domain: sde sdu

text:
Patricia Poblete-Grant
email: patricia.poblete@ufrontera.cl

Paula Cartes
email: paula.cartes@ufrontera.cl

Sofía Pontigo
email: sofia.pontigo@ufrontera.cl

Philippe Biron
email: philippe.biron@sorbonne-universite.fr

María De La

Luz Mora
email: mariluz.mora@ufrontera.cl

Cornelia Rumpel
email: cornelia.rumpel@inrae.fr

María De

La Luz Mora

Luz Mora

Phosphorus fertiliser source determines the allocation of root-derived organic carbon to soil organic matter fractions

Keywords: Poultry manure, Rock phosphate, C isotopes, SOM density fractionation

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

To enhance soil organic carbon (SOC) sequestration in agricultural soils it is necessary to increase plantderived organic carbon inputs [START_REF] Chenu | Increasing organic stocks in agricultural soils: Knowledge gaps and potential innovations[END_REF], requiring nitrogen (N) and phosphorus (P) fertiliser addition. This is problematic especially for P, since P fertilisers are made from rock phosphate (RP) and may have limited availability in the future due to their global resource distribution and depletion of global P reserves [START_REF] Reijnders | Phosphorus resources, their depletion and conservation, a review[END_REF]. In the context of a circular economy, alternative P fertilisers may include poultry manure, a waste material that is available in large quantity due to a growing broiler industry (FAO, 2018). This material may be recycled into soil amendments by composting to prevent environmental concerns. Indeed, poultry manure compost (PM) is a suitable fertiliser to improve yield and enhance soil physical properties and SOC sequestration [START_REF] Li | Effect of poultry litter and livestock manure on soil physical and biological indicators in a rice-wheat rotation system[END_REF][START_REF] Malik | Organic amendments differ in their effect on microbial biomass and activity and on P pools in alkaline soils[END_REF][START_REF] Mierzwa-Hersztek | Influence of Poultry Litter and Poultry Litter Biochar on Soil Microbial Respiration and Nitrifying Bacteria Activity[END_REF][START_REF] Mubeen | Integrated phosphorus nutrient sources improve wheat yield and phosphorus use efficiency under sub humid conditions[END_REF][START_REF] Poblete-Grant | Synergistic and antagonistic effects of poultry manure and phosphate rock on soil P availability, ryegrass production, and P uptake[END_REF][START_REF] Spohn | Phosphorus and carbon in soil particle size fractions: A synthesis[END_REF]. To optimise resource use and to prevent adverse effects (e.g., contamination with heavy metals), combined application of PM and RP has been suggested [START_REF] Mubeen | Integrated phosphorus nutrient sources improve wheat yield and phosphorus use efficiency under sub humid conditions[END_REF][START_REF] Poblete-Grant | Synergistic and antagonistic effects of poultry manure and phosphate rock on soil P availability, ryegrass production, and P uptake[END_REF].

The use of organic or inorganic fertilisers may affect the functioning of plant and soil microbial communities differently because of contrasting nutrient availability and input of exogenous organic matter. Indeed, organic fertilisers were shown to increase root growth [START_REF] Zhang | Benefits of organic manure combined with biochar amendments to cotton root growth and yield under continuous cropping systems in Xinjiang[END_REF] and labile SOC (Datta et al., 2010). Root-derived compounds together with available nutrients may be taken up and incorporated into the soil microbial biomass. The latter process was found to be a pre-exquisite for SOC stabilisation [START_REF] Cotrufo | The Microbial Efficiency-Matrix Stabilization (MEMS) framework integrates plant litter decomposition with soil organic matter stabilization: Do labile plant inputs form stable soil organic matter?[END_REF]. A recent study showed that different organic fertilisers alter root carbon input and its use by the soil microbial biomass [START_REF] Vidal | Promoting plant growth and carbon transfer to soil with organic amendments produced with mineral additives[END_REF] and therefore most probably SOC sequestration.

Accordingly, organic fertilisation strategies should be investigated in order to improve OC transfer from plant to soil. To the best of our knowledge, the effect of contrasting P fertilisers on these processes has never been assessed.

Therefore, in this study, we examined the influence of rock phosphate (RP), poultry manure compost (PM) and their mixture (PMRP) on OC transfer from plant to soil and allocation of plant-derived OC (rootderived OC) into particular SOC pools. To investigate the influence of edaphic properties on the general nature of the response pattern and its magnitude, we used two soil types differentiated by their P status and soil reaction. Our conceptual approach included continuous 13 C labelling of ryegrass plants grown on a Luvisol and a Neoluvisol in a growth chamber under controlled conditions, followed by destructive sampling and soil physical fractionation. We separated the unprotected OC fraction (FLf) from the one occluded in aggregates (OLf) and OC in organo-mineral associations (Hf). We quantified plant-derived OC, in the three different SOC pools with contrasting stability by using the isotope mass balance approach. We made two hypotheses: (1) in treatments with organic P fertilisers we will observe greater microbial biomass and more available nutrients than in those with mineral P fertilisers because of greater plant-derived OC input and (2) among the three P fertiliser treatments, we will observe contrasting plantderived OC allocation to SOM fractions because of their specific effects on OC flow from plant to soil.

Materials and Methods

Materials

We used two soils differing in pH, available P (Olsen-P) and bulk density. Both soils belong to the French observatory SOERE PRO (https://www6.inra.fr/valor-pro/SOERE-PRO-les-sites) and were collected at 0-30 cm depth at Colmar (Eastern France) and Le Rheu (northwestern of France). They were classified as Neoluvisol (NEO) (pH 6.1) in Colmar, and Luvisol (LUV) (pH 8.5) in Le Rheu [START_REF] Baize | Référentiel pédologique[END_REF].

The NEO soil was characterised by a bulk density of 1.54 g cm -3 and an available (Olsen's) P concentration of 60 mg kg -1 . Total SOC concentration was 10 mg g -1 , and total N concentration was 1 mg g -1 ,. The soil had a silty texture with a clay content of 14.6 %. The LUV soil was characterised by a bulk density of 1.26 g cm -3 and an available (Olsen's) P concentration of 11 mg kg -1 . Its SOC concentration was 12 mg g -1 , and its N concentration was 1 mg g -1. Additionally this soil contained 128 mg g -1 CaCO 3 .

The soil had a silty texture with a clay content of 20.7%.

Poultry manure (PM) had a P concentration of 13.2 mg g -1 , and a humidity of 22%. It showed a carbon concentration of, 288 mg g -1 OC, a nitrogen concentration of 45 mg g -1 total N, and a C:N ratio of 6.4. The mineral fertiliser phosphate (RP) showed a P concentration of 300 mg g -1 and a Ca content of 500 mg g -1

as CaO. It had a residual C content of 20 mg g -1 , 0.5 mg g -1 total N, and a C:N ratio of 45.9.

Growth chamber experiment

Perennial ryegrass (Lolium perenne) was grown in pots with 500 g of soil (unaltered or amended with organic and/or inorganic P fertilisers). The species was chosen because of its high yields and ease of establishment, making it a popular choice for permanent grasslands [START_REF] Rivero | Does the "high sugar" trait of perennial ryegrass cultivars express under temperate climate conditions?[END_REF]. A total of 97 ryegrass seeds were sown in the pots containing the two different soil types. Organic and/or inorganic fertilization was applied in form of dry powder to match a P input of 100 mg P kg -1 . We had the following treatments: composted poultry manure (PM), rock phosphate (RP), a mixture of composted poultry manure and rock phosphate (PMRP), unfertilized soil (control), and. unplanted soil (bare soil). To account for the nutrient input from PM, N and potassium (K) in the control and treatments with RP were amended with NH 4 NO 3 and KCl, respectively (262 mg N and 221 mg K per kg soil) to reach similar concentrations as in the PM treatment. PM addition was added based on its P content and corresponded to an application of 14-and 9.8-ton ha -1 in PM and PMRP treatments.. In addition, the OC inputs were 443 and 310 mg C g -1 from PM and PMRP respectively. The fertilization rate of RP was equivalent to 0.8 and 0.25 t ha -1 in RP and PMRP treatments.

We used a completely randomized factorial experimental design with four replicates per treatment and set up the experiment in a growth chamber (SERVATHIN Rubic V, iEES Paris, Thiverval-Grignon).

Temperatures were 24°C and 17°C during the day and night, respectively. They were selected as optimal growth temperatures of temperate perennial ryegrass species, which ranges between 15 and 25°C [START_REF] Perera | Using leaf temperature to improve simulation of heat and drought stresses in a biophysical model[END_REF]. For the first 13 days, the light intensity was 650 mol m -2 s -1 , with a day length of 8 h and 11 h thereafter until the end of the experiment. Plant development took place over the course of three weeks in controlled soil moisture conditions. Moisture was monitored through the use of ten humidity probes (Decagon Echo 5) randomly distributed in the pots. It was maintained on average at 40% of the field capacity by regular watering. Twenty days after seeding (3 weeks), the growth chamber was artificially labelled during the following 4 weeks using continuous 13 C-CO 2 enrichment of the atmosphere (approximately 145 g of 10 atom-% 13 C-CO 2 ), leading to a mean 13 C ratio of 868‰ (standard error = 83).

The pots were harvested 7 weeks after seeding when plants were at tillering stage. Shoots and roots were removed from the soil, washed with distilled water, and their fresh weight was determined. After oven drying at 65°C for 48 h the plant material was ground to pass a 0.84 mm sieve. The remaining soil was classified as rhizosphere soil and was oven-dried at 40°C and sieved at 2 mm. An aliquot of all samples was ground for further analyses.

Microbial biomass carbon

We used the chloroform fumigation-extraction method [START_REF] Vance | An extraction method for measuring microbial biomass C[END_REF] to determine microbial biomass C (MBC) in 5 g of fresh soil. Briefly, fumigated and unfumigated soils were extracted with of K 2 SO 4 -and the TOC concentration of the extracts measured with a TOC analyser (TOC-VCSH by Shimadzu with a sampler ASI-V by Shimadzu).. The microbial biomass was determined using the difference between fumigated and unfumigated samples.

Soil organic matter density fractionation

Soil organic matter (SOM) was divided into three fractions by density fractionation using 20 g of soil and 25 ml polytungstate following a method proposed by [START_REF] Adams | An investigation of the distribution of phosphorus between free and mineral associated soil organic matter, using density fractionation[END_REF]. We isolated the free light fraction (FLf), occluded light fraction (OLf), and the heavy fraction (Hf). The FLf and OLf fractions were overdried at 40°C, and the Hf fraction was freeze-dried. Thereafter, an aliquot was ground and analysed for elemental and isotopic composition.

