state[END_REF], have suggested that continuous recrystallization occurs in specimens where a crucial HAGB fraction was formed during deformation (~64%). Thus, in the case of sufficiently large strains, the HAGB fraction rises to a reasonable value to maintain the microstructure against discontinuous recrystallization. In the present work, the AA6060 alloy has exhibited its maximum value of ~55% in HAGB fraction after deformation up to 8 ECAP passes [START_REF] Khelfa | Microstructure and strengthening mechanisms in an Al-Mg-Si alloy processed by equal channel angular pressing (ECAP)[END_REF] which seems to be insufficient to achieve continuous recrystallization.

Figures 6 and7 summarize the grain size evolution and the recrystallized grains, respectively.

Based on the grain orientation spread approach (GOS) available in OIM-software, the recrystallized grain fraction was determined. GOS represents the average deviation between every point orientation in the grain and the average grain orientation [START_REF] Wouters | Effects of crystal structure and grain orientation on the roughness of deformed polycrystalline metals[END_REF]. The recrystallized grains are distinct from the deformed ones by a threshold GOS value of 1° [START_REF] Cho | Determination of a mean orientation in electron backscatter diffraction measurements[END_REF]. In the present study, GOS ≤1° are considered to be fully recrystallized.

As plotted in Figure 6, the average recrystallized grains is gradually increasing with rising annealing temperature. Initially, the measured grain sizes are marked by a stage of relatively slow evolution until 300 °C. Further annealing up to 450 °C shows a faster evolution, and the smallest grain size is obtained after 8 passes, which is inconsistent with several previous findings [START_REF] Ferrasse | Texture evolution during equal channel angular extrusion (ECAE): part II. An effect of post-deformation annealing[END_REF][START_REF] Abib | Thermal stability of Cu-Cr-Zr alloy processed by equal-channel angular pressing[END_REF]. From this standpoint, it can be considered that the control of grain growth is more efficient after larger strain.

According to Humphreys predictions [START_REF] Humphreys | A unified theory of recovery, recrystallization and grain growth, based on the stability and growth of cellular microstructures-I. The basic model[END_REF], as the HAGB fraction produced on deformation increased, the microstructure gradually becomes stable against the catastrophic grains growth that bounded by HAGBs because of the energies and mobilities of HAGBs are not sensitive to boundary misorientation comparing to low angle boundaries. The average grain sizes after 1, 2, 4 and 8 ECAP passes after annealing at 450 ° C are approximately identical to 18 μm. This grain size is well below than the one of the as-received alloy (52 µm) [START_REF] Khelfa | Microstructure and strengthening mechanisms in an Al-Mg-Si alloy processed by equal channel angular pressing (ECAP)[END_REF] and could enhance its superplasticity ability.

The recrystallized grains fraction evolution in Figure 7 shows curves with a sigmoidal shape composing mainly of three distinct stages. The recrystallization fraction rate in the first stage ranges from 150 °C to 250 °C, and it grows slowly due to the incubation period. The second stage highlights a roughly considerable increase of the recrystallized grain fraction up to 350 °C. After that, a flat region with quite constant evolution takes place since the recrystallization is complete.

Texture evolution

Texture of the initial state

Figure 8 presents the texture of the as-received AA6060 alloy in terms of recalculated pole figure {111} and ODF section at φ2 = 0 and 45°, respectively. The as-received alloy exhibits a typical extrusion texture, where a typical <001> fiber is formed. In this texture, the directions <001> are preferably aligned along the extrusion direction [START_REF] Hirsch | Texture and Anisotropy in Industrial Applications of Aluminum Alloys[END_REF]. It also becomes apparent that the texture is dominated by the presence of components like Cube and Goss.

Texture evolution after ECAP and subsequent annealing

The ideal ECAP orientations for face-centered cubic (FCC) alloys utilizing a channel angle  = 90° are presented and their characteristic positions in Euler space (1,, 2) are given in Table 1. Copper and Al alloys using route Bc [START_REF] Higuera | Texture Analysis in Ultrafine Grained Coppers Processed by Equal Channel Angular Pressing[END_REF][START_REF] Reyes-Ruiz | Texture and Lattice Distortion Study of an Al-6061-T6 Alloy Produced by ECAP[END_REF].

