Bernard G Barthès
email: <bernard.barthes@ird.fr>

Aurelien Penche

Edmond Hien

Philippe Deleporte

Cathy Clermont-Dauphin

Laurent Cournac

Raphaël J Manlay

Aurélien Penche

Effect of ramial wood amendment on sorghum production and topsoil quality in a Sudano-Sahelian ecosystem

Keywords: Organic amendments, branches, soil carbon, soil phosphorus, termites, tropical dry Africa

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

Introduction

Natural and cultivated ecosystems differ in the degree of closure of biogeochemical cycles, due to differences in structural and functional complexity [START_REF] Odum | The strategy of ecosystem development[END_REF][START_REF] Perry | Bootstrapping in ecosystems[END_REF].

Agrosystems are particularly open systems for energy and nutrient balance: their viability depends strongly on external soil management to compensate for nutrient and carbon (C) losses whereas the resilience of natural ecosystems is achieved largely through self-sustained processes of biological maintenance. The conventional management of agrosystems generally aims at maintaining some soil physical and chemical properties within acceptable ranges based on "substitution maintenance" (sensu [START_REF] Izac | On agricultural sustainability and its measurement in small-scale farming in sub-Saharan Africa[END_REF]. Indeed, tillage and mineral fertilizers may compensate for soil physical degradation and nutrient losses, respectively; but they may also have detrimental agronomic and environmental effects, such as soil erosion and degradation, groundwater pollution, eutrophication of aquatic habitats, greenhouse gas emissions, etc. [START_REF] Tilman | Agricultural sustainability and intensive production practices[END_REF]. Furthermore, in many instances, tillage and fertilizers are unable to avoid soil degradation, which has been estimated to affect more than 560 Mha of agricultural land [START_REF] Lal | Soil degradation by erosion[END_REF]. Soil degradation involves physical, chemical and/or biological processes, which all result in the decline in soil organic C; but some of these processes may be reversed by increasing soil organic C [START_REF] Lal | Agricultural activities and the global carbon cycle[END_REF]). This has important consequences for the ecosystem services provided by soils: for example, increasing topsoil organic C by 1 Mg ha -1 may increase cereal grain production by 32 Mt in developing countries, where there is a need to increase production by 477 Mt between 2000 and 2025 to meet human food demand [START_REF] Lal | Enhancing crop yields in the developing countries through restoration of the soil organic carbon pool in agricultural lands[END_REF].

With the development of soil ecology, which underlines the role of biological factors in soil functioning, the application of organic amendments has regained interest as a sustainable alternative for agrosystem management by improving soil physical, chemical and biological properties [START_REF] Manlay | Historical evolution of soil organic matter concepts and their relationships with the fertility and sustainability of cropping systems[END_REF]). However, this practice is often limited by the availability and competition for use of organic resources, transportation costs, and the delicate control of decomposition dynamics and nutrient bioavailability [START_REF] Kumar | Crop residues and management practices: Effects on soil quality, soil nitrogen dynamics, crop yield, and nitrogen recovery[END_REF]. The emergence of agroecology as a scientific discipline responds to the need to overcome these limitations, among others [START_REF] Altieri | Agroecology: the science of natural resource management for poor farmers in marginal environments[END_REF]. One basic hypothesis in agroecology is that the sustainability of natural ecosystems relies on structural and functional features that are largely specific to local environmental conditions; and that, once strategies to imitate these features have been developed, they can be transferred to agrosystems to enhance their sustainability [START_REF] Ewel | Natural systems as models for the design of sustainable systems of land use[END_REF].

The grassland ecosystem served as a model for perennial grain cropping in North America [START_REF] Jackson | Natural systems agriculture: a truly radical alternative[END_REF] and for direct seeding mulch-based cropping systems (CIRAD 1999). Forest ecosystems served as model for agroforestry. The strategy of using tree branches, or ramial wood (RW), as soil amendment also mimics tree ecosystems.

The use of RW for soil amendment has been developed in Canada for over 20 years and then in other countries [START_REF] Lemieux | Cet univers caché qui nous nourrit : le sol vivant[END_REF]. It consists in burying small-diameter tree branches in the upper layer of cultivated soils in a "sink" (target) agroecosystem. The initial recommendations, established under cool climate conditions, mentioned the use of branches less than 7 cm in diameter, which were chipped, hence the expression "ramial chipped wood" to describe the material itself and the technique. The aim of applying RW is to improve the organic and biological status of the sink soils so that it becomes closer to that of the forest soils from which they would derive (although the ecosystem preceding cropping is not necessarily forest). The developers of this technique claim that applying RW would increase crop yields by modifying the soil physical and chemical properties, and, to a greater extent, stimulating soil biological activities, especially by encouraging fungi at the expense of bacteria. Applying RW may also buffer the impact of climate on plant performance, which is of special interest for smallholder farmers in the dry tropics [START_REF] Vries | Land use alters the resistance and resilience of soil food webs to drought[END_REF].

However, rather little statistical information is available in the literature on the effects of RW amendments on crops and soil, according to a review by [START_REF] Barthès | Effets de l'apport de bois raméal sur la plante et le sol : une revue des résultats expérimentaux[END_REF]. All studies in temperate conditions reviewed in this paper were carried out in Canada, on coarse-textured soils, and involved burying chipped RW. The few studies carried out under tropical conditions often involved the application of roughly fragmented or unfragmented RW, which was buried or sometimes used as mulch. On the whole, the review indicated that RW application increases the soil nutrient and organic matter content, stimulates soil biological activities, especially those of fungi, which in turn improves nutrient availability for crops. However, owing to high carbon-to-nitrogen and carbon-to-phosphorus ratios, RW application might lead to nitrogen (N) and phosphorous (P) immobilization in the year of application, and thus to a decrease in the yield of the following crop. This has been observed when RW was buried in sandy soils, particularly in temperate conditions, but research is required to extend this result to tropical conditions [START_REF] Barthès | Effets de l'apport de bois raméal sur la plante et le sol : une revue des résultats expérimentaux[END_REF]. However, the review reported that N immobilization could be offset by additional N input; and that applying RW tends to increase the yields of subsequent crops, even with repeated RW application. Furthermore, the application of RW, especially as mulch, improves soil physical properties (moisture, porosity, structure, etc.), which, in semi-arid conditions, is closely linked with termite activity. There has not been any report of possible effects on crop pests and diseases.