Elemental and isotopic analyses

We determined the OC and N content of the bulk soil and physical fractions with an elemental analyser Variopyrocube (Elementar). Before the analyses, decarbonation of the LUV soil was carried out by HClfumigation [START_REF] Harris | Acid fumigation of soils to remove carbonates prior to total organic carbon or carbon-13 isotopic analysis[END_REF]. We used an Isotope Ratio Mass Spectrometer (Micromass Isoprime) to determine the stable carbon isotope composition of soil and plant material relative to the Vienna Pee Dee Belemnite (VPDB) international standard.. It was calculated as follows:

! !" ! ‰ = ( ! !"#$%& ! !"#$%#&% ) ×10 ! (1)
where R = 13 C/ 12 C (molar ratio).

Calculations of the contribution of the two different carbon types (plants and poultry manure compost)

Plant-derived OC in soil,, which we hypothesised to be of root origin (RDC), was computed for bulk samples and density fractions from the natural δ 13 C abundance by using the following equation:

!"# % = (! !" !" !"#$!%#&!! ! !" !" !"#$ !"#$) (! !" !"#$%!!" !""#! ! !" !" !"#$ !"#$ ! 100 (2)
where ''δ 13 C treatment'' refers to all treatments sampled at the end of the experiment, ''δ 13 C bare soil'' is the soil without plants, and ''δ 13 C enriched roots'' is the mean δ 13 C value of ryegrass roots of each treatment.

For the PM and PMRP treatment, poultry manure-derived OC (PMC) was estimated as the difference in total SOC following the treatments and the sum of the bare soil OC and root-derived OC (RDC) (Zhang et al., 2015). Additionally, RDC (mg) and PMC (mg) allocated in each SOM fraction was calculated similarly by using the mass balance.

Data analysis

The data sets were checked for normality (Shapiro-Wilk test) and homogeneity of variance (Levene test).

Statistical differences of means (95% significance level) were analysed using analyses of variance (ANOVA). We tested the factors: soil type (S), treatment (T) and their interaction (S x T). Post hoc tests with the function Tukey-test were made for the explanatory variables independently when the ANOVAs detected significant differences. The relationships between soil physicochemical parameters (Poblete-Grant et al., 2019) were tested by Pearson correlation analyses. Furthermore, a multiple linear regression model was performed by selecting manually predictors from the full model to generate a simplified model.

Models were checked for normality and homogeneity using "nortest" and "lmtest" packages. Furthermore, collinearity was tested using the "vif" function from the "car" package. Models were selected based on the their R value the Akaike criterion (AIC). Finally, we performed a principal component analysis to identify the response variables that best explained the treatment and soil type effect using the "facto-extra", "devtools", and "ade4" packages. The statistical software R Foundation for Statistical Computing Version 3.6.3 was used to conduct the tests (R Development Core Team 2009-2018 RStudio, Inc). Effects were declared significant at p ≤0.05.

Results

Elemental content and stable isotope ratios of plant tissues and soils amended with different fertiliser types

Plant OC concentration ranged from 334 to 410 mg g -1 for shoots and from 153 to 355 mg g -1 for roots (Table 1). Shoot and root OC were lower for plants grown on LUV soil compared to those grown on NEO soil (Table 1). In addition, root OC content significantly increased after PM and PMRP amendment of both soils. Plant N concentrations ranged from 16.2 to 58.7 mg g -1 in shoots and from 5.52 to 18.4 mg g -1 in roots (Table 1). The nitrogen concentrations of shoots of plants grown on the LUV soil were significantly increased by RP followed by PMRP. However, root N concentrations were lowest in the RP treatment (Table 1). When grown on the NEO soil, shoot and root N concentrations were most increased by RP followed by PM and PMRP. The C:N ratios ranged from 5.80 to 24.8 and 13.9 to 28.1 for shoots and roots, respectively. Soil type influenced C:N ratios of shoots and roots, being higher when plants were grown on the NEO soil as compared to the LUV soil. The PM and PMRP amendments increased the C:N of roots for the LUV soil, while it was decreased for the NEO soil (Table 1). When RP was added to both soils, shoot C:N ratio decreased. The stable C isotope ratio (δ 13 C) ranged from 940 to 1,078 for shoots and from 611 to 1,047 for roots (Table 1). It was similar for controls, PM, and PMRP treatments of both soils. The δ 13 C of shoots was highest for plants grown on the LUV soil amended with PM and PMRP, whereas for plants grown on the NEO soil, RP and PMRP treatments showed the highest δ 13 C followed by PM and control.

The root δ 13 C was increased only in the RP treatment of the NEO soil (Table 1).

Insert Table 1, please

The total SOC concentrations ranged between 10.5 and 16.5 mg g -1 (Table 1). PM amendment led to significantly increased SOC values in both soils, whereas PMRP addition increased SOC only in the NEO soil (Table 1). Soil N concentrations ranged between 1.22 to 2.01 mg g -1 and were significantly increased only in the NEO soil by using PM. On the other hand, soil C:N ratios ranged from 7.7 to 8.9 and were lowest for the NEO soil amended with RP (Table 1). Soils differed in δ 13 C, being higher in the NEO soil as compared to the LUV soil (Table 1). The δ 13 C increased in the LUV soil by using PM and PMRP.

However, for the NEO soil, RP and PMRP treatments showed the greatest δ 13 C. The RP treatment showed lower δ 13 C for the LUV soil as compared to the NEO soil.

In addition, soil type and treatments significantly affected root-derived OC (RDC) input (p≤0.001; Table 1).

It ranged from 0.11 and 6.67% corresponding to …. And … mg in all treatments (Table 1). RDC input was higher in the NEO than the LUV soil. The PM and PMRP treatments increased RDC in both soils as compared to the control and RP treatments.

Microbial biomass, and elemental content of SOM pools as influenced by fertiliser types

Microbial biomass carbon (MBC) ranged from 102 to 484 mg kg -1 . It showed significant differences for soil type, treatment, and their interaction (p≤0.001; Fig. 1; Table S2). We found that the application of PM significantly increased MBC in the LUV and NEO soils, with regards to the control. The RP treatment had a negative effect on MBC leading to lower values (14-38%) for both soils as compared with the unfertilised control. However, combination of RP with PM induced the greatest MBC increases (2-fold) for the NEO soil. In the LUV soil amended with PMRP, similar MBC concentrations as in the control were recorded.

Insert Figure 1, please

SOC concentrations and distribution in physical fractions

The SOC concentrations of the FLf fraction ranged between 147 to 237 mg g -1 (Table 2). They were lower than the SOC concentrations of the OLf fraction and higher than those of the Hf fraction, ranging between 206 to 328 mg g -1 and 7.61 to 12.2 mg g -1 , respectively. Treatments showed significant differences for the SOC concentration of the FLf, OLf, and Hf fractions (p≤0.01; Table 2), while only the SOC concentration of the OLf fraction showed differences according to soil type (p≤0.001; Table 2). The interaction between soil type and treatment was significant for the SOC concentration of the FLf fraction (p≤0.001; Table 2).

Total N concentrations ranged from 8.60 to 20.1 mg g -1 in the FLf and from 11.6 to 26.2 mg g -1 in the OLf fractions. They were strongly increased by PM and PMRP addition in both soils (Table 2). In the Hf fraction total N concentrations ranged from 1.01 to 1.33 mg g -1 and showed no differences between treatments.

The C:N ratio ranged between 12.4 to 19.3 in the FLf and between 11.8 to 16.2 in the OLf (Table 2). The FLf and OLf of the PM and PMRP treatments showed the lowest C:N ratios. On the other hand, the C:N ratio of the Hf ranged between 7.61 to 12.2 with no differences among the treatments.

The allocation of SOC to FLf was increased from 9% to 20 and 14% in the LUV soil amended with PM and PMRP, respectively (Fig. 2A). Interestingly, RP reduced the SOC allocation to the FLf of the LUV soil. On the other hand, the allocation of SOC to the OLf of the LUV soil was slightly increased by PM and PMRP as compared to RP and control,. In the NEO soil, the allocation of SOC to the FLf was increased from 15% to 31 and 27% by the PM and PMRP, respectively.

The allocation of plant and poultry manure compost-derived OC to SOM fractions is depending on the fertiliser type

PM and PMRP treatments increased RDC contribution to both soils, compared to the control soil, while RP treatments showed no change. In the FLf, RDC contribution ranged from 3.66% to 17.6%, and was highest in PM and PMRP treatments (Table 2). In RP treatments, values were similar as those of the control. RDC allocated to the OLf, was lower, varying between 1.18 and 6.56% (Table 2). The PM treatment showed the highest RDC in the OLf of the LUV soil, while PMRP showed the greatest RDC in the OLF of the NEO soil. In the Hf fraction, RDC accounted for 2.09 to 5.82% (Table 2). It was higher than the control in the PM treatment and lower than the control in the RP treatment for the NEO soil, while in the LUV soil only PM treatment showed significantly higher values than the control.

Insert Figure 2, please

The total quantity of poultry manure-derived OC (PMC) was higher in the PM treatment (3.03 and 7.82 mg) as compared to PMRP (1.43 and 5.04 mg) in both soils (Fig. 3A). In PM and PMRP treatments of the NEO soil, highest amounts of PMC were found in the FLf (47.7 and 26.9 mg) followed by the Hf (24.4 and 19.3 mg), and finally in the OLf (4.31 and 2.11 mg) (Fig. 3B-D). In contrast, the LUV soil showed differences of PMC allocation in these two treatments. The PMC in the LUV soil amended with PM was mainly accumulated in the FLf (18.8 mg) and in the Hf (16.0 mg), and the less PMC was found in the OLf

(1.50 mg). However, PMRP treatment showed the highest accumulation of PMC (43.7 mg) in the Hf as compared to the PM treatment (16.0 mg) (Fig. 3D). Moreover, in the LUV soil amended with PMRP, the accumulation of PMC was highest in Hf followed by FLf (8.02 mg), and finally in the OLf (1.10 mg) (Fig. 3D). Total PMC and PMC allocated in the FLf were significantly affected by soil type, treatment, and their interaction (p≤0.01; Table S2). We also observed significant differences for the PMC allocated to the OLf and Hf due to the treatment (p≤0.001; Table S2).