These texture components seem to be stable until the 4 ECAP pass. Additionally, the ODF section of the sample extruded twice (Figure 11a) shows a weak A component. The intensity of the present orientations rises with rising the ECAP passes number up to 4 passes with a deviation from their ideal positions about 5-10° toward 1. This deviation is attributed to the effect of friction and dies geometry [START_REF] Beyerlein | Texture evolution in equal-channel angular extrusion[END_REF].

The presentation of ODF sections in the range of 0-360° in 1 revealed that the monoclinic symmetry is not evident. The monoclinic symmetry loss is frequently reported in route Bc due to a rotation around the ED axis between each pass [START_REF] Shaeri | Microstructure and texture evolution of Al-7075 alloy processed by equal channel angular pressing[END_REF]. After 8 ECAP passes, Figure 12 allows seeing significant texture changes. The texture intensity decreases and A component appears after annealing (Figure 12). This behavior leads to the formation of B fiber {hkl}<110> (containing the A, A , B and B components) [START_REF] Beyerlein | Texture evolution in equal-channel angular extrusion[END_REF]. Conversely, the A fiber {111}<uvw> (containing A, A , * A was reported in copper, and Al-7075 alloy with increasing the number of ECAP passes using route Bc [START_REF] Shaeri | Microstructure and texture evolution of Al-7075 alloy processed by equal channel angular pressing[END_REF][START_REF] Li | Texture evolution during multi-pass equal channel angular extrusion of copper: Neutron diffraction characterization and polycrystal modeling[END_REF]. Several authors [START_REF] Beyerlein | Texture evolution in equal-channel angular extrusion[END_REF][START_REF] Li | Texture evolution during multi-pass equal channel angular extrusion of copper: Neutron diffraction characterization and polycrystal modeling[END_REF] reported that the texture components in route Bc are not stable (both in position and intensity) during the first 4 passes because of the complexity in the imposed deformation, which makes the texture characterization relatively tricky compared to route A. Accordingly, the development of the different shear texture components in the first ECAP passes strongly depends on the initial texture [START_REF] Beyerlein | Texture evolution in equal-channel angular extrusion[END_REF][START_REF] Li | Texture evolution during equal channel angular extrusion: Effect of initial texture from experiment and simulation[END_REF][START_REF] Suwas | Evolution of crystallographic texture during equal channel angular extrusion of copper: The role of material variables[END_REF]. Thus, the ECAP processing with route Bc of material with strong <100> fiber leads to the development of a strong C component in the first passes [START_REF] Suwas | Evolution of crystallographic texture during equal channel angular extrusion of copper: The role of material variables[END_REF]. This observation is in good agreement with the present finding, where the initial texture of the alloy developed a welldefined <100> fiber (Figure 9). The texture weakening with increasing ECAP passes was reported in various FCC alloys processed by ECAP via route Bc [START_REF] Reyes-Ruiz | Texture and Lattice Distortion Study of an Al-6061-T6 Alloy Produced by ECAP[END_REF][START_REF] Shaeri | Microstructure and texture evolution of Al-7075 alloy processed by equal channel angular pressing[END_REF][START_REF] Abib | On the microstructure and texture of Cu-Cr-Zr alloy after severeplastic deformation by ECAP[END_REF]. According to some researchers [START_REF] Reyes-Ruiz | Texture and Lattice Distortion Study of an Al-6061-T6 Alloy Produced by ECAP[END_REF], this texture evolution is attributed to the formation of new grains with different or random orientations.

The texture after 1 ECAP pass seems to be stable up to annealing at 350 °C for 1 h. As the annealing temperature rises, the C component remains the dominant texture although their intensities decrease. Interestingly, annealing at 450 °C leads to some changes in the texture of the specimen with one pass where C component tends to disappear and B to appear. The A-fiber and B-fiber are partially present due to the absence of A and B component. It is important to note that this typical ECAP texture was evaluated upon annealing at 300 °C in the 2 ECAP passes specimen and rapidly after annealing at 250 °C in case of the 4 ECAP passes specimen. The development of shear texture with decreasing annealing temperature could be attributed to the increase of stored energy introduced by high strain, thereby the acceleration of static recrystallization.

A significant shear texture weakening can be noticed in all annealed specimens. The texture weakening and randomization were also reported after annealing of deformed pure copper via route Bc with increasing ECAP passes [START_REF] Molodova | Thermal stability of ECAP processed pure copper[END_REF]. Besides, intensity differences of A/ A and B/ B components are observed in the annealed samples implying the absence of the monoclinic symmetry.