The effects of RW application can be affected by several factors such as the tree species and amendment characteristics, including quantity, frequency, and chip size, but the available information cannot be used to draw up precise recommendations. Furthermore, there is little information on the advantages of using RW rather than organic, non-woody amendments [START_REF] Barthès | Effets de l'apport de bois raméal sur la plante et le sol : une revue des résultats expérimentaux[END_REF].

Most of the studies reviewed tested the application of RW at rates higher than 5 Mg ha -1 (dry matter basis, DM), which may raise concerns about the availability of ramial resources and about transportation and possible chipping of RW, especially in conditions where no machines are available.

The objective of the study was to assess the effects of amendment with RW or straw, buried or mulched, at a low application rate, on some topsoil chemical properties, on termite cast coverage (as a proxy of their activity), and on sorghum production, in a manual cropping system under semi-arid conditions.

Materials and methods

Study site

The study was carried out in 2007, 2008 and 2009, at the experimental station of Gampéla (12°24'35" N, 01°21'05" W), 15 km east of Ouagadougou, in the central region of Burkina Faso.

In the area, rainfall is between 600-900-mm yr -1 , from May to October (INERA 1995). In 2007, 2008and 2009, the annual rainfall at Gampéla was 751, 865 and 854 mm yr -1 , respectively, which was higher than the mean of the previous ten years (692 mm yr -1 ). The rainfall distribution was well-balanced in 2008, whereas 2009 was characterized by major flooding on September 1 st , with 233 mm rainfall within a few hours (Fig. 1), when the sorghum was at flowering stage. The mean annual temperature is 28°C.

The landscape is slightly undulating, with a slope less than 2%, and is developed on granites and migmatites [START_REF] Hottin | Carte géologique du Burkina Faso, échelle l/l,000,000 e[END_REF]. The soil at the experimental station is an endogleyic Acrisol (IUSS Working Group WRB 2006). The texture is silty sand in the topsoil and silty clay in the subsoil. This soil type is common in Burkina Faso [START_REF] Gómez | Sustainable farming systems in the sub-Sahelian zone of Burkina Faso -Key factors[END_REF].

Most of the land around the experimental station is cultivated. The main crops are cereals such as sorghum, millet and maize, and legumes such as niebe (black-eyed pea) and peanut.

Experimental design

The trial was set up in an area that had been under natural fallow for four years, and then uniformly cropped with sorghum for one season in 2006 before starting the trial (manual cropping, no organic or nutrient inputs). The trial had a randomized complete block design, with four blocks perpendicular to the main slope. Each block had six 6 × 5 m 2 plots, which were separated by 1 m wide alleys.

The experiment compared six types of applications, located at random in each block, all plots being cropped with sorghum [Sorghum bicolor (L.) Moench, var. Sariasso]. Three treatments involved the application of RW from Piliostigma reticulatum (DC.) Hochst, which is a nonnitrogen-fixing Caesalpiniaceae shrub that is very common in the Sudano-Sahelian area [START_REF] Arbonnier | Arbres, arbustes et lianes des zones sèches d'Afrique de l'Ouest[END_REF]. After being cut when the land is cleared, it regenerates rapidly when the land is left fallow [START_REF] Yelemou | Gestion traditionnelle et usages de Piliostigma reticulatum sur le Plateau central du Burkina Faso[END_REF]). The experiment used leafy branches less than 2 cm in diameter, collected in late May. Their composition was determined on five composite samples, each comprising 12 branches cut from several places on each of five shrubs more than 10 m apart (see analytical methods in the next section). These Piliostigma branch samples (including leaves) had mean (± standard error, n = 5) C, N, P and potassium (K) contents of 46.2 (± 0.2), 1.31 (± 0.02), 0.088 (± 0.005) and 0.88 (± 0.09) g 100 g -1 DM, respectively (C/N = 35.3); water content was 49% (as measured after drying at 50°C until constant mass was achieved). These branches were cut into small pieces (< 5 cm long) using a machete and applied immediately to the plots concerned. Other treatments involved the application of dry sorghum straw, in which C, N, P and K contents averaged 43.2, 0.59, 0.042 and 0.82 g 100 g -1 DM, respectively (C/N = 73.2); water content was 2%. Straw was also cut into small pieces. The six treatments studied were:

-Ctrl (control): sorghum cropping according to local practices, without any input; the soil was hoed manually to a depth of 5 cm at the beginning of June and sorghum was sown in late June or early July, when the soil moisture was considered appropriate; the plots were then hoed manually every two weeks for six weeks to weed and break the crust caused by heavy rains on the silty soil; the sorghum was harvested in October or November, and crop residues were removed; -WoBu (wood buried): as for Ctrl but RW was applied at a rate of 1.5 Mg DM ha -1 yr -1 (i.e. 0.69 Mg C ha -1 yr -1 ); in late May the RW was collected, chipped, applied the same day, and buried at a depth of 5 cm using a hoe; -WoMu (wood mulched): as for WoBu but RW was mulched and lightly covered with soil from the plot to avoid dispersal by wind or runoff; -WoBuN (wood buried with N): as for WoBu with an additional application of 9.6 kg N ha -1 yr -1 as urea two weeks after emergence; the aim was to offset possible N immobilization by soil microorganisms; -StBu (straw buried): as for WoBu but the 1.5 Mg DM ha -1 yr -1 of RW was replaced by straw at a rate of 1.6 Mg DM ha -1 yr -1 (i.e. 0.69 Mg C ha -1 yr -1 , as for RW) with the addition of 10.47 N, 0.65 P and 0.75 K kg ha -1 yr -1 applied as mineral fertilizer (14.23.14) and urea, two weeks after emergence; this mineral complement aimed at achieving similar C, N, P and K application rates in WoBu and StBu; -StMu (straw mulched): as for StBu but the straw was mulched and lightly covered with soil to avoid dispersal by wind or runoff.