Insert Figure 3, please

Relationships between RDC allocation to physical fractions and biological and physicochemical parameters

The multiple linear regression models showed that readily available organic P was the explanatory variable with the greatest positive effect on the contribution of RDC in all SOM fractions of both soils (Table 3). MBC and MBP also contributed to the development of these models. In the LUV soil, the readily available organic P and MBP explained the 89% of the variability of RDC in the FLf, while in the NEO soil both variables explained the 76% and 69% of the variability of RDC in the OLf and Hf. The readily available organic P and MBC predictors, on the other hand, explained 66% and 94% of the variability of RDC in the OLf and Hf of the LUV soil, respectively, as well as 96 of the variability of RDC in the FLf of the NEO soil (Table 3). For the OC-FLf, the variables RDC-FLf and PMC-FLf explained the 72% and 74% of its variability. In the NEO soil, readily available organic P explained 54% and 64% of the variability of OC-FLf and OC-OLf, respectively. However, MBC and RDC-OLf were the variables, which were most important and explained (78%) the variability of OC-OLf (Table 3).

Insert Table 3, please

Discussion

Soil microbial biomass is enhanced by organic P fertilisers though substrate provision and stimulation of plant-derived organic matter input

Our findings showed that poultry manure compost fertilisation increased MBC in both amended soils (Fig. 1), which is consistent with the findings of [START_REF] Malik | Organic amendments differ in their effect on microbial biomass and activity and on P pools in alkaline soils[END_REF], who investigated two alkaline soils (pH 8.3)

and [START_REF] Mierzwa-Hersztek | Influence of Poultry Litter and Poultry Litter Biochar on Soil Microbial Respiration and Nitrifying Bacteria Activity[END_REF], who studied a moderately acid soil (pH 6.4) amended with PM. On the other hand, we found that RP amendment of both soils decreased the MBC compared to the unfertilised control (Table 2). Similar negative fertiliser effects on soil microorganisms have previously been reported [START_REF] Lupwayi | Soil microbial biomass , functional diversity and crop yields following application of cattle manure, hog manure and inorganic fertilizers[END_REF]. According to [START_REF] He | Effect of phosphate rock , lime and cellulose on soil microbial biomass in acidic forest soil and its significance in carbon cycling[END_REF], the primary mechanism responsible for the decreased MBC following RP amendment may be a rise in soil pH caused by liming materials containing CaO such as RP.

Stimulation of MBC by organic fertilisers may be related to high plant-derived OC input resulting from increased root biomass following the enhancement of the availability of both inorganic and organic P by PM amendment [START_REF] Poblete-Grant | Synergistic and antagonistic effects of poultry manure and phosphate rock on soil P availability, ryegrass production, and P uptake[END_REF]. Thus, MBC increase might be explained by either, i) the enhanced availability of P and other nutrients, which allowed for higher root carbon input in the active SOM pool (FLf) or ii) a direct input of PM (Fig. 3), which could provide a readily available OC-source and thus stimulate MBC [START_REF] Li | Response of soil organic carbon fractions, microbial community composition and carbon mineralization to high-input fertilizer practices under an intensive agricultural system[END_REF][START_REF] Lupwayi | Linking soil microbial biomass and enzyme activities to long-term manure applications and their nonlinear legacy[END_REF]. It may also increase the available macronutrients inputs, which may be benefitting MBC [START_REF] Mierzwa-Hersztek | Influence of Poultry Litter and Poultry Litter Biochar on Soil Microbial Respiration and Nitrifying Bacteria Activity[END_REF][START_REF] Spohn | Microbial gross organic phosphorus mineralization can be stimulated by root exudates -A 33P isotopic dilution study[END_REF].

Soil microorganisms and SOM account for more than 10% of the plant-derived OC input belowground.

Microorganisms in the rhizosphere, whether alive or dead, may serve as a significant sink for root carbon and promote the development of soil microstructures [START_REF] Vidal | Linking 3D soil structure and plant-microbe-soil carbon transfer in the rhizosphere[END_REF]. Plants rely on soil microorganisms like bacteria and fungi to depolymerize and mineralize organic nutrient forms for their nutrient uptake [START_REF] Jacoby | The role of soil microorganisms in plant mineral nutrition-current knowledge and future directions[END_REF]. Due to a lack of readily available substrates, a large number of soil microorganisms may not be active. The readily accessible OC produced by roots most likely drives microbial activity and growth in the rhizosphere, resulting in the production of extracellular enzymes and, as a result, improved SOM decomposition and nutrient mineralisation [START_REF] Kuzyakov | Competition between roots and microorganisms for nitrogen: Mechanisms and ecological relevance[END_REF]. In these conditions, microbial biomass can compete for nutrients with plants. Microbial P and N uptake can prevent P fixation by soil colloids or N leaching. When the readily available OC is depleted, microbial biomass N and P become available to plants through microbial turnover. Microorganisms can thus be conceptualised as slow-release fertilisers that store P and other nutrients when their concentrations are high, and then release them when nutrient concentrations are low [START_REF] Malik | Organic amendments differ in their effect on microbial biomass and activity and on P pools in alkaline soils[END_REF].

In addition, we found that MBC was greatly affected by soil type. For example, the LUV soil amended with PM showed the highest increase in MBC as compared to the control (Fig. 1), whereas in the NEO soil, the MBC for the PM treatment was lower than in the combined treatment PMRP. [START_REF] Malik | Organic amendments differ in their effect on microbial biomass and activity and on P pools in alkaline soils[END_REF] also found differences of MBC associated to soil type responses to PM amendment. These findings were attributed to contrasting soil texture and organic matter content. However, in our experiment, the two soils had comparable organic matter content and soil texture, but they differed in pH and initial available P status.

Therefore, soil reaction and nutrient availability may have affected microbial growth response to the amendments. Furthermore, it was previously observed that RP was inefficient to promote belowground growth [START_REF] Poblete-Grant | Synergistic and antagonistic effects of poultry manure and phosphate rock on soil P availability, ryegrass production, and P uptake[END_REF], which might diminish the release of easily available OC from roots needed to increase the abundance of soil microorganisms.

Organic fertilisers improve carbon transfer from plant to soil organic matter fractions

Our study shows differences of root-derived organic carbon (RDC) contribution among soils and treatments (Table 1 and2). Our results indicated that organic fertilisers, such as poultry manure alone (PM) or combined with rock phosphate (PMRP) were able to improve RDC similarly in both soil types.This is in contrast to mineral fertilisers in form of rock phosphate (RP) . Our findings are consistent with the results of [START_REF] Qiao | Improving soil nutrient availability increases carbon rhizodeposition under maize and soybean in Mollisols[END_REF] and Dou et al. (2016a, b), who found that pig manure can stimulate rhizodeposition by increasing nutrient availability in several soil types. According to their findings higher nutrient availability increased CO 2 fixation via photosynthesis, root biomass, and nutrient status in plants.,.

This agrees with the contrasting contribution of RDC reported for the NEO and LUV soils, which might be related to their contrasting initial P availability. These results are in agreement with previously reported differences in ryegrass-derived OC as influenced by soil type [START_REF] Domanski | Carbon flows in the rhizosphere of ryegrass (Lolium perenne)[END_REF][START_REF] Kaštovská | Fate and dynamics of recently fixed C in pasture plant-soil system under field conditions[END_REF][START_REF] Kuzyakov | Carbon partitioning and below-ground translocation by Lolium perenne[END_REF]. Indeed edaphic factors such as soil available P, N and pH may influence SOC dynamics and allocation to stabilised pools [START_REF] Di | Combinations of soil properties, carbon inputs and climate control the saturation deficit dynamics of stable soil carbon over 17-year fertilization[END_REF].

Our results showed a higher allocation of RDC to the FLf and OLf fractions of soils amended with PM and PMRP compared to those amended with RP (Table 2 and Fig. 3B). The FLf is known as the active SOM pool, while the Hf is generally regarded as the mineral-associated and thus stabilised SOM pool (Von [START_REF] Lützow | SOM fractionation methods : Relevance to functional pools and to stabilization mechanisms[END_REF]. Moreover, both soil types amended with PM showed an increased RDC contribution to OLf (Table 2). This fraction consists of partly decomposed organic matter occluded in soil aggregates [START_REF] Adams | An investigation of the distribution of phosphorus between free and mineral associated soil organic matter, using density fractionation[END_REF][START_REF] Wagai | Nature of the "occluded" low-density fraction in soil organic matter studies: A critical review[END_REF] providing physical protection and thus limiting OC availability for microbial decomposition [START_REF] Rasse | Is soil carbon mostly root carbon? Mechanisms for a specific stabilisation[END_REF]. Accordingly, previous data stated that long-term application of PM may increase SOC accumulation by its positive effect on soil aggregate formation (Poblete-Grant et al., 2020). In the Hf pool we found that the content of RDC was more increased in LUV and NEO soil amended with PMRP as compared with RP amendment and the unfertilised soil (Table 2). In general, RDC allocation in the three SOM pools was strongly influenced by the treatments containing PM; however, the two soil types showed differences betweenamong FLf and either OLf or Hf.

The positive effect on RDC allocation to the SOM fractions could be explained by the enhancement of soil organic P belonging to the readily fraction (extracted with H 2 O and NaHCO 3 ). Multiple linear regression models supported our findings, which showed that readily available organic P coupled with MBC or MBP explained from 66% to 96% of the variability of RDC from all SOM fractions in both soils (Table 3).

Therefore, we suggest that short-term organic fertiliser application may improve SOC storage in all SOM fractions due to their organic P input. This is in line with a recent literature review by [START_REF] Spohn | Phosphorus and carbon in soil particle size fractions: A synthesis[END_REF], who showed the importance of organic P for SOC storage, indicating that large amounts of P are required to achieve high levels of SOC accumulation promoted by initiatives such as 4 per mille [START_REF] Soussana | Matching policy and science : Rationale for the ' 4 per 1000 -soils for food security and climate ' initiative[END_REF].

In contrast, RP did not promote the transfer of RDC to SOM fractions even though in our previous study showed an increase of root biomass in the NEO soil [START_REF] Poblete-Grant | Synergistic and antagonistic effects of poultry manure and phosphate rock on soil P availability, ryegrass production, and P uptake[END_REF]. We found that RP addition decreased RDC in the Hf of the LUV soil, and in all fractions from the NEO soil as compared to their unfertilised control (Table 2). Thus, application of organic fertilisers may support both crop yield as well as SOC sequestration through direct OC input and the stimulation of plant growth by nutrient and OC input [START_REF] Zhao | Effect of compost and inorganic fertilizer on organic carbon and activities of carbon cycle enzymes in aggregates of an intensively cultivated Vertisol[END_REF].