The orientation changes during different annealing temperatures could be an indication of the discontinuous recrystallization occurrence. Indeed, it is well established that the evolution of the recrystallization texture is strongly related to the recrystallization mechanism [START_REF] Jazaeri | The transition from discontinuous to continuous recrystallization in some aluminium alloys: I -the deformed state[END_REF][START_REF] Jazaeri | The transition from discontinuous to continuous recrystallization in some aluminium alloys: IIannealing behavior[END_REF]. The deformation texture was found retained during continuous recrystallization [START_REF] Abib | Thermal stability of Cu-Cr-Zr alloy processed by equal-channel angular pressing[END_REF], while discontinuous recrystallization could lead to new orientations [START_REF] Tirsatine | On the recrystallization and texture of Fe-36%Ni alloy after accumulative roll bonding and annealing at 600[END_REF]. Indeed, the weakening of the recrystallization texture was associated with the appearance of large grains due to the occurrence of discontinuous recrystallization [START_REF] Molodova | Thermal stability of ECAP processed pure copper[END_REF].

Figure 12 illustrates the texture during the isochronal annealing of the 8 ECAP pass specimen. This figure highlights that the texture intensity is higher than the other samples. The B, A and * 2 A components disappeared upon annealing at 150 °C, and 250 °C. Figure 12d indicates a typical ECAP texture beginning from 300 °C.

The texture strengthening can be measured using the following equation [START_REF] Bunge | Texture Analysis in Materials Science: Mathematical Methods[END_REF]:

I = 1 8𝜋 2 ∫G 𝑓 2 (𝑔). 𝑑𝑔 (1)
where f(g) is the ODF intensity at g orientation and G is the Euler space.

Figure 13 displays the texture index evolution against the annealing temperature of AA6060 alloy after ECAP processing and subsequent annealing.

This figure demonstrates that the texture indexes is almost stable during annealing up to 250 °C for all ECAPed samples, except for the 4 ECAP pass one where the texture index decreases rapidly from 8.5 to 3.2 upon annealing at 150 °C. The decrease in the texture strength can be noticed after annealing at 250 °C followed by stabilization until annealing at 450 °C where the texture index value is similar for almost all the ECAPed samples (~ 1.4).

Microhardness and dislocation density evolution after ECAP processing and annealing

Figure 14 shows the microhardness variation after annealing treatments. Before annealing, the microhardness values rise steadily with rising the ECAP passes number. The tendencies of microhardness-temperature relation for all specimens are similar, i.e., the microhardness values vary slightly till the temperature of 150 °C. Then, the microhardness sharply drops during annealing temperatures between 150 °C and 300 °C, and finally, it stabilizes for temperatures higher than 350 °C.

As shown in Figure 14, the microhardness of specimens after 1 and 2 passes undergo a gradual increase during annealing up to 150 °C. This evolution is related to the creation and the precipitation of solute rich clusters which create a pinning effect of precipitates on dislocations causing the strengthening of the material. An analysis of differential scanning calorimetry (DSC) carried out on the identical material and under the identical ECAP conditions [START_REF] Khelfa | Microstructure and strengthening mechanisms in an Al-Mg-Si alloy processed by equal channel angular pressing (ECAP)[END_REF] demonstrated the presence of an exothermic peak in the range of 150-225 °C. This peak was associated with the GP zones precipitation.

A comparison with materials ECAPed 4 and 8 times revealed that the softening of these latter began at slightly lower annealing temperatures than the samples processed once and twice. In the range of temperature between 150-300 °C, the lowest microhardness value has been recorded for the four passes sample. In keeping with the rule of Johnson-Mehl-Avrami-Kolmogorov (JMAK) [START_REF] Humphreys | Recrystallisation and Related Annealing Phenomena[END_REF], this decrease becomes faster as the level of deformation rises. After registering almost the same microhardness values before annealing, the 8 ECAP pass sample seems to be more stable against annealing up to 300 °C having higher microhardness value than the 4 ECAP pass sample. The character of the curve indicates a shift in the process that governs the mechanical properties' deterioration. According to Hall-Petch relation [START_REF] Hall | The deformation and ageing of mild steel: III discussion of results[END_REF][START_REF] Petch | The cleavage strength of polycrystals[END_REF], the strength of severely deformed UFGed material is affected mainly by the grain size and the dislocation densities [START_REF] Tóth | A dislocation-based model for all hardening stages in large strain deformation[END_REF]. Therefore, it is expected that the coarsening of grain and the elimination of dislocations during annealing would regulate the hardness and strength of the material. Figure 6, displaying the evolution of grain size, and Figure 15, displaying the evolution of dislocation density with isochronal annealing, need to be checked at the same time.