The application rate of 1.5 Mg DM ha -1 for ramial wood was selected as being between the rates of 1 and 2 Mg DM ha -1 tested by [START_REF] Wezel | Mulching with branches of an indigenous shrub (Guiera senegalensis) and yield of millet in semi-arid Niger[END_REF] in neighbouring Niger.

Measurements and analyses

Sorghum grain and straw production were measured every year at harvest in the central 16.4 m² (out of 30 m²) of every plot. The two outside rows and the two end sheaves on the rows were not taken into account. The grains and straw were weighed after air drying for 2 months and oven drying of aliquots at 60°C for 2 days (until constant mass was achieved).

In 2008 and 2009, the weight of 1000 seeds was determined on an aliquot of air-dried grains.

The grain yield and weight of 1000 seeds were used to calculate the number of seeds per hectare. The seed number and the weight of 1000 seeds have been proposed as indicators of cereal yield limiting factors: the former would reflect water and nutrient availability during the vegetative phase of the crop cycle, and the latter, possible water stress and/or pest and disease intensity during the period from flowering to maturity [START_REF] Doré | Methodological progress in regional agronomic diagnosis[END_REF].

Composite soil samples were collected on each plot in April 2007, before starting the trial, at 0-5 cm depth, and again at harvest in 2008 and 2009, at 0-5 and 5-15 cm depth, using an auger. Three separate samples were taken from sorghum rows in different parts of the plot and mixed thoroughly to form the composite sample. The soil samples were air-dried and then gently broken up using a pestle and mortar, and sieved at 2 mm. Aliquots were ground to 0.2 mm for further analyses.

The C and N concentrations in the soil and plant samples were determined on 0.2 mm ground aliquots by dry combustion using an elemental analyzer (CHN Fisons/Carlo Erba NA 2000, Milan, Italy). However, in 2007, the soil N could not be determined owing to analytical problems. As all of the soils were carbonate-free, total C was equal to organic C. The soil available P (Pav) concentration was determined on 0.2 mm ground aliquots using the Olsen procedure for samples collected in 2007 and 2008 (extraction using sodium bicarbonate at pH 8.5), and the Olsen-Dabin procedure for samples collected in 2009 (extraction using sodium bicarbonate and ammonium fluoride at pH 8.5), both with colorimetric assay. The latter procedure, which complexes iron and aluminum, tends to extract more P and has sometimes been considered more suitable for soils from tropical regions [START_REF] Pansu | Handbook of soil analysis: mineralogical, organic and inorganic methods[END_REF]. The P and K concentrations in plant samples were determined after dry mineralization by inductively coupled plasma atomic emission spectrometry (ICP-AES; Pansu and Gautheyrou 2006).

Termite activity was characterized at the beginning of the cropping season in 2008 by semiquantitative assessment of the abundance of termite casts on the soil surface. On each plot, a 1 × 1 m square frame with one hundred 10 × 10 cm squares was put on the soil, and the number of squares with termite casts was counted. The count was replicated 10 times on the plot, along two perpendicular transects. The count was carried out seven days after applying RW or straw and again one and two weeks later. It was not possible subsequently because the termite casts were destroyed by rainfall.

Statistical analyses

An analysis of covariance (ANCOVA; Yang and Juskiw 2011) was carried out to determine the significance of treatment effects on sorghum grain and straw production, and on topsoil C, N and Pav (at p < 0.05). An analysis of variance (ANOVA) was carried out to determine the significance of treatment effects on the interannual variations of grain yield and on the abundance of termite casts (at p < 0.05). ANOVA considered block and treatment as explicative variables. ANCOVA additionally considered the initial (April 2007) C and Pav contents at 0-5 cm as explicative variables, in order to remove possible effects of initial topsoil conditions. The significance of explicative variables was assessed by type III sum-ofsquare. A Duncan's paired test was used to assess the significance of differences between treatments at a given date, and a Student's paired test was used to assess the significance of differences between dates for a given treatment. The correlation coefficient was calculated between the initial C and Pav contents at 0-5 cm. The correlation coefficient between the weight of 1000 seeds and number of seeds per hectare was calculated over two years (data were available for 2008 and 2009); although less informative due to the small number of plots, it was also calculated per year. All analyses were carried out using the XLSTAT 2008 v6.01 software (Addinsoft, Paris).

Results

Effects of treatments on soil total carbon, total nitrogen, and available phosphorus

At the start of the trial (April 2007), the total C and Pav at 0-5 cm depth were significantly higher upslope than downslope (block effect), and were significantly correlated (r = 0.43, p < 0.05; Table 1). This was the reason for using initial C and Pav as additional explicative variables in ANCOVA when studying treatment effects.

There were few significant differences between treatments (Table 1). Firstly, in 2008 at 5-15 cm, C was significantly lower in WoBuN than in most other treatments, and significantly lower in StBu than in WoBu. Secondly, in 2009 at 0-5 cm, Pav was significantly lower when the amendments were buried (WoBu, WoBuN, StBu) than when they were mulched (WoMu, StMu); for this date and depth layer, differences between plots were significantly affected by initial Pav, and to a lesser extent, by initial C and block.