Moreover, soil structure through aggregate stability induced by PM could play an important role in SOC dynamics [START_REF] Poblete-Grant | Soil available P, soil organic carbon and aggregation as affected by long-term poultry manure application to Andisols under pastures in Southern Chile[END_REF]. Accordingly, the increased accumulation of poultry manure-derived OC (PMC) in the OLf fraction could be associated to its physical effect on enhancing soil macroaggregates. Differences in PMC content between soils, on the other hand, may be attributed to differences in soil mineralogy (Fig. 3). Accordingly, the high pH and concentration of CaCO 3 in the LUV soil could explain the high allocation of PMC in the Hf of the PMRP treatment highlighting the role of Ca from carbonates in the mineral bonds and interactions between minerals and SOM mediated by cation bridges [START_REF] Zhang | Soil aggregation and aggregating agents as affected by long term contrasting management of an Anthrosol[END_REF].

In general, P fertiliser types greatly influenced the contribution of plant-derived carbon and its allocation to SOM pools. Our study highlighted the important role of the soil biomass in the soil transfer of plant derived OC. Moreover, our study indicated that the availability of organic P might promote the release of root derived OC improving its incorporation to soil. We conclude that depending on soil type, the short-term application of poultry manure or its combination with rock phosphate as P fertiliser could represent an interesting alternative to stimulate plant-and poultry manure compost-derived carbon transfer to stabilised SOC fractions. The general pattern of these effects was less influenced by soil type than their magnitude Organic fertilisers may have beneficial effects on (1) the soil microbial biomass, crucial to ensure OC transfer from plant to soil and (2) biomass production. . Field trials are required to determine the effects of poultry manure alone and its combination with rock phosphate under real-world conditions in order to assess the long-term importance of these mechanisms.

Acknowledgments:

We are grateful with the financial support of the Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT) in Chile (projects n° 3210228, 1181050, 1201257, and 3200901), as well as to the Chilean ANID scholarship n° 21150715, and ECOS C13U02. In addition, authors acknowledge the support towards of the IEES laboratory in France. The SOERE-PRO and KOMECO companies for supplying the soils and poultry manure compost used in this study. Finally, authors are greatly grateful with the reviewers of Soil Biology and Biochemistry journal for their contribution in the improvement of the manuscript.

Author's contributions CR, MLM, PB and PPG designed the experiment. PPG and PB carried out the experiment. PPG, CR, MLM analysed the data and wrote the manuscript. CR, MLM, PC, and SP reviewed the manuscript.

Declarations

Conflict of interest

The authors mentioned that they do not have any financial or non-financial conflict of interest, and that this article does not contain any studies with human participants or animals.


Mathis Duport

Guilhem Martin
email: guilhem.martin@simap.grenoble-inp.fr

Pierre Lhuissier

Jean-Jacques Blandin

Frédéric Prima

Rémy Dendievel

Stabilizing post-yielding behavior of a stretching dominated lattice structure through microstructural optimization

Keywords: Ti-alloy, Additive Manufacturing, Microstructure, Lattice structures, Mechanical behavior, Energy absorption

Lattice structures are a class of cellular structures made of a periodic assembly of struts that can deform either by the bending or stretching of the struts [1,2]. Such structures were found interesting because they are good candidates to fill in gaps in the materials properties space as suggested in [1,3]. Stretching-dominated structures were developed for lightweight structural applications, i.e. for high strength and stiffness at low weight [1,3]. Bendingdominated structures are preferred in applications where energy absorption capacity is of utmost importance such as for crush applications [1,3,4]. Different properties related to the energy absorption capacity can be of interest: absorbed energy or energy efficiency. Based on the schematic shown in Figure 1 and representing a typical compressive response of a lattice structure, we recall the definitions of the different properties related to the energy absorption capacity. The peak-stress corresponds to the maximum stress before the first localization (σpeak) and the volumetric energy absorption (UV) is estimated by integrating the area under the stressstrain curve until reaching densification strain (εD). There is no consensus on the definition of εD. It is defined here as the strain corresponding to σpeak. The absorption energy efficiency is defined as follows: E(%) = UV /Uid with Uid being the volumetric energy absorbed in presence of perfect plateau stress while approaching the peak stress, see Figure 1. Finally, the specific energy absorption (SEA in J/g) is calculated as follows: SEA = UV /ρlattice with ρlattice the density of the lattice structure. Energy efficiency is particularly relevant when the energy must be absorbed while keeping the maximum transmitted stress (peak stress) below a given threshold stress that would cause injury or damage. For instance, mechanical responses exhibiting a postyielding strengthening or softening can be considered as less efficient in comparison with a perfectly plastic response. The advent of Powder Bed Fusion (PBF) additive manufacturing (AM) has made easier the fabrication of complex geometries such as lattice structures, see e.g. [3][4][5][6]. Absorbed energy or energy efficiency are related to the post-yielding behavior. This behavior depends of course both on the architecture (e.g. stretching-dominated or bending-dominated) and on the constitutive material, but also on microstructure and defects inherited from AM. For example, pores and surface defects such as notch-like defects can lead to a reduction of ductility of the constitutive material [7] and thus to the early failure of lattice structures. The latter is particularly true when the constitutive material shows a limited ductility and a lack of workhardening capacity. Using post-fabrication surface treatments can be considered to partially address those limitations [8][9][10][11]. In particular, it is difficult to homogeneously apply such treatments to lattice structures. That is why it is thought that using a defect-tolerant constitutive material would be more efficient because the presence of defects would not necessarily lead to early catastrophic failure [12][13][14]. Damage-tolerant alloys are often materials exhibiting deformation mechanisms such as TRIP (Transformation Induced Plasticity) or TWIP (TWinning Induced Plasticity) because their excellent work-hardening capacity drastically improve their resistance to strain localization. Some authors have recently reported the successful fabrication of TRIP [12,[15][16][17] or TWIP [13,14,18] steels by AM. For lightweight applications, Ti-alloys are good candidates due their good specific strength. However, the heritage Ti-alloys such as the Ti-6Al-4V or the Ti-5Al-5V-5Mo-3Cr alloy are known to suffer from a limited ductility and a poor work-hardening capacity. To overcome such limitations, βmetastable Ti-alloys showing good strength-ductility trade-off with large work-hardening rates thanks to the TRIP/TWIP effects have been developed, see e.g. [19][20][21]. Binary Ti-Mo alloys have been proven promising candidates with the TRIP/TWIP Ti-12Mo [20,[22][23][24] and the TWIP Ti-15Mo alloy [25].

Tuning the mechanical response of lattice structures by playing with the architecture [26,27] or by varying the relative density [1,2] has been the focus of many research studies, see e.g. [3][4][5]. However, tailoring the mechanical response of such structures by taking advantage of the metallurgy of the constitutive materials has received little interest and most studies were focused on lattice structures made of Inconel 718, see e.g. [28][29][30]. Here, we demonstrate that the mechanical behavior of one of the most popular stretching-dominated lattice structures, namely the octet-truss, can be tailored through microstructural optimization of a defect-tolerant binary Ti-14Mo alloy to accommodate plastic buckling of the constitutive struts.

Pre-alloyed powder of binary Ti-14Mo (wt%) alloy was loaded into an E-PBF ARCAM A1 machine operating under vacuum (2.10 -3 mbar) and at an accelerating voltage of 60 kV. The processing conditions were preliminary optimized (Tprocess = 660°C ± 30°C, P =270 W, Speed Function = 130) so that samples with a relative density exceeding 99.90% were fabricated. Bar samples with dimensions 10×10×60 mm 3 were produced vertically using a snake-like scanning strategy (hatch spacing = 0.1 mm) with a 90° rotation of the scanning pattern at each layer. Octet-truss lattice structures consisting of 3×3×3 unit cells with struts of 1 mm diameter were also fabricated. Bulk samples and lattice structures were subjected to different heat treatments to produce homogeneous microstructures in an Ar-atmosphere to limit oxygen contamination. Annealing above the β-transus at 900°C for 30 minutes was followed by oil quenching (OQ) to achieve a full β-metastable microstructure [22]. Some samples with a full β-metastable microstructure were also subjected to additional ageing at 150°C for 8h to trigger nanoprecipitation of the ωiso phase as proposed in [22,23].

Samples were prepared for optical microscopy: grinding down to 2400 using SiC abrasive papers and polishing with a solution consisting of 90% colloidal silica suspension (0.04 μm particle size) and 10% H2O2. The polished cross-sections were etched with Kroll's reagent to reveal the microstructures. Optical micrographs were taken with an Olympus DSX 510 microscope. SEM images were acquired with a ZEISS GEMINI SEM500 with an operating voltage of 15 kV. For EBSD analysis, additional surface preparation was performed using the VIBROMET from Buehler for 12h using a mixture of a colloidal silica suspension and H2O2. Electron Back-Scattered Diffraction (EBSD) maps were collected using a step size of 5 µm and post-treated using the OIM Analysis software. X-ray computed tomography was employed to assess the material integrity and to determine the volume of the as-fabricated lattice structure (voxel size 25 μm) using a EasyTom Nano-XL tomograph from RX solutions. Microhardness measurements were performed using a Wilson Tukon 1212 Vickers Tester with a load of 1 kg (HV1). Dog-bone specimens with a gauge length of 15 mm were extracted from the bar by electron discharge machining to determine the tensile response of the various microstructures using a DY35 testing machine equipped with a 20 kN load cell at a strain rate of 10 -3 s -1 . Three samples for each condition were tested to ensure statistical consistency. Uniaxial compression tests using an MTS 810 equipped with a 100 kN load cell were carried out at 10 -3 s -1 to characterize the energy absorption performances of the octet-truss structures subjected to different heat treatments. Digital Image Correlation (DIC) was employed for strain measurements throughout the tests. 6 Mpixels cameras with 50 mm lenses and GOM Correlate Software were used.

Figure 2 gives an overview of the microstructures investigated in this work. The asbuilt microstructure is a two phase α+β microstructure as revealed by the SEM-BSE image and EBSD phase map shown respectively in Figure 2a and Figure 2b. Due to preheating during E-PBF, the as-built microstructure varies along the building direction with a gradient in α-laths size. Here, we provide a typical micrograph taken in the region corresponding to the gauge length of an as-built specimen. β-grains show a typical columnar morphology with an average width between 50 and 80 μm (Figure 2a). Such columnar grains result from epitaxial growth from the underlying layers. Two populations of α-laths can be distinguished based on their size. Coarse α-laths are thought to have nucleated and grown in the intergranular regions as well as within the β-grains at a temperature close to the preheating temperature during E-PBF (Figure 2b). Intragranular fine α-laths revealed using high magnification images as typically shown in Figure 2c have likely formed during the final cooling stage once the build was completed. A full metastable β-microstructure results from annealing at 900°C/30 min followed by OQ (Figure 2d). The columnar grain morphology was preserved but their size has increased in comparison with the ones observed in the as-built conditions (average width between 100 and 200 μm). Finally, the full β-microstructure was subjected to additional low temperature ageing at 150°C/8h to strengthen the material via nanoprecipitation of the ωiso phase. At the grain scale, no difference with the β-metastable microstructure can be evidenced, see comparison between Figure 2d and Figure 2e. However, the hardness has substantially increased from 280 ± 5 HV1 in the β-metastable microstructure to 350 ± 5 HV1 after ageing at 150°C/8h because of the nanoprecipitation of the ωiso, as documented in [22,23].