The Kernel Average misorientation (KAM) method characterizing the local misorientation is used to calculate the Geometrically Necessary Dislocations (GND) density in all the microstructure. Its principle is well explained in the literature [START_REF] Betanda | Measurement of stored energy in Fe-48%Ni alloys strongly cold-rolled using three approaches: Neutron diffraction, Dillamore and KAM approaches[END_REF]. From misorientation measurements, the density of dislocation (ρGND) can be calculated using the next equation [START_REF] Takayama | Stored energy and Taylor factor relation in an Al-Mg-Mn alloy sheet worked by continuous cyclic bending[END_REF][START_REF] Liu | Effect of grain orientation on deformation structure in coldrolled polycrystalline aluminium[END_REF][START_REF] Calcagnotto | Orientation gradients and geometrically necessary dislocations in ultrafine grained dual-phase steels studied by 2D and 3D EBSD[END_REF]:

𝜌 𝐺𝑁𝐷 = 𝛼 𝜃 𝐾𝐴𝑀 𝑋𝑏 ( 2
)
where α is the parameter depending on the type of grain boundary and it is is equivalent to two for the tilting boundaries, four for twist ones and three for the mixture of the two types, X = nd is the kernel size (n the closest neighbors to be identified (n=3)) and d is the scan step (d=0.1 µm for specimens annealed at 150 °C and d= 0.2 µm for specimens annealed at 250 to 450 °C ) and b is the Burgers vector (b= 0.286 nm in aluminum).

The grain growth is negligible in the low temperature region (i.e.,150 °C-250 °C), while the dislocation density exhibits a dramatic collapse suggesting regeneration of the dislocation structure associated with UFG structure creation. In this temperature region, the most likely re-arrangement and mutual annihilation of dislocations with opposite signs occur during annealing. This observation ties well with earlier reports [START_REF] Stráská | Microstructure stability of ultra-fine grained magnesium alloy AZ31 processed by extrusion and equal-channel angular pressing (EX-ECAP)[END_REF][START_REF] Zi | Response of aluminum processed by extrusion preceded ECAP to isochronal annealing[END_REF]. The fine grain structure is unstable during annealing and the grain sizes of recrystallized grains increase at temperatures 250 < T < 300 °C (see Figure 6). For all samples, the dislocation density declines abruptly in the same temperature range. Indeed, both dislocations annihilation and coarsening of grain are considered to contribute concurrently to the alloy softening.

At elevated temperatures (T > 350 °C), the grain growth is likely to greatly affect the hardness of the studied alloy because no significant change occurs on dislocation density evolution.

Grain Growth Kinetics

The mechanism of grain growth requires a specific driving force, which is either supplied by diverse microstructural modifications ( recrystallization, precipitate evolution, annihilation) or by external heat sources. Thus, normal grain growth is commonly characterized by a form relationship.

[61]:

𝑑 2 -𝑑 0 2 = 𝑘 𝑡 (3)
where d is the mean grain size after annealing, d0 is the original grain size [START_REF] Khelfa | Microstructure and strengthening mechanisms in an Al-Mg-Si alloy processed by equal channel angular pressing (ECAP)[END_REF], t is the annealing duration (where t=1h is used in those studies) and k is a parameter that describes the mobility of the grain boundary and the interface. The grain growth rate is correlated with the factor of boundary mobility (k) and can be calculated through the use of the conventional Arrhenius equation [START_REF] Wang | An Investigation of Microstructural Stability in An Al-Mg Alloy with Submicrometer Grain Size[END_REF]:

𝑘 = 𝐴 exp( -𝑄 𝑅𝑇 ) (4)
where A is a pre-exponential constant, R is the gas constant, Q is the activation energy for boundary mobility that is correlated with the driving force needed to activate grain growth, and T is the absolute temperature of annealing.

The present data can be interpreted by constructing the plot of ln (

𝑑 2 -𝑑 0 2 𝑡
) versus l000/RT.