Effects of treatments on crop production

The mean overall yields in 2007, 2008 and 2009 were 0.87, 0.37 and 0.28 Mg DM ha -1 for grain, and 3.18, 1.94 and 0.72 Mg DM ha -1 for straw, respectively. Grain yields were low, and both yields decreased noticeably with time (Table 2). Using ANCOVA, the mean grain and straw yields and the mean total aboveground biomass (sum of grain and straw yields) did not differ significantly between treatments in 2007. In 2008, the mean grain and straw yields were significantly lower in Ctrl than in WoBuN and StBu, although ANCOVA showed that the treatments had little effect on grain yield (p = 0.1); nevertheless it showed that the total aboveground biomass was significantly lower in Ctrl than in WoBuN and StBu (data not shown). In 2009, there was no significant effect of treatment on yields but there were significant effects of block and initial topsoil Pav, which was not the case in 2007 and 2008.

The mean three-year cumulative grain and straw yields were not affected significantly by the treatment; but, as in 2007 and 2008, there was an overall trend towards lower grain and straw yields in Ctrl and WoBu than in the other treatments (Table 2). The interannual variation in the grain yield was not significantly dependant on the treatment.

The mean weight of 1000 seeds was significantly lower in Ctrl than in the other treatments in 2008, but was not significantly affected by the treatments in 2009. The mean number of seeds per hectare was also significantly smaller in Ctrl than in WoBuN and StBu in 2008 (13 vs. 38 and 36 10 6 seeds ha -1 , respectively; data not shown), and was significantly higher in StMu than in StBu in 2009 (21 vs. 11 10 6 seeds ha -1 ; data not shown). Taking 2008 and 2009 together, there was a negative relation between seed weight and seed number (R = -0.52, p < 0.1), particularly if the data for Ctrl in 2008 were removed (R = -0.74, p < 0.01), and, to a lesser extent, if the data for WoBu in 2008 were also removed (R = -0.85, p < 0.01; Fig. 2).

However, there was a positive correlation between the seed weight and number for 2008 on its own (R = 0.81, p < 0.1), and no significant correlation for 2009 on its own (R = -0.36, p > 0.1); but this was less informative owing to the small number of plots per year. There were more grains in 2008 than in 2009 but these were smaller. For both years, the seed weight was highest for StBu and smallest for Ctrl.

Termite cast abundance

According to ANOVA, the number of 10 × 10 cm squares with termite casts at the beginning of the cropping season in 2008 was 4-18 times higher for treatments with RW (8 to 11% of the total number of squares, standard error < 2%) than for the other treatments (0.6% for Ctrl, 1.2% for StBu, 2.1% for StMu, standard error < 1%), the difference being very significant (p < 0.001).

Discussion

Topsoil C, N and Pav

The main significant differences in the topsoil properties between the treatments were: (i) C at 5-15 cm at the 2008 harvest, which was lower in WoBuN than in the other treatments (the trend was also noticeable at 0-5 cm and in 2009), and (ii) Pav at 0-5 cm at the 2009 harvest, which was lower when the amendment was buried than when it was mulched. The aboveground biomass production tended to be higher in WoBuN than in the other treatments in 2007 and 2008 (this was significant only in 2008 when compared to Ctrl). The aboveground residues were exported, but it is likely that root production and thus plant C input into the soil were also higher in WoBuN, especially in 2008. It could thus be expected that topsoil C would be higher with this treatment than with the others, but this was not the case. Furthermore, soil C at 0-5 cm tended to decrease in all treatments over the three-year experiment, and the decrease was significant for WoMu, indicating that mulching did not prevent soil C loss.

An increase in topsoil C has often been reported in RW experiments. However, there are very few studies for sandy soils in dry tropical regions and these are much less conclusive [START_REF] Barthès | Effets de l'apport de bois raméal sur la plante et le sol : une revue des résultats expérimentaux[END_REF]. At the second tomato harvest after burying RW at a rate of 8 to 31 Mg DM ha -1 , [START_REF] Soumare | Effects of Casuarina equisetifolia composted litter and ramial-wood chips on tomato growth and soil properties in Niayes, Senegal[END_REF] observed that topsoil C did not differ significantly from the control (though 40% higher) and did not depend on the RW application rate (± 4% variation). Taken in conjunction with our results, this suggests that adding high rates of fresh organic matter to sandy soils, at least in semi-arid conditions, has no clear effect on topsoil C, and that adding low rates causes the topsoil C to decrease. This may be explained by the weak protection of organic matter against mineralization [START_REF] Barthès | Texture and sesquioxide effects on waterstable aggregates and organic matter in some tropical soils[END_REF], and also probably by priming effect [START_REF] Kuzyakov | Review of mechanisms and quantification of priming effects[END_REF][START_REF] Fontaine | The priming effect of organic matter: a question of microbial competition?[END_REF].

In this study, topsoil N was not significantly affected by the treatments. This does not agree with the results reported by [START_REF] Soumare | Effects of Casuarina equisetifolia composted litter and ramial-wood chips on tomato growth and soil properties in Niayes, Senegal[END_REF] in a sandy soil under tomato in Senegal (similar climate and soil conditions to the present study), where burying RW (8 to 31 Mg DM ha -1 ) caused the topsoil N to increase tenfold compared to the control. This positive effect might be attributed to the much higher RW application rates than in the present study, as well as the much higher N content in RW. High RW application rates are not necessarily sufficient to increase topsoil N: for wetter conditions and a more clayey soil, topsoil N under banana plantation was not significantly affected two years after applying woodchip mulch at a high-rate (50 Mg DM ha -1 ; [START_REF] Salau | Effects of mulching on soil properties, growth and yield of plantain on a tropical ultisol in southeastern Nigeria[END_REF].