Figure 2. Overview of the as-built microstructure before and after heat treatment: (a) Low magnification SEM-BSE micrograph showing intergranular and intragranular coarse α-laths embedded in a matrix β; (b) EBSD-phase map confirming the presence of the α and β phase; (c) High magnification SEM-BSE micrograph showing the presence of nanoscale fine α-laths. EBSD-IPF maps revealing the grain structure of (d) the β-metastable microstructure after 900°C/30 min + OQ, and (e) the β-metastable microstructure strengthened via nanoprecipitation of the ωiso phase (900°C/30 min + OQ + 150°C/8h).

The tensile response associated with the different microstructures shown in Figure 2 is displayed in Figure 3a-b. The mechanical properties of the different microstructures are summarized in Table 1. The two-phase α+β microstructure is the one showing the largest yield strength (nearly 850 MPa) but the smallest elongation to failure (≈10%) due to a limited workhardening (absence of TWIP effect). On the opposite, the β-metastable microstructure has a yield strength of 450 MPa but an extended ductility (elongation to failure exceeding 43%) thanks to an outstanding work-hardening capacity (Figure 3a). Such properties are consistent with the ones reported in the literature for the Ti-12Mo [20,22,24] and Ti-15Mo [25] fabricated by more conventional processing routes. Here, the high work-hardening capacity is essentially due to a TWIP-effect as illustrated in Figure 3c-d. The latter microstructure can thus be qualified as defect-tolerant. The β-metastable microstructure strengthened via nanoprecipitation of the ωiso phase shows an increased yield strength (640 MPa) in comparison with the βmetastable microstructure (450 MPa) while maintaining a relatively good ductility because the TWIP effect is still activated. Examples of octet-truss lattice structures fabricated by E-PBF are shown in Figure S 1a. XCT scans of the lattice structures (see example in Figure S 1b) allow the material integrity to be assessed but also the density (ρlattice = mlattice/Vbounding box) and relative density (ρlattice / ρTi14Mo, here ≈ 11%) of the fabricated lattices to be estimated. The latter is important if one aims at properly estimating the specific energy absorption (in J/g) using the density of the fabricated lattice (ρlattice). Figure S 1c is a cumulative projective view of all slices along the lattice thickness proving the successful fabrication with a nearly internal defect-free lattice structure.

The compressive mechanical response of the octet-truss lattice structure having an asbuilt microstructure is first compared to the one showing a β-metastable microstructure, see Figure 4a. The microstructures in the lattice structure were found to be very similar to the ones shown in Figure 2 and observed on bulk samples. The mechanical response of the octet-truss lattice structure having a β-metastable microstructure strengthened by nanoprecipitation of ωiso is also given in Figure 4a. Different mechanical properties related to the energy absorption capacity and defined in Figure 1 have been extracted from Figure 4a. The absorption energy properties of the octet-truss structure showing different microstructures are given in Table 2. The octet-truss structure exhibiting a two-phase α+β microstructure shows an unstable stressstrain response where different regimes can be seen. Elastic regime up to the peak stress is followed by an abrupt drop in stress due to the occurrence of strain localization in a crush band oriented at 45° with respect to the loading direction (Figure 4b). The breaking of struts along the crush band is evidenced as illustrated in Figure 4b. Then, successive increases and decreases in stress are observed and related to the occurrence of sequential strain localization that goes along with the brittle crush of additional struts, see snapshots at selected strains in Figure 4b. The full video is given in Multimedia 1.

The octet-truss structure exhibiting a β-metastable microstructure shows a mechanical behavior that contrasts with that of the α+β. Interestingly, the mechanical response of one of the most popular stretching-dominated lattice structure, namely the octet-truss, is tailored through microstructural optimization by stabilizing its post-yielding behavior, the peak stress being immediately followed by plateau stress, see Figure 4a. After reaching the peak stress, the abrupt stress decrease is not detected because the struts can sustain large plastic strain before breaking thanks to the activation of the TWIP effect. Most of the struts can be deformed plastically by buckling (see snapshots at selected strains in Figure 4c, full video available in Multimedia 2). While the stiffness was found nearly unchanged, the strength of the structure (peak strength) was decreased by nearly 30% (24 MPa for the α+β microstructure vs. 16 MPa for the β-metastable microstructure) but the absorption energy efficiency has been significantly increased from about 35% to nearly 80%, see Table 2. The recent work of Banait et al. [28] is particularly relevant to discuss our results because they investigated the effect of microstructure on the mechanical response of Inconel 718 BCC-lattice structures processed by L-PBF. They show that there is a transition in the mechanical behavior of 3D printed BCC-lattice structures resulting from precipitation-strengthening of the constitutive material upon ageing with a more unstable post-yielding regime. Here, we show using an octet-truss geometry that the postyielding regime can be stabilized by promoting the formation of a β-metastable microstructure undergoing a TWIP effect instead of the two phase α+β microstructure present in the as-built lattice structure. The mechanical response of the octet-truss with a β-metastable microstructure was further tuned by strengthening the matrix via nanoprecipitation of the ωiso phase. An intermediate mechanical behavior (Figure 4a) is obtained. Through nanoprecipitation, the strength of the lattice structure exhibiting the α+β microstructure is almost recovered but, at the same time, its sensitivity to strain localization is greatly reduced since struts were still able to sustain large plastic strain (Figure 4d) thanks to a preserved TWIP effect. This β-metastable microstructure strengthened with ωiso precipitates allows to further increase the volumetric absorption: UV = 10.5. mJ/mm 3 vs. UV = 7.8 mJ/mm 3 for the β-metastable microstructure. However, the post-yielding behaviour is more unstable, therefore the energy absorption efficiency is reduced in comparison with the β-metastable microstructure (70% vs. nearly 80%). Finally, it worth noticing that in the postyielding regime, the amplitude of stress variations seems related to the work-hardening capacity of the constitutive material. The microstructure showing the poorest work-hardening capacity is the one exhibiting the largest stress variations. On the opposite, the β-metastable microstructure shows smooth variations of stress in the post-yielding regime.

Using defect-tolerant materials to build stretching-dominated lattices structures broadens the range of applications of such materials because their microstructure can be tuned to become tolerant to plastic buckling of the constitutive struts in order to stabilize the postyielding behavior. Optimizing the microstructure of lattice structures processed by additive manufacturing paves the way towards lattice structures with optimized energy absorption performances and extend the applications of stretching-dominated structures.

Beta_metastable.mp4 file attached to this submission

Multimedia 2: Video recorded during compression of the lattice structure having a β-metastable microstructure (TWIPeffect).

Figure 1 .

 1 Figure 1. Schematic of the mechanical response of a lattice structure including the definition of different mechanical properties allowing to quantify the energy absorption performances.

Figure 3 .

 3 Figure 3. (a) True stress-strain curves and (b) Engineering stress-strain curves for the different microstructures shown in Figure 2. (c) Optical micrographs taken after different plastic strain increments in the same region of interest showing the formation of a twinning network. (d) EBSD-IPF map revealing the nature of the twins: {332} <113> with their typical misorientation of 50°.

Figure 4 .

 4 Figure 4. (a) Compressive engineering stress-strain response of octet-truss lattice structures with an as-built α+β microstructure, a β-metastable microstructure, and with a β-metastable microstructure strengthened by nanoprecipitation of ωiso. Images of the lattice structure taken during mechanical testing of the octet-truss with (b) an as-built α+β microstructure, (c) a β-metastable microstructure, and (d) a β-metastable microstructure strengthened by nanoprecipitation of ωiso.

Figure S 1 .

 1 Figure S 1. (a) Images of as-built octet-truss lattice structures fabricated by E-PBF. (b) 3D rendering of an as-built lattice structure. (c) Axial cumulative projective view of all the slices throughout an as-built lattice structure revealing the presence of very few pores (in red).

Table 1 .

 1 Summary

		σy,0.2%	σUTS	Δσ = σUTS -σy,0.2%	εu	%A
		(MPa)	(MPa)	(MPa)		(%)
	Two-phase α+β	845 ± 14	920 ± 7	71 ± 7	0.05 ± 0.02	10 ± 2
	As-built
	β-metastable	450 ± 2	1046 ± 26	598 ± 28	0.32 ± 0.04	43 ± 6
	900°C/30 min + OQ
	β-metastable + ωiso	643 ± 5	1034 ± 18	390 ± 20	0.29 ± 0.02	39 ± 3
	900°C/30 min + OQ
	+ Ageing 150°C/ 8h

of the mechanical properties of the different microstructures. Average values over the three tests performed are given. σy,0.2%, σUTS , Δσ and εu have been extracted from true-stress-strain curves, and %A from engineering-stress-strain curves.

Table 2 .

 2 Energy absorption properties of the octet-truss lattice structure with different microstructures.

		σpeak (MPa)	σpeak/ σy,0.2%	UV	SEA	E
				(mJ/mm 3 )	(J/g)	(%)
	Two-phase α+β	24.4	0.029	5.2	9.7	35.1
	As-built
	β-metastable	16.0	0.036	7.8	14.6	79.7
	900°C/30 min + OQ
	β-metastable + ωiso	22.8	0.035	10.5	19.6	69.9
	900°C/30 min + OQ +
	Ageing 150°C/ 8h

Acknowledgements

This work was funded by the Center of Excellence of Multifunctional Architectured Materials "CEMAM" n°AN-10-LABX-44-01. This work has benefited from the facilities available on the characterization platform CMTC of Grenoble INP.

Declaration of competing interest

The authors declare that they have no competing interests.