Figure 16 represents the annealing influence on grain growth kinetics of AA6060 alloy. This plot allows distinguishing two temperature ranges with different activation energies. The difference in the rate of grain growth from lower to higher temperatures, which reveals distinct growth regimes, has been extensively reported in the literature [START_REF] Wang | An Investigation of Microstructural Stability in An Al-Mg Alloy with Submicrometer Grain Size[END_REF][START_REF] Tighiouaret | Thermal Stability of an Mg-Nd Alloy Processed by High-Pressure Torsion[END_REF][START_REF] Park | Microstructural characteristics and thermal stability of ultrafine grained 6061 Al alloy fabricated by accumulative roll bonding process[END_REF][START_REF] Mohamed | Deformation mechanism maps based on grain size[END_REF][START_REF] Dhal | Insight into the microstructural evolution during cryo-severe plastic deformation and post-deformation annealing of aluminum and its alloys[END_REF]. At low annealing temperature (T < 300 °C), Q is the energy needed for complete recrystallization of the unrecrystallized microstructure, while at the elevated annealing temperature (T > 300 °C) Q is the activation energy for coarsening of the initial UFG microstructure [START_REF] Dhal | Insight into the microstructural evolution during cryo-severe plastic deformation and post-deformation annealing of aluminum and its alloys[END_REF].

One can easily observe that the activation energy continually reduces with increasing ECAP passes. A similar observation was obtained earlier [START_REF] Molodova | Thermal stability of ECAP processed pure copper[END_REF][START_REF] Higuera-Cobos | Mechanical, microstructural and electrical evolution of commercially pure copper processed by equal channel angular extrusion[END_REF]. This finding was correlated with a rise in HAGBs fraction with an increasing ECAP passes number allowing less activation energy of recrystallization.

The main activation energy (Q) determined from the slopes of the experimental results in the unrecrystallized grains at lower temperatures (≤ 300 °C), is about ~ 75 kJ mol -1 . This value is significantly smaller than the predicted value for lattice self diffusion in pure Al (~143.4 kJ mol -1 [START_REF] Mohamed | Deformation mechanism maps based on grain size[END_REF]), or the activation energy for Mg impurity diffusion in Al (~130.5 kJ mol -1 [START_REF] Rothman | Tracer Diffusion of Magnesium in Aluminum Single Crystals[END_REF]), neither that for lattice diffusion of Si in Al (~125-154 kJ mol -1 [START_REF] Huang | Subgrain growth and low angle boundary mobility in aluminium crystals of orientation {110}〈001[END_REF]). However, it is quite closer to the value expected for grain boundary diffusion in Al (~86 kJ mol -1 [START_REF] Mohamed | Deformation mechanism maps based on grain size[END_REF]) and that for diffusion across dislocation cores in Al (~82 kJ mol -1 [START_REF] Murr | Interfacial Phenomena in Metals and Alloys[END_REF]). Previous studies have claimed that low activation energy for grain boundary diffusion could be accomplished in SPD-produced UFG materials. One reason is that the nonequilibrium grain boundaries are unstable requiring a slight additional driving force to allow the grains and grain boundaries to be rearranged [START_REF] Valiev | Deformation behaviour of ultra-fine-grained copper[END_REF][START_REF] Lian | On the enhanced grain growth in ultrafine grained metals[END_REF]. Other researchers attributed the low energies to the improved atomic mobility of the nonequilibrium grain boundaries owing to the enhanced diffusivity resulting from the existence of multiple extrinsic dislocations [START_REF] Roy | Thermal Stability in Bulk Cryomilled Ultrafine-Grained 5083 Al Alloy[END_REF]. Besides, aluminum with its strong stacking fault energy favors the dislocation annihilation and offers the required driving force for the annihilation of dislocation and recrystallization [START_REF] Dhal | Shunmugam Precipitation phenomena, thermal stability and grain growth kinetics in an ultra-fine grained Al 2014 alloy after annealing treatment[END_REF].