The low application rates might also explain the limited effects of treatments on topsoil Pav in 2007and 2008. Nevertheless, in 2009, topsoil , topsoil Pav was lower in treatments involving burying, which might be explained by P immobilization by soil microorganisms. This immobilization might result from repeated treatments over three years (the topsoil Pav decreased from harvest 2007 to harvest 2009) and/or from the major flooding in September 2009, which might have stimulated mycelial proliferation. The burying of RW at a high rate (> 18 Mg DM ha -1 yr -1 ) in a sandy soil under potato in Canada also caused topsoil Pav to be significantly lower than in the control, which was attributed to P immobilization [START_REF] Tremblay | Fractionnement de la fertilisation azotée d'appoint à la suite de l'incorporation au sol de bois raméaux fragmentés : modifications de certaines propriétés biologiques et chimiques d'un sol cultivé en pomme de terre[END_REF]. A significant difference in topsoil Pav was also observed under tomato in Senegal between the control and plots where RW had been buried at rates of 8 to 31 Mg DM ha -1 , and, according to analyses of P concentration in leaves, it was similarly attributed to microbial immobilization [START_REF] Soumare | Effects of Casuarina equisetifolia composted litter and ramial-wood chips on tomato growth and soil properties in Niayes, Senegal[END_REF]. However, the topsoil Pav was not clearly affected in more clayey soils in humid tropics, with RW or ligneous inputs either buried or applied as mulch at a rate between 5 and 50 Mg DM ha -1 yr -1 [START_REF] Obiefuna | Establishment of pineapple orchards and soil loss control systems for erodible tropical ultisols of southeastern Nigeria[END_REF][START_REF] Salau | Effects of mulching on soil properties, growth and yield of plantain on a tropical ultisol in southeastern Nigeria[END_REF][START_REF] Kwabiah | Phosphorus availability and maize response to organic and inorganic fertilizer inputs in a short term study in western Kenya[END_REF].

It should be borne in mind that the soil C, N and Pav contents were very low at the site studied here, as is often the case in central Burkina Faso: over the three years, on the composite samples analyzed (not averaged over blocks), C ranged from 2.2 to 4.3 g kg -1 , N from 0.17 to 0.31 g kg -1 , and Pav from 1 to 20 mg kg -1 . This study did not characterize repeatability or reproducibility for soil C, N and Pav analyses; nevertheless, especially for C and N, the differences between the treatments or dates were not much greater than analytical precision (e.g. around 0.5 g kg -1 for C), which limited the interpretation of results.

Crop production

Few differences in the crop properties studied were significant between treatments, probably due to the low application rates and to spatial heterogeneity. However, some significant differences in yield were recorded in 2008: more grain and straw were harvested in WoBuN and StBu than in Ctrl. The lower yield in Ctrl might be explained by the lack of inputs. By contrast, for both WoBuN and StBu the amendment was buried and mineral N was added, which probably resulted in higher nutrient bioavailability. The grain yield in 2008 varied mainly according to the number of grains (cf. Fig. 2), which depends on water and nutrient availability during flowering; whereas the grain weight was low, suggesting that the water supply was limiting during the grain-filling period [START_REF] Doré | Methodological progress in regional agronomic diagnosis[END_REF]). It should be noted that rainfalls in August and September were low in 2008 in comparison with 2007 and 2009 (cf.

Fig. 1). The trend changed in 2009: the yield seemed to depend firstly on Pav, which was significantly lower in treatments where the amendment was buried (see discussion on soil properties). Despite fairly high seed weight, the grain yield was very low due to the low number of grains (cf. Fig. 2), which confirms that the supply of minerals was a limiting factor. Significantly smaller grains in Ctrl than in the other treatments in 2008 suggests that straw and RW amendments had a general positive effect on late season conditions, during the grainfilling period, probably owing to a better water supply (see discussion regarding termites).

The lack of any significant effect of treatment on the cumulative grain and straw yield over the three years suggested that the experimental design (in particular the number of blocks) might not be appropriate for assessing the impact of a low organic matter application rate.

Furthermore, the lack of any significant effect of RW application on interannual yield variation might indicate either that the RW application was not effective at buffering climatic variations, or that the experimental conditions (duration and design) were not suitable for showing any buffering effect. However, the strong decrease in yield from 2007 to 2009 seemed to be caused by nutrient depletion, very possibly a lack of P, rather than by interannual variations. Unnoticed pest attacks might also have affected the yield from year to year.

According to the literature (cf. review from [START_REF] Barthès | Effets de l'apport de bois raméal sur la plante et le sol : une revue des résultats expérimentaux[END_REF], burying chipped RW in coarse-textured soils decreases the yield of the first crop after application, as observed in Canada (N'dayegamiye [START_REF] Dubé | L'effet de l'incorporation de matières ligneuses sur l'évolution des propriétés chimiques du sol et sur la croissance des plantes[END_REF][START_REF] Beauchemin | Effets d'apport d'amendements ligneux frais et humifiés sur la production de pomme de terre et sur la disponibilité de l'azote en sol sableux[END_REF][START_REF] Beauchemin | Effets d'amendements ligneux sur la disponibilité d'azote dans un sol sableux cultivé en pommes de terre[END_REF][START_REF] Larochelle | L'impact du bois raméal fragmenté sur la dynamique de la mésofaune du sol[END_REF]; the crops studied were cereals and potato) and in the only peer-reviewed paper reporting RW burying in sandy soils under tropical conditions (Soumare et al. 2002, for tomato). The trend appears, therefore, to be well established for temperate conditions but unconfirmed for tropical conditions, and it is not confirmed by the present study. The lower first yield with buried RW has been attributed to the decrease in N uptake by crops after N immobilization by soil microorganisms (N'dayegamiye [START_REF] Dubé | L'effet de l'incorporation de matières ligneuses sur l'évolution des propriétés chimiques du sol et sur la croissance des plantes[END_REF][START_REF] Beauchemin | Effets d'apport d'amendements ligneux frais et humifiés sur la production de pomme de terre et sur la disponibilité de l'azote en sol sableux[END_REF]), and additionally, in tropical conditions, to P immobilization [START_REF] Soumare | Effects of Casuarina equisetifolia composted litter and ramial-wood chips on tomato growth and soil properties in Niayes, Senegal[END_REF]. The latter point has been discussed in the previous section. However, published papers have reported that the yield of the subsequent crops tends to be higher with the application of RW than without, even when RW has been applied repeatedly (N'dayegamiye [START_REF] Dubé | L'effet de l'incorporation de matières ligneuses sur l'évolution des propriétés chimiques du sol et sur la croissance des plantes[END_REF][START_REF] Gasser | Short-term effects of crop rotations and wood-residue amendments on potato yields and soil properties of a sandy loam soil[END_REF][START_REF] Soumare | Effects of Casuarina equisetifolia composted litter and ramial-wood chips on tomato growth and soil properties in Niayes, Senegal[END_REF]. Burying chipped RW in less sandy soils or mulching RW, even in sandy soils, increases crop yields from the first season, as observed in tropical conditions [START_REF] Aman | Effects of chopped twig wood on maize growth and yields in the forestsavanna transition zone of Côte d'Ivoire[END_REF][START_REF] Wezel | Mulching with branches of an indigenous shrub (Guiera senegalensis) and yield of millet in semi-arid Niger[END_REF][START_REF] Gómez | Sustainable farming systems in the sub-Sahelian zone of Burkina Faso -Key factors[END_REF]; the crops studied were maize and millet). However, most of the studies referred to above applied RW at much higher rates than the present study (4-50 Mg DM ha -1 yr -1 vs. 1.5 Mg DM ha -1 yr -1 here), which very probably explained the greater effects on crop yield. It should be noted that the only study carried out in conditions similar to the present study (two cereal cropping seasons after one 1-or 2-Mg DM ha -1 application of mulched RW on a sandy soil in semi-arid Niger) was not very conclusive [START_REF] Wezel | Mulching with branches of an indigenous shrub (Guiera senegalensis) and yield of millet in semi-arid Niger[END_REF]: the millet grain yield was higher with the application of RW than without (+80% in average) but this was hardly significant owing to soil variability; furthermore, the grain yield did not vary to any great extent when the RW application rate was doubled (+6% in average).