SUPPLEMENTARY MATERIALS


Jiyuan Lyu

Tuan Le

Aurélie Claraz

Clémence Allain

Pierre Audebert

Géraldine Masson
email: geraldine.masson@cnrs.fr

Tuan Le Aurélie Claraz

s-Tetrazine: robust and green photo-organocatalyst for aerobic oxidation of N,N-disubstituted-hydroxylamines to nitrones

Keywords: Tetrazine, photo-organocatalyst, nitrones, sequential oxidative (3+3) cycloaddition

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Nitrones are an important class of functional group that have been extensively used as versatile intermediates in many organic reactions such as nucleophilic additions, radical additions, cycloaddition reactions, electrocyclization reactions etc. 1 In addition, nitrones such as α-phenyl N-tertiary-butyl nitrone (PBN) and 5,5-dimethyl-pyrroline N-oxide (DMPO) are widely used as spin-trapping reagents for the detection and identification of radicals formed in biological systems or chemical reactions. 2 Traditionally, nitrones are prepared by the condensation reaction between N-substituted hydroxylamines and carbonyl compounds in the presence of dehydrating agents. 3 The alternative approach is the direct oxidation of easily available N,N-disubstituted-hydroxylamines 4 to nitrones which avoids the manipulation of unstable aldehydes. 5 However, although efficient, this latter oxidative procedure usually utilizes stoichiometric amounts of oxidants 1e, 6 or catalytic amounts of toxic metals. 7 To date, only one photocatalytic 8 oxidation process using an expensive iridium complex has been reported. 9a Therefore, the development of efficient and more environmentally friendly synthesis of nitrones by oxidation of N,N-disubstituted hydroxylamines is still of great importance. Herein we communicate the first photo-organocatalytic aerobic oxidation of N,N-disubstituted-hydroxylamines, affording diverse nitrones in good yields under very mild reaction conditions using tetrazines, as cheap and easily available organic redox catalysts (Scheme 1). As a part of our continuous interest in the development of photocatalytic and/or organocatalytic aerobic oxidation, 10 we have recently disclosed a new class of photooxidants, 3,6disubstituted s-tetrazines. 11 Their efficiency was successfully demonstrated by their ability to mediate oxidative C-S bond functionalizations under visible light irradiation. 12 We also found that 3,6-disubstituted s-tetrazines in their ground states were efficient organocatalysts for the aerobic oxidative nitroso-Diels-Alder (NDA) reaction of N-arylhydroxylamines with dienecarbamates. 13 Based on our previous works, 12,13 we envisioned that 3,6-disubstituted s-tetrazines could catalyze the oxidation of N,N-disubstituted-hydroxylamines to nitrones. To test our hypothesis, we examined the oxidation of N,Ndibenzylhydroxylamine 2a in the presence of 3,6-bis(2,2,2trifluoroethoxy)-1,2,4,5-tetrazine (10 mol %) as a photocatalyst 1a and 2,6-lutidine (2.0 equiv) as a base under visible light irradiation (green LEDs) and oxygen atmosphere. Pleasingly, the desired nitrone 3a was obtained in 62% yield (Table 1, entry 1). Reaction with other s-tetrazines 1b-1d resulted in lower yields (entries 2-4). The effect of solvent was also examined, and protic solvents such as 2,2,2-trifluoroethanol (TFE), ethanol (EtOH) and hexafluoroisopropanol (HFIP) improved significantly the yield (entries 7-9). Subsequently, a series of bases were investigated, revealing that DMAP was best, affording 3a in 95% yield (entry 10). Yield decreased by using 1.0 equiv of DMAP (entry 13).

Template for SYNLETT Thieme

Replacing oxygen gas by air atmosphere (entry 14) diminished significantly the reaction efficiency. Inspired by our previous work, we were interested to evaluate the possibility to generate in situ the active photocatalyst 1a from commercially available 3,6-dichlorotetrazine 1d. Gratefully, this protocol using 10 mol% of 1d and 2 equivalents of DMAP delivered 3a in same yield (entry 15). None or only trace of nitrone was observed in the absence of photosensitizer 1, DMAP or oxygen (entries 16-18). However, when the reaction was carried out in the dark, the desired product was isolated in 52% yield (entry 19). Although less efficient than at its excited state, 1a in the ground state in TFE can oxidize the N,N-disubstituted-hydroxylamines 2. Finally, the photocatalytic aerobic oxidation was successfully performed on 1 mmol scale to produce the corresponding nitrone in 82% yield after 8 h and in 95% after 16 h (entry 20). With the optimum conditions in hand, the scope of the photooxidation was next examined with different N,Ndibenzylhydroxylamines 3b-3g in the presence of 3,6dichlorotetrazine 1d under green LEDs irradiation (Scheme 2).

As a result, all reactions provided the corresponding nitrones in excellent yields (>90%). Symmetric benzylhydroxylamines bearing electron-rich and -poor aromatic substituents reacted smoothly to give the corresponding nitrones 3b and 3c in nearly quantitative yields.

Template for SYNLETT Thieme

The oxidation of unsymmetrical dibenzylhydroxylamines afforded 82-100% yields of two easily separable regioisomeric nitrones (3d-3h) in a ratio going from 2:1 to 1:1. These results indicate that the electronic properties of aromatic rings have little effect on the regioselectivity in the nitrone formation. On the other hand, the unsymmetrical mono-benzylated secondary hydroxylamines (2k) led to the regioselective synthesis of stabilized nitrones 3k in excellent yield. In addition, the cyclic hydroxylamine 2j worked well to afford a suitable precursor 3j for the construction of isoquinoline alkaloids. It worth noticing that this photooxidative process was also compatible with free alcohol (3i). The N-phenyl hydroxylamine 2l worked also well to form the corresponding nitrone 3l in moderate yield. The presence of heteroaromatic rings such as thiophene was also tolerated. The commonly used free-radical spin trapping agent, α-phenyl N-tertiary-butyl nitrone (3m) was synthesized in quantitative yield from easily available N-benzyl-N-(tertbutyl)hydroxylamine (2m). 14 Pleasingly, replacing DMAP with potassium hydrogenocarbonate as a base allowed the isolation of nitrones 3o-p in good yields from symmetrically disubstituted N,N-dialkylhydroxylamines (2o-p).

To demonstrate the applicability of this photocatalytic oxidative reaction, we were interested to explore the feasibility of developing oxidative 1,3-dipolar (3+3)-cycloaddition between N,N-disubstituted hydroxylamines 2 and oxyallyl cations. In 2018, Archambeau 15 and Rawal 16 groups independently reported an efficient (3+3)-cycloaddition of nitrones and α-tosyl ketones as stable precursors of oxyallyl cations. Given that cycloadditions were carried out in presence of a base and protic solvents, we reasoned that this reaction could be compatible with our photooxidative process. Toward this end, we initially tested the cycloaddition of α-tosyl cyclohexanone 4 with preformed nitrones 3a (Table 2). Using our optimized conditions for nitrone formation, the desired 1,2-oxazinane 5 was isolated in 15% yield (entry 1). Variations of the base and the solvent revealed that the yield slightly increased from 15% to 35% using KHCO3 in a mixture of HFIP/xylenes (entries 2-3). 16 It was subsequently found that reaction with 3 equivalents of 4 improved the yield to 58% (entry 4). a General condition: 3a (0.1 mmol), 4 (y x 0.1 mmol), base (0.2 mmol), in solvent (1 mL) for 15 h. b Yields refer to chromatographically pure products.

Encouraged by this result, the photocatalytic oxidative (3+3)cycloaddition between 2a and 4 with tetrazine 1d was further evaluated under these reaction conditions. While no reaction took place when the reagents were combined all together, a sequential one-pot two step oxidation/cycloaddition procedure afforded the desired 1,2-oxazinane 5 in 58% yield (Scheme 3).

Scheme 3 Tetrazine-catalyzed aerobic oxidative (3+3) cycloaddition.

The following control experiments were conducted to probe the mechanism. In the presence of the superoxide radical anion quencher TEMPO, the formation of nitrone 3a was inhibited, suggesting that superoxide radical anion (O2 •-) is involved in this reaction. 17 Then, the oxidation potential of N,Ndibenzylhydroxylamine 2a was determined by cyclic voltammetry as +0.14 V vs SCE in DCM (see supporting information). This data indicated that the oxidation of 2a by excited 3,6-bis(2,2,2-trifluoroethoxy)-1,2,4,5-tetrazine ( 1a) (E 0 (1a*/1a •-)=1.59 V vs SCE in DCM). 12,13 is thermodynamically favorable. To confirm this, Stern-Volmer fluorescence quenching studies were performed with s-tetrazine 1a (see supporting information). Fluorescence intensity dramatically decreased when hydroxylamine 2a was introduced. These linear relationships mainly support that the excited organocatalyst is reductively quenched by hydroxylamine. On the basis of the above experiments and previous mechanistic studies, 8,12,13 a plausible mechanism for visible-light-induced aerobic oxidation of hydroxylamines can be proposed (Scheme 4). Initial oxidation of hydroxylamine 2 by the visible-light-excited (in situ generated) photocatalyst 1a produces the hydroxylammonium radical cation 8 and tetrazine anion-radical 7. The ground state catalyst 1a is regenerated by the reduction of oxygen generating the oxygen superoxide radical anion (O2 •-). Simultaneously, the radical hydroxylammonium cation-radical 8 undergoes deprotonation by DMAP, O2 •-or HOO -to afford an O-centered radical 9. This resulting radical is quickly further oxidized by either oxygen or any ROS like the hydroperoxide radical (HOO • ) affording 10. Finally, the base-mediated deprotonation of 10 leads to the desired nitrone 3. Another possible mechanism without light irradiation was proposed (See supporting information). In the dark, the tetrazine (E 0 (1a/1a •-)=-0.58 V vs SCE in DCM) 12,13 in presence of DMAP 18 could simultaneously oxidize and deprotonate the hydroxylamine (via a protoncoupled electron transfer (PCET)-type mechanism) 19 to form the final nitrone. 13 Next, the reduced organocatalyst is oxidized by oxygen, thereby completing the catalytic cycle (See supporting information).

Template for SYNLETT Thieme

Scheme 4 Proposed mechanism for the aerobic photooxidation of hydroxylamines to nitrones.

In conclusion, we have devised a photo-organocatalytic protocol for the synthesis of nitrones through aerobic oxidation of N,Ndisubstituted-hydroxylamines. The in situ formed 3,6-bis(2,2,2trifluoroethoxy)-1,2,4,5-tetrazine was found to exhibit a high catalytic activity in forming these products. This catalytic transformation allowed atom economical and direct synthesis of nitrones. The reaction has a broad substrate scope affording various nitrones in excellent yields. We have also developed an oxidative (3+3) cycloaddition of an oxyallyl cation with a nitrone formed in situ from oxidation of a N,N-disubstitutedhydroxylamine.

Scheme 2

 2 Scheme 2 Scope of the photooxidation of N,N-disubstituted-hydroxylamines 2. a,b a General condition: 2 (0.1 mmol), DMAP (0.21 mmol), 1d (0.01 mmol) in TFE (1 mL), irradiated overnight with 7.2 W green LEDs at rt. b Isolated yield after column chromatography. c Reaction time: 6 h instead of overnight. d KHCO3 instead of DMAP.