The activation energy is unexpectedly low (~20 kJmol -1 ) in the completely recrystallized structure at high temperatures (> 300 °C). This value is inconsistent with numerous findings in the literature obtained after several SPD techniques of different Al alloys like Al-Mg processed by ECAP [START_REF] Wang | An Investigation of Microstructural Stability in An Al-Mg Alloy with Submicrometer Grain Size[END_REF], AA6061 subjected to ARB [START_REF] Park | Microstructural characteristics and thermal stability of ultrafine grained 6061 Al alloy fabricated by accumulative roll bonding process[END_REF], AA5083 fabricated by cryo-milling [START_REF] Dhal | Insight into the microstructural evolution during cryo-severe plastic deformation and post-deformation annealing of aluminum and its alloys[END_REF] or AA2014 processed by cryo-rolling [START_REF] Dhal | Insight into the microstructural evolution during cryo-severe plastic deformation and post-deformation annealing of aluminum and its alloys[END_REF]. Nevertheless, several other studies suggest a similar change to a high-temperature regime with very low activation energy [START_REF] Abbasi-Baharanchi | Thermal Stability Evaluation of Nanostructured Al6061 Alloy Produced by[END_REF][START_REF] Gleiter | Observations suggesting a transformation in the structure of high-angle grain boundaries in lead[END_REF][START_REF] Gondi | Grain Growth in Polycrystalline Materials[END_REF][START_REF] Pandey | Thermal and microstructural stability of nanostructured surface of the aluminium alloy 7075[END_REF]. E.g., studying the thermal stability of AA6061 aluminum alloy developed by cryo-rolling followed by optimal aging heat treatment [START_REF] Abbasi-Baharanchi | Thermal Stability Evaluation of Nanostructured Al6061 Alloy Produced by[END_REF], it has been recorded a decrease in the activation energy in the elevated temperature range (300-500 °C). They attributed to the dissolution of coherent and semi-coherent precipitates and to the diminution in the pinning impact that inhibits the migration of grain boundaries and then the amount of activation energy. Likewise, there have been reports of a change to quite a low activation energy in an elevated temperature regime which is near to that for boundary diffusion in high purity lead [START_REF] Gleiter | Observations suggesting a transformation in the structure of high-angle grain boundaries in lead[END_REF] and to that for liquid diffusion in zinc [START_REF] Gondi | Grain Growth in Polycrystalline Materials[END_REF]. The first study [START_REF] Gleiter | Observations suggesting a transformation in the structure of high-angle grain boundaries in lead[END_REF] has suggested the occurrence of change to various boundary structure. Nevertheless, in the second investigation [START_REF] Gondi | Grain Growth in Polycrystalline Materials[END_REF], the low energy was described as a transition in the boundary migration process from one dominated by diffusion to other requiring collaborative shuffles of atomic. Recently, a one grain growth regime was observed in the AA7075 alloy treated by ultrasonic shot peening and annealed at different temperatures up to 350°C where the activation energy for grain growth was determined to be 37.9 kJmol -1 [START_REF] Pandey | Thermal and microstructural stability of nanostructured surface of the aluminium alloy 7075[END_REF]. The latter research considered that the dominant role in grain coarsening was played by grain boundary self-diffusion.

Grain growth requires the migration of HAGBs and the kinetics are strongly influenced by many factors, including solute (impurity) drag, second phase (Zener) drag, the temperature dependence of grain boundary mobility, and grain boundary segregation [START_REF] Pandey | Thermal and microstructural stability of nanostructured surface of the aluminium alloy 7075[END_REF][START_REF] Liu | Mechanical properties and thermal stability of nanocrystallized pure aluminum produced by surface mechanical attrition treatment[END_REF]. The driving force for grain growth is typically quite low at high temperatures, particularly in comparison to the recrystallization regime [START_REF] Humphreys | chapter 11 -Grain Growth Following Recrystallization[END_REF].

Conclusion

Series of AA6060 alloy samples for thermal stability investigations were subjected through Equal Channel Angular Pressing using the Route Bc for 1, 2, 4 and 8 passes and followed by isochronal annealing at various temperatures range between 150 and 450 °C for 1 h.

The following conclusions were drawn.

 The specimens showed recrystallization and grain growth. In addition, the recrystallized microstructure typically has the randomly scattered large grains and small grains indicating the appearance of the discontinuous recrystallization process.  The grain growth kinetics of AA6060 alloy are followed by two stages that rely upon the range of annealing temperature. Average order values of ~75 and ~20 kJ mol -1 of activation energies were estimated in the elevated temperature 300-450 °C range and the low temperature 150-300 °C range, respectively. At elevated temperatures, the grain growth is controlled by lattice self diffusion. While at low temperatures, the grain growth is governed by high mobility of the nonequilibrium grain-boundary.

Figures 1 to 4

 4 Figures 1 to 4 demonstrate the TD inverse pole figure (TD-IPF) maps from EBSD illustrating

Figure 5 .