Termites as ecosystem engineers?

The application of RW promoted termite activity markedly and significantly, and to a much greater extent than the application of straw. The positive effects of the presence of dead woody material on termite colonization have already been reported in wet tropical conditions [START_REF] Davies | Successional response of a tropical forest termite assemblage to experimental habitat perturbation[END_REF]. By contrast, [START_REF] Rouland | Experimental manipulation of termites (Isoptera, Macrotermitinae) foraging patterns in a Sahelo-Sudanese savanna: effect of litter quality[END_REF] found that more termite casts were produced on straw than on woody material in an experiment in Senegal during the dry season, under drier conditions than in the present study. Since they also showed that litter preference was strongly driven by termite species, further characterisation of termite diversity in Gampéla would be needed for further comparison with other works. [START_REF] Mando | Effect of termites and mulch on the physical rehabilitation of structurally crusted soils in the Sahel[END_REF] demonstrated on crusted, uncropped soils in Burkina Faso that the positive role of mulches (straw or/and branches) on soil porosity, infiltration and water status was mainly due to their stimulation of termite activity. The rate of RW application was higher than in the present study (6 vs. 1.5 Mg DM ha -1 ), but it is likely that the RW mulch also had a positive but less marked effect on the soil hydrophysical properties in the present study.

Ecological intensification due to ramial wood?

This experiment was set up to determine the specificity of woody amendment (vs. herbaceous amendment) by applying similar amounts of C, N, P and K to plots with RW and straw amendment. To compensate for the lower nutrient content of straw, small amounts of mineral N, P and K were applied in addition to the straw. Actually this was not very relevant because these elements were much more easily available for plants than in the organic amendments, which probably favoured the crops in the straw-amended treatments, distorting the comparison with RW-amended treatments.

The strong mediation of RW by termites in this study suggests that patterns of soil improvement previously observed using woody amendment (involving the stimulation of a diffuse fungal network in the topsoil; [START_REF] Barthès | Effets de l'apport de bois raméal sur la plante et le sol : une revue des résultats expérimentaux[END_REF] could not be reproduced under the experimental protocol of this study. There were probably beneficial effects through improved soil hydrophysical properties and soil biochemical and nutrient enrichment by casts. However, termite foraging results in the partial transfer of applied organic matter to their nests, and thus in its partial removal from the crop-soil organo-mineral balance, and in heterogeneous shaping of the foodweb. Establishing fungi-mediated soil improvement patterns would probably require RW to be applied at the onset of the rainy season, when termite activity is at its lowest [START_REF] Manlay | Post-fallow decomposition of woody roots in the West African savanna[END_REF]). However, this would not only suppress any positive benefit from termite foraging but might also decrease nutrient (especially N and P) availability to plant due to microbial immobilization.

Conclusion and perspectives

Over the three years of the experiment, burying or mulching ramial wood or straw at a rate of 0.69 Mg C ha -1 yr -1 had little significant effect on topsoil C, N and Pav or on sorghum grain and straw yields. This was mainly attributed to the low application rate, which was probably so low that the effects were masked by spatial heterogeneity, particularly that of the soil.

Nevertheless, at the third harvest, the topsoil Pav in treatments with buried amendment was significantly lower than in the other treatments, and lower than at the start of the experiment, both probably due to P immobilization by soil microorganisms. The low application rate was not sufficient to increase topsoil C, N, and Pav, which, on the contrary, tended to decrease, due to the mineralization of soil organic matter and probably to priming effect and transfer of organic matter by termites to their nests. Further studies may be needed to take account of the spatial patterns of redistribution of organic matter inputs by termites. Indeed, termite activity was strongly stimulated by the application of ramial wood.

Yields were low, particularly the grain yield, and decreased with time, suggesting that the treatments studied were not sustainable. On the whole, the application rate was not sufficient to cause significant differences in yields between treatments. Nevertheless, at the second harvest, Ctrl produced significantly less than some of the other treatments; it also produced significantly fewer, smaller grains than all other treatments. This suggested that organic applications had a beneficial effect on grain filling, probably owing to better water at the end of the cropping season, which is usually rainless. This also suggested that organic applications had a beneficial effect on grain initiation (number of grains per cropping area) when rainfall was limited during the flowering season.