Table 1

 1 Survey of reaction conditions for the photocatalyzed oxidation of 2a. a Yields refer to chromatographically pure products.c With 1.0 equiv of base. d Reaction conducted under air atmosphere. e Reaction conducted under argon atmosphere. f Reaction performed in the dark. g 1 mmol scale relative to 2a.

	Entry	R (1)	Base	Solvent	3a Yield (%) b
	1	CF3CH2O-(1a)	2,6-lutidine	DCM	62
	2		2,6-lutidine	DCM	55
		(1b)
	3		2,6-lutidine	DCM	44
		(1c)
	4	Cl (1d)	2,6-lutidine	DCM	38
	5	1a	2,6-lutidine	CH3CN	47
	6	1a	2,6-lutidine	Toluene	24
	7	1a	2,6-lutidine	EtOH	80
	8	1a	2,6-lutidine	TFE	80
	9	1a	2,6-lutidine	HFIP	78
	10	1a	DMAP	TFE	95
	11	1a	K2CO3	TFE	71
	12	1a	Cs2CO3	TFE	58
	13 c	1a	DMAP	TFE	49
	14 d	1a	DMAP	TFE	65
	15	1d	DMAP	TFE	95
	16 e	1d	DMAP	TFE	trace
	17	1d	-	TFE	26
	18	-	DMAP	TFE	-
	19 f	1d	DMAP	TFE	52
	20 g	1d	DMAP	TFE	82(95) h
	a General conditions: N,N-dibenzylhydroxylamine 2a (0.1 mmol), base (0.21 mmol),
	1 (0.01 mmol), in solvent (1 mL) irradiated overnight with 7.2 W green LEDs (λ = 535
	nm) at rt.

b h After 16 h.

Table 2

 2 Survey of reaction conditions for the (3+3) cycloaddition. a

	Entry	Base	Solvent	y	5 Yield (%) b
	1	DMAP	TFE	1.5	15
	2	KHCO3	TFE	1.5	26
	3	KHCO3	HFIP/xylenes (1:1)	1.5	35
	4	KHCO3	HFIP/xylenes (1:1)	3	58

Acknowledgment

TL thanks Saclay University for the doctoral fellowships. JL thanks china scholarship council for the doctoral fellowship. PA thanks the Institut Universitaire de France for personal and financial support.

Funding Information

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Primary Data

NO.

Supporting Information

YES (this text will be updated with links prior to publication)

Conflict of Interest

The authors declare no conflict of interest.


Camille Pauzon
email: camille.pauzon@simap.grenoble-inp.fr

Maxence Buttard

Arthur Després

Béchir Chehab

Jean-Jacques Blandin

Guilhem Martin

A novel laser powder bed fusion Al-Fe-Zr alloy for

Keywords: Additive manufacturing, Laser powder bed fusion, Alloy design, Aluminium alloys, mechanical performance, electrical conductivity

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Combining low density, high corrosion resistance and desirable physical and mechanical properties, Al-alloys are a workhorse of the transport and energy industries. Laser powder bed fusion (LPBF) offers new perspectives to achieve near-net shape complex components in few manufacturing steps, compared to the traditional routes of cast and wrought products. The preferred approach to produce Al-based LPBF components has been to use casting alloys [1], for example AlSi7Mg and AlSi10Mg. Unfortunately, these grades do not deliver high mechanical performance when LPBF-processed [START_REF] Eos Gmbh | EOS Aluminium AlSi10Mg Material Data Sheet EOS Aluminium AlSi10Mg Good Strength & Dynamic Load Bearing Capacity[END_REF][START_REF]EOS Aluminium AlF357 Material Data Sheet Metal Solutions[END_REF]. Al-alloys from the 6xxx series provide both good strength and conductivity after a T6 heat-treatment and would be excellent candidates to manufacture heat exchangers. However, they suffer from hot cracking during autogenous laser welding because of their large freezing range and coefficient of thermal expansion [START_REF] Zhao | [END_REF], and hot cracking also occurs during LPBF [5][6][7][8]. Uddin et al. [9] managed to suppress hot cracking using powder bed heating to 500°C. Despite its success, this approach is not compatible with industrial LPBF systems. Besides, adjustments to the alloy composition have been considered, with addition of grain refiners that promote the heterogeneous nucleation of Al grains, making the alloy more resistant to cracking. These refiners can either be inoculating agents admixed to the powder like TiB2 and TiC [10,11], or primary phases formed in situ such as Al3X (X=Zr, Sc) [5,6,8,12]. In this case, the benefit is two-fold with the additional potential for precipitation strengthening through post-ageing treatment [13,14]. It has also been suggested to tune the powder composition to favour elements which do not increase the solidification range [START_REF] Mehta | High Performance Aluminium Alloys for Laser Powder Bed Fusion: Alloy Design and Development[END_REF][START_REF] Buttard | [END_REF]. These approaches have addressed to some extent the cracking issues of the 6xxx series. But other challenges remain, connected to laser-induced solute evaporation and the need for further solutionizing and quenching. Briefly, there is a necessity to develop novel Al-alloys with strength and conductivity comparable or surpassing that of the 6xxx series combined with good LPBF processability. This work introduces a new Al-alloy designed for LPBF to achieve a superior strengthconductivity trade-off, by leveraging the unique characteristics of the process: the relatively high solid-liquid interface velocities and high thermal gradients resulting in high cooling rates. The multiscale characterization reveals unique microstructural features of the Al-Fe-Zr alloy inherited from LPBF and their evolution upon ageing, further connected to the material tensile response and conductivity.

One criterion for alloying element selection is a low vapour pressure in comparison to common solutes like Zn and Mg, which tend to restrict the processing window to low energy inputs. The loss of such elements can lead to local composition variations and for example reduced precipitation hardening for 6xxx or 7xxx alloys. Their vaporization can affect the melt pool stability, lead to porosity formation, and generated vapours may interact with the laser, resulting in diminished and varying energy input. These challenges will become more predominant as production capacities are boosted with high power, multi-laser systems, and high layer thicknesses [17]. An additional criterion is to favour elements with low diffusivity in Al to limit the extent of in situ precipitation during thermal cycles and retain a supersaturated solution in the as-built state. Finally, toxic elements such as Be are avoided as well as certain Critical Raw Materials (CRMs) like Sc bringing high costs and supply risks.

With these criteria, Zr and Fe were shortlisted as they combine both low vapour pressure and low diffusivity in solid Al. Zr plays a dual role with the formation of primary Al3Zr resulting in grain refinement, limiting hot cracking, and its contribution to precipitation strengthening through post-ageing treatment [18]. It diffuses in Al more slowly than Sc [19].

Similarly, Fe exhibits a low diffusivity in Al [20]. Fe is of special interest for its very low equilibrium solubility in Al (0.044 wt.% at 654°C, computed with Thermo-Calc, TCAL6). Its minor content in the solid solution is expected to not significantly reduce the matrix conductivity. In addition, from Scheil-Gulliver calculations, Fe is expected to form Al13Fe4 (Fig. S1). Based on this rationale, gas atomised powder with chemical composition given in Samples were produced with an EOS M290 under Argon. The baseplate made of a 5XXX-serie Al-alloy was preheated to 100°C, and the stripe scanning pattern was rotated by 67° between layers. Using a 370 W power, a 1400 mm/s scan speed, 0.13 mm hatch distance and a 60 µm layer thickness, near-fully-dense samples (> 99.7 %) were manufactured. The specimens were heat treated for stress-relief at 300°C for 2 hours before baseplate removal. Such stress-relief post-processing is commonly applied to LPBF Al components to avoid unwanted deformation, see e.g. [21,22]. Samples were then subjected to a post-fabrication ageing treatment at 400°C for 4 hours, identified as the peak ageing with 110 ± 2 HV, vs. 65 ± 2 HV after stress-relief. The low diffusivity of Zr in the matrix explains the high ageing temperature. Microstructure characterization was performed using a scanning electron microscope (Zeiss Gemini SEM 500 FEG) equipped with an electron backscattered diffraction (EBSD) detector. The samples were polished until a 1µm diamond suspension step, and a final vibratory polishing. EBSD maps were acquired with a 0.3 µm step size at 20 kV. High angle grain boundaries plotted in black on the orientation maps are defined by a misorientation >10°. The MATLAB toolbox MTex was used to compute the grain sphericity defined as the ratio of the grain area to the product of the perimeter and equivalent radius (equal to 0.5 for a perfect circle). Transmission electron microscopy (TEM) was conducted with a TEM FEG JEOL 2100 and perform energy-dispersive X-ray spectroscopy (EDS) by operating at 200 kV. Thin foils were prepared by electropolishing using a solution of 30% nitric acid and 70% methanol at -35°C and 20V. Phase and orientation information of specific location identified at the melt pool scale were retrieved using ACOM (Automated Crystallographic Orientation Mapping) with the ASTAR TM hardware and software from Nanomegas. This technique presented in [23] and applied to LPBF materials [START_REF] Buttard | [END_REF]24], consists in scanning the region of interest with a nano-beam and collecting for each point the diffraction pattern acquired by a CCD camera. This diffraction dataset is post-processed, to compare the diffraction pattern in each point to the simulated one of candidate phases with a template matching approach. Since this technique only finds correlation between the collected patterns and that of the listed candidates, the definition of the candidate bank must be done cautiously. In the present work, it was established based on the Thermo-Calc calculations and XRD analysis (Fig. S1). Electrical conductivities were measured by the eddy current method with a SIGMATEST 2.069 operating at 60kHz. From these, the thermal conductivities were derived from the Smith-Palmer law [25]. Finally, the macroscopic tensile behaviour was evaluated using dog-bone samples, machined from vertical cylinders to obtain a gauge length of 25 mm and diameter of 4 mm. The room temperature tests were performed according to ISO 6892 standard [START_REF]Metallic Materials -Tensile Testing -Part 1: Method of Test at Room Temperature[END_REF] on a Zwick/Roell Zmart Pro machine equipped with a 100 kN load cell.