 5 Figure 5. HAGB fraction increases sharply and continually for the one pass specimen, at temperatures

Figures 9 -

 9 Figures 9-12 show the texture in term of recalculated pole figures (111) and ODF sections at φ2 = 45°

Figure

  Figure 9a indicates the development of a strong C component and weak B component after 1

  is not complete due to the absence of * 2 A component. Molodova et al.[START_REF] Molodova | Thermal stability of ECAP processed pure copper[END_REF] also reported the strengthening of C and B components with a rising ECAP passes number for UFG pure copper via route Bc. In the meantime, the domination of * 1



  ECAP process through 1, 2, and 4 ECAP passes lead to the development of strong C component and weak B component. Meanwhile, the typical shear ECAP texture of FCC metals is observed after 8 ECAP passes. The texture intensity increases with increasing ECAP processing up to 4 passes and then decreases after 8 ECAP passes. The calculation of the texture index shows the propensity for texture weakening and randomization with rising annealing temperature up to 450 °C.  It was noticed from the observed transitions distinguished by sudden softening and degradation of the deformation texture components, which occurred after annealing above 150°C while the UFG microstructure of the AA6060 ECAPed material remained stable up to this temperature. The 8 ECAP pass sample has shown a limited grain growth and a restricted evolution after 300°C. A relatively homogeneous microstructure after annealing up to 450 °C has been obtained with reasonably small mean grain size ~ 12.5 µm compared to the 4 ECAP pass sample (~16 µm) despite their similar behavior after ECAP processing in terms of achieved grain sizes (0.57 and 0.68 µm, respectively).

Figures captionsFigure 1 :

 1 Figures captions

Figure 2 :

 2 Figure 2: IPF maps of AA6060 after 2 ECAP passes and subsequent annealing for 1 h at: (a) 150 °C,

Figure 3 :

 3 Figure 3: IPF maps of AA6060 after 4 ECAP passes and subsequent annealing for 1 h at: (a) 150 °C,

Figure 4 :

 4 Figure 4: IPF maps of AA6060 after 8 ECAP passes and subsequent annealing for 1 h at: (a) 150 °C,

Figure 5 :

 5 Figure 5: Volume fraction of HAGBs of AA6060 processed by ECAP through 1, 2, 4 and 8 passes as

Figure 6 :

 6 Figure 6: Recrystallized grain size evolution of AA6060 processed by ECAP through 1, 2, 4 and 8

Figure 7 :

 7 Figure 7: Evolution of the recrystallized grain percentage of AA6060 processed by ECAP through 1,

Figure 8 :

 8 Figure 8: Recalculated pole figure (111) and ODF sections at φ2 = 0 and 45° of the as-received

Figure 9 :

 9 Figure 9 : Recalculated pole figure (111) and ODF section at φ2 = 45° of AA6060 alloy after: a) 1

Figure 10 : 27 Figure 11 :

 102711 Figure 10 : Recalculated pole figure (111) and ODF section at φ2 = 45° of AA6060 alloy after: a) 2

Figure 12 :

 12 Figure 12: Recalculated pole figure (111) and ODF section at φ2 = 45° of AA6060 alloy after: a) 8

Figure 13 :

 13 Figure 13: Evolution of the texture index before and after annealing.

Figure 14 :

 14 Figure 14: Hardness evolution the of AA6060 alloy processed by ECAP through 1, 2, 4 and 8 passes

Figure 15 :

 15 Figure 15: Evolution of the dislocation density in AA6060 alloy as a function of annealing

Figure 16 ::

 16 Figure 16: : Evolution of ln(d 2 -d0 2 /t) as a function of 1000/RT for AA6060 alloy processed by ECAP

Figure 13 Figure 14 39Figure 15

 131415 Figure 2

Table captions Table 1 :

 captions1 Ideal ECAP orientations for FCC alloys using a channel angle  of 90°

	Figure 1
	28

Acknowledgments

Tarek khelfa and Fuguo Li acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 51275414, No.51605387). This work also was supported by the PHC-Maghreb program N o 16MAG03. JMC thanks CONACYT (Mexico) for the economic partial funding of his sabbatical leave at UMSNH. JAMB gratefully acknowledges the financial support of the Ministry of Science and Higher Education of the Russian Federation in the framework of Increase Competitiveness Program of NUST «MISiS» (No. К4-2019-045), implemented by a governmental decree dated 16th of March 2013, N 211.

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.