Larger application rates need to be tested, but this will raise issues about the availability of the branches, which also have to be studied. Long-term studies, involving farm-and territoryscales, are also needed to address both the application efficiency and the availability of resources.

Table 1 Effects of treatments on the total carbon (C), total nitrogen (N), and available phosphorus (Pav; mean ± standard error) in the topsoil as estimated by ANCOVA. For a given variable and date, different letters indicate significant differences between treatments (p < 0.05). T0 stands for the start of the experiment (April 2007)

Few

  significant changes in topsoil C and N were observed over time: C decreased in WoMu at 0-5 cm from April 2007 to the 2009 harvest, and N increased in StBu at 5-15 cm from the 2008 harvest to the 2009 harvest (Table 1). By contrast, Pav decreased significantly and noticeably at 0-5 cm in Ctrl and in treatments with buried amendment (WoBu, WoBuN, StBu) from April 2007 to the 2009 harvest, although not monotonically: it increased between April 2007 (dry season) and the 2008 harvest and then decreased. Partial data at the 2007 harvest even suggested that Pav increased strongly between April 2007 and the 2007 harvest and then decreased (data not shown). At 5-15 cm the Pav decrease over time was significant for all treatments. It is worth noting that the method used for determining Pav involved procedures that tended to extract more P in 2009 than in 2007 and 2008; so the decrease in Pav from 2007 to 2009 might be underestimated.

Fig. 1 Fig. 2

 12 Fig. 1 Monthly rainfall at Gampéla from 2007 to 2009

Table 2

 2 Effects of treatments on grain and straw yield and on the weight of 1000 seeds (mean ± standard error) as estimated by ANCOVA. For a given variable and period, different letters indicate significant differences between treatments (p < 0.05). T0 stands for the start of the experiment (April 2007), C and Pav for total carbon and available phosphorus, respectively, at 0-5cm depth

	Year and variable

Acknowledgements

This work was supported by the joint research unit UMR Eco&Sols (Ecologie fonctionnelle & biogéochimie des sols & des agro-écosystèmes, i.e. Functional ecology & biogeochemistry of soils & agroecosystems; Montpellier SupAgro -Cirad -Inra -IRD), by AgroParisTech, and by the University of Ouagadougou. It was also supported by the Wassa project (Woody amendments for Soudano-Sahelian agriculture) funded by the FP7 -Era ARD (Seventh framework program for research, European research area, Agricultural research for development). We would also like to thank the two anonymous reviewers and Tony Tebby for English correction.


Tom Sanviemvongsak
email: tom.sanviemvongsak@ensiacet.fr

Daniel Monceau

Martin Madelain

Clara Desgranges

James Smialek

Bruno Macquaire

Cirimat

Cyclic oxidation of alloy 718 produced by additive manufacturing compared to a wrought-718 alloy

published or not. The documents may come

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Introduction

The high temperature oxidation resistance of Ni-base alloys produced by Additive Manufacturing (AM) is a new and active topic, and several publications can be found in the opened literature since 2014. The alloy 718 is the one the most popular nickel base superalloys produced by additive manufacturing and one of the most studied in the oxidation field. Although the initial microstructures of AM-718 alloys are different from cast or wrought alloys, available experimental results do not show much difference regarding their high temperature isothermal oxidation resistance [START_REF] Sanviemvongsak | High temperature oxidation of IN 718 manufactured by laser beam melting and electron beam melting: Effect of surface topography[END_REF][START_REF] Juillet | Characterization and oxidation resistance of additive manufactured and forged IN718 Ni-based superalloys[END_REF][START_REF] Yu | Study of Formed Oxides in IN718 Alloy during the Fabrication by Selective Laser Melting and Electron Beam Melting[END_REF]. However, when these alloys produced by AM have a poor metallurgical state -a low density with the presence of cracks or poresthey present much higher oxidation kinetics than a wrought alloy [START_REF] Jia | Selective laser melting additive manufacturing of Inconel 718 superalloy parts: Densification, microstructure and properties[END_REF]. Optimized AM process parameters are now available to produce dense 718 alloys. Heat treatments are used to release 2 most of the internal stress and to get closer to the microstructure of a conventional-718 alloy (wrought or cast). The literature results regarding the effect of heat-treatments on the oxidation resistance are not unanimous. Some researchers showed that standard heat treatments carried out on the alloy 718 produced by AM do not have significant effect on its high temperature oxidation resistance [START_REF] Juillet | Characterization and oxidation resistance of additive manufactured and forged IN718 Ni-based superalloys[END_REF][START_REF] Kang | Effect of post-treatment on the microstructure and high-temperature oxidation behaviour of additively manufactured inconel 718 alloy[END_REF][START_REF] Calandri | Solution Treatment Study of Inconel 718 Produced by SLM Additive Technique in View of the Oxidation Resistance[END_REF]. Conversely, Sadeghi et al. showed that the conventional heat treatment (AMS5662) and a high isostatic pressure (HIP) treatment increase the oxidation resistance at 850 °C of the alloy 718 produced by electron beam melting (EBM) [START_REF] Sadeghi | Influence of thermal post treatments on microstructure and oxidation behavior of EB-PBF manufactured Alloy 718[END_REF]. This result was mainly attributed to densification and the homogenization of the microstructure compared to the as-built sample.