The typical tensile response of the stress-relieved and the aged conditions are presented in Figure 1a, b, and the derived properties in Table 2. A distinct increase in YS of about 140% from the stress-relieved (~130 MPa) to the aged conditions (~310 MPa) is achieved. The strength of the new alloy is superior to that reported for Al-Fe LPBF alloys developed for high conductivity [START_REF] Qi | [END_REF]28]. Upon ageing, the electrical and thermal conductivities increased by about 50% (see Table 2). The Ashby chart in Figure 1c permits to locate the performance of the new Al-Fe-Zr alloy among the cast and wrought Al series with respect to YS and thermal conductivity. High thermal conductivity comes usually at the expense of strength for the conventional alloys, and LPBF materials tend to exhibit intermediate performance, for example AlSi10Mg with YS of 200 MPa and κ of 160 W/m·K when stress-relieved [START_REF] Eos Gmbh | EOS Aluminium AlSi10Mg Material Data Sheet EOS Aluminium AlSi10Mg Good Strength & Dynamic Load Bearing Capacity[END_REF]. The Al-Fe-Zr alloy with a YS of 310 MPa and κ of 180 W/m·K after direct ageing of 4 hours at 400°C, will be of interest for heat exchanger applications with complex design requiring both high strength and conductivity. The 6xxx series after a T6 heat-treatment are the only conventional grades with performance approaching the present alloy (see white bubbles in Figure 1c). However, as mentioned in the introduction these alloys suffer from poor LPBF-processability and cannot benefit from the associated design freedom.

Condition YS [MPa] A [%]

Au An overview of the microstructure along the building direction (BD) for the stressrelieved and the aged material is displayed in Figure 2. Large areas including several melt pool boundaries were mapped using EBSD. A complex grain structure is revealed with regions of fine equiaxed grains at the bottom of the melt pools and columnar grains elongated along the BD towards the centre of the melt pools (Figure 2a andd). These features are common to other Al-alloys developed for LPBF and forming Al3X-L12 primary inoculating phases, see e.g. [8,13]. The grain colouring highlights their weak crystallographic texture. The BSE-SEM images confirm that the alloy is dense, crack-free and well-processed by LPBF. The presence of fine equiaxed zones (FEZ) displayed in Figure 2c, with grains of mean equivalent diameter below 1 µm, is partly attributed to the formation of primary Al3Zr particles (L12), leading to heterogeneous nucleation of Al grains at the melt pool boundary where the solid-liquid front velocity is relatively low, as verified using thermal calculations by many authors see e.g. [29][30][31]. Examples of Al3Zr with cuboid morphologies are presented in Figure S2.

Towards the centre of the melt pools, the solid-liquid interface velocity increases leading to the suppression of the nucleation of the Al3Zr particles due to solute trapping, and thus to the columnar growth of Al grains along the local thermal gradient as already shown in [32,33]. The grain structure is stable upon ageing. With a sphericity index threshold at 0.3, the FEZ fraction was estimated to be only in the range of 0.1 to 0.2 in the stress-relieved and aged samples. In addition, the BSE-SEM images highlight the presence of bright grain boundary particles. Their presence is rather sporadic in the columnar zones (CZ) of the stress-relieved material (~0.3% estimated with ImageJ on Figure 2b), while the FEZ grain boundaries appear decorated with thin films and particles (~3% on Figure 2c). In the aged condition, both the FEZ and CZ grain boundaries are decorated with coarser bright particles with sizes up to a few hundred nanometres (Figure 2e andf). Using TEM and ACOM, images can be produced by virtually placing apertures of various shapes on the diffraction patterns and integrating the intensity within the aperture for each point on the recorded map. A Virtual Bright Field (VBF) TEM image can be generated with a virtual aperture on the transmitted beam. Figure 3a displays a VBF image of a region of interest in the stress-relieved sample containing a grain boundary within a CZ. Experimental nanodiffraction patterns are displayed overlaid with the best candidate phase. The good agreement between the recorded and simulated templates (Figure 3f), together with the consistency with EDS maps (Figure 3b-e), identifies the presence of Al13Fe4 (monoclinic), Al matrix and an Al-rich oxide. No Zr-enrichment was found in the second-phase particles. Locally in the stress-relieved sample, metastable Al6Fe (orthorhombic) could also be identified within the globular Fe-rich intermetallic. The presence of coarser intergranular Fe-rich precipitates in the aged condition revealed in Figure 2e-f suggests the growth and coalescence of the fine globular Fe intermetallics from the stress-relieved sample seen in Figure 3a. As shown in [34], the diffusion of Fe in Al matrix is enhanced by the presence of dislocations and grain boundaries. It is also reasonable to assert that the intergranular Fe-rich particles pin the grain boundaries as no substantial change in grain size could be detected in the EBSD maps. Figure 4 presents the EDS and ACOM analyses for the aged sample (400°C, 4 hours) within a single grain in a CZ. As for the stress-relieved sample, the globular Al13Fe4 and the oxide phase are present. Following ageing, high-aspect ratio plate-like Al13Fe4 precipitates are identified, with lengths up to hundreds of nanometres and thicknesses down to a few nanometres. Similarly, as for VBF images, Virtual Dark Field (VDF) images can be generated using virtual apertures integrating the diffracted intensities. Figure S3 generated with such virtual apertures highlights the presence of many crystal orientations within the globular Al13Fe4 particles. These nanocrystals explain the broad XRD peak observed at low angles (Figure S1b). The precipitation of the intragranular plate-like Al13Fe4 is associated with the decomposition of the supersaturated matrix obtained from LPBF. Note that the EDS map shown in Figure 4c is not enough resolved to reveal the possible precipitation of Zr-rich nanosized precipitates. It highlights that despite its low solubility in Al, some Fe is trapped in the solid solution upon LPBF. Al13Fe4 typically leads to large intermetallic particles and embrittlement for cast Al-alloys [35]. Upon LPBF and ageing, the plate-like Al13Fe4 are extremely fine and the material retains good ductility. TEM bright field images and selected area diffraction patterns (SAED) of the aged specimen were collected in a FEZ in <110> zone axis (Figure 5a,b) and a CZ in <211> zone axis (Figure 5c,d), on which are identified the signature of a coherent phase. Fe has a very low solubility in Al and appears to have been consumed by the formation of globular Fe-rich intermetallics upon LPBF and the precipitation of plate-like Al13Fe4 upon ageing. Besides the primary Al3Zr, no other Zr-rich phases have been identified with ACOM and EDS in the stressrelieved and aged specimens. It is reasonable to propose that the coherent precipitates are spherical nano-sized Al3Zr as in [13,14]. The high conductivity is attributed to the matrix depletion in solute atoms, namely Fe and Zr, caused by the dual precipitation of Al13Fe4 and Al3Zr.

Additional ACOM recordings were collected at higher magnification and smaller step size within the matrix. Many of the recorded nanodiffraction patterns exhibit superlattice reflections. The diffracted intensities along a virtual annular aperture where the superlattice reflections are observed (see Figure 5f) were used to generate the VDF image in Figure 5e. This image reveals the presence of the coherent nanoprecipitates since contrast partly results from the surrounding strain fields. The density of these nano-Al3Zr appears significantly greater than that of plate-like Al13Fe4 also formed upon ageing. The strengthening associated with the ageing treatment is thus primarily attributed to the nanoprecipitation of coherent Al3Zr-L12. To a minor extent, the precipitation of the plate-like Al13Fe4 may additionally contribute to the recorded strengthening. In summary, a novel Al-1Fe-1Zr alloy achieving a superior strengthconductivity trade-off and good LPBF-processability was presented. The primary Al3Zr ensure no hot cracking occurs, while direct ageing leads to the dual precipitation of coherent nano-Al3Zr and plate-like Al13Fe4 and YS increase up to 310 MPa. This depletion of the supersaturated matrix obtained from LPBF results in a concomitant conductivity increase.

Figure 1 :

 1 Figure 1: Typical (a) engineering and (b) true tensile stress-strain curves of LPBF Al-Fe-Zr in the stress-relieved (SR) and peak-aged conditions. (c) Ashby chart of yield strength as a function of thermal conductivity plotted with CES Selector GRANTA 2010, showing typical performance of wrought Al alloy series and the novel Al-Fe-Zr introduced in this work.

Figure 2 :

 2 Figure 2: EBSD orientation maps in inverse pole figure (IPF) colouring, (a) in the stress-relieved and (d) the peak-aged (4h at 400°C) conditions. Typical microstructures imaged with the BSE detector within (b, e) the CZ and (c, f) the FEZ.

Figure 3 :

 3 Figure 3: Local solute enrichments and phase distribution of a region of interest containing a grain boundary in the CZ after stress-relief (a) Virtual Bright Field image. EDS maps of the following elements: (b) Aluminium, (c) Zirconium, (d) Iron and (e) Oxygen. (f) Phase map showing the morphology and distribution of phases present within the frame highlighted in (a). The phase reliability in grey scale is overlaid on the map. Selected nanodiffraction patterns are displayed with their corresponding R index.

Figure 4 :

 4 Figure 4: Local solute enrichments and phase distribution of a region of interest in the CZ after ageing at 400°C for 4h. (a) Virtual Bright Field image. EDS maps of the following elements: (b) Aluminium, (c) Zirconium, (d) Iron and (e) Oxygen. (f) Phase map showing the morphology and distribution of phases present within the frame highlighted in (a). The phase reliability in grey scale is overlaid on the map. Selected nanodiffraction patterns are displayed with their corresponding R index.

Figure 5 :

 5 Figure 5: TEM imaging in zone axis of the aged sample: (a, b) bright field image within the FEZ and its associated SAED pattern, (c, d) corresponding image and pattern for the CZ.(e) Virtual Dark Field image within the CZ generated with ACOM and the virtual aperture displayed in (f) on an individual nanodiffraction pattern.

Table 1

 1

	was produced. This alloy is designed and supplied by Constellium under the trademark
	Aheadd® CP1. A particle size between 26 µm (D10) and 64 µm (D90) was measured following
	ISO 13320.
	Element Wt. %	Method
	Al	Balance N. A.
	Fe	1.02	ICP
	Zr	1.05	ICP
	O	0.098	LECO
	N	<0.005	LECO
	H	0.0019	LECO

Table 1 :

 1

Chemical composition of the Al-Fe-Zr alloy gas atomised powder for LPBF, measured by inductively coupled plasma optical emission spectrometry (ICP) and carrier gas hot extraction (LECO).

[%] σ [10 6 S/m] κ [W/m·K]

	SR	128	25	7.2	17.80 ± 0.02 122
	4h/400°C 309	13	5.9	27.02 ± 0.02 180

Table 2 :

 2 Tensile

	properties (the minimal performances over the conducted 3 tests are reported), electrical σ and
	thermal conductivity κ of LPBF-processed Al-Fe-Zr in the stress-relieved and peak-aged conditions. Uniform
	elongation (Au) is determined just before the test piece necking. Elongation to failure (A) was measured on the
	engineering stress-strain curves.

Acknowledgements

This work has been conducted in the framework of the Aéroprint project supported by La Région Auvergne-Rhône-Alpes. The authors would also like to acknowledge the support from Constellium. Gilles Renou and Patricia Donnadieu are acknowledged for their technical support and fruitful discussions.

Declaration of competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.