In aircraft turbines, alloy 718 components are exposed to thermal cycles (take-off, cruising, landing) which induce thermal strains of the oxide layer and its metallic substrate. These deformations generate stresses that can be relaxed by plastic deformation of the oxide layer and/or the metal, by cracking of the oxide layer (shear stress), by decohesion of the oxide layer [START_REF] Tien | Oxide spallation mechanisms[END_REF], or by creep of the metal and the fine grained oxide scale [START_REF] Evans | Creep effects on the spallation of an alumina layer from a NiCrAlY coating[END_REF]. Laboratory cyclic air oxidation tests are carried out to simulate these operating cycles in a simplified manner. They allow to evaluate the growth kinetics of the oxide layers and their adherence to the alloy. Up to now, only one publication reports cyclic oxidation behaviour of the alloy 718 produced by AM. The Oak Ridge laboratory showed that EBM-718 alloys present lower mass losses than a wrought-718 alloy, when they were exposed to cyclic humid air oxidation at 750 °C [START_REF] Unocic | High-Temperature Performance of UNS N07718 Processed by Additive Manufacturing[END_REF].

This result was attributed to the formation of iron oxides on the surface of the AM samples, which would result in a greater mass gain for the EBM samples. However, this result remains unquantified and it is not clear whether the mass gain due to iron oxide formation is greater than the mass loss due to spalling. More recently, in the same research group, Romedenne et al. studied the cyclic oxidation in air at 950 °C of Hastelloy X alloy produced by LBM and EBM, comparing their behaviour to that of the wrought alloy [START_REF] Romedenne | High temperature air oxidation behavior of Hastelloy X processed by Electron Beam Melting (EBM) and Selective Laser Melting (SLM)[END_REF]. They found out that the oxide scale of the Hastelloy X alloy produced by EBM and LBM spalled faster than the wrought alloy. This result was mainly attributed to the precipitation of a thin layer of SiO 2 at the metal/Cr 2 O 3 interface.

One of the main characteristics of AM parts is their high roughness when compared to machined ones. High roughness has some specific effects such as enhanced isothermal oxidation kinetics in terms of mass gain due to a higher reactive surface area [START_REF] Sanviemvongsak | High temperature oxidation of IN 718 manufactured by laser beam melting and electron beam melting: Effect of surface topography[END_REF][START_REF] Casadebaigt | Influence of Microstructure and Surface Roughness on Oxidation Kinetics at 500-600 °C of Ti-6Al-4V Alloy Fabricated by Additive Manufacturing[END_REF], or accelerated protective element depletion (Cr depletion here) in convex areas [START_REF] Ramenatte | A comparison of the high-temperature oxidation behaviour of conventional wrought and laser beam melted Inconel 625[END_REF]. As AM technology users would like to use as-built parts, it is important to study as-built samples.

Nevertheless, it is also important to study ground or polished samples to dissociate the effects of chemistry, microstructure and surface geometry.

In the present paper, the cyclic air oxidation resistance of the alloy 718 produced by LBM and EBM process is compared to that of a commercial heat-treated wrought alloy (AMS5662).

The effect of the surface roughness, the heat treatment and the chemical compositions on the oxide scale adhesion will be discussed.

Materials and experimental procedures

The alloy 718 samples were produced using laser beam melting (LBM, EOS M290) and electron beam melting (EBM, Arcam A2X). They were produced at Safran (Safran Additive Manufacturing, Magny-les-Hameaux, France) with the machine manufacturer's standard settings, respectively EOS TM and ARCAM TM . The alloys were studied in their as-built form and compared to those heat-treated. The heat-treatment of LBM samples consisted of a homogenization treatment under vacuum at 1095 °C for 1 h, followed by natural cooling in the cold section of the furnace until they reached room temperature. The p O 2 was estimated to be below 0.01 mbar during this heat treatment. Then followed a precipitation hardening treatment at 760 °C during 5 h under vacuum, followed by 1 °C/min cooling rate in order to reach 650 °C and held 1 h, then finally cooled in the chamber until they reached room temperature. The AM samples were compared to the heat-treated wrought (WRG) alloy 718 (AMS 5662) provided by Aubert et Duval (Ancize, France), their 718 ingots were hot rolled.

A more detailed description of these alloys can be found in a previous paper [START_REF] Sanviemvongsak | High temperature oxidation of IN 718 manufactured by laser beam melting and electron beam melting: Effect of surface topography[END_REF], which contains the characterization of their chemical composition, microstructure and the description of the AM parameters used to build these samples. Their dimensions approached 15 × 10 × 2 mm. Some samples surfaces were ground using P600-SiC paper and others were studied with their raw surfaces. To remove the initial roughness, the grinding needed to remove less than 150 ± 40 μm from the raw LBM sample and less than 600 ± 60 μm deep from the raw EBM sample. After grinding, samples were ultrasonically cleaned in acetone and ethanol, then dried.

The initial chemical compositions of the three alloys were quantified using an electron probe microanalyser (EPMA, SX Five FE CAMECA) calibrated on standards at 7 kV and 20 nA.

Moreover, the concentration of minor and impurity elements (Y, La, Ce, Re, Co, Si, B, P, S, Ta and Mn) were measured using glow discharge mass spectrometry (GDMS, Astrum Nu Ametek ® ). This measurement has 20 % relative uncertainty regardless of the nature of the analysed element and its quantity. The concentration of carbon was measured with instrumental gas analyser (IGA, Horiba EMIA 820V) with a measurement uncertainty of around 2% (for 500 ppm wt. of C). The grain size evaluations were performed using a scanning electron microscope (JEOL JSM-7100TTLS LV) equipped with an EBSD camera (NordlysNano, Oxford Instruments).

The cyclic air oxidation tests were conducted in a vertical cyclic oxidation rig at the CIRIMAT laboratory (Toulouse, France). The samples were hanged with FeCrAlY hooks on FeCrAl ODS plates. Hooks were preoxidised 1 h at 1100 °C in order to form a very adherent and slow growing alumina scale. The temperature was set using a type-K thermocouple welded to a spare 718 sample. The monitoring of the temperature was assured by a type-S thermocouple located near the samples. Samples underwent cycles with 1 h hot dwell at 900 °C in low velocity laboratory air (vertical partially opened tube) and 15 min of cooling under pulsed lab air flow. The temperature of all the samples reached the room temperature in less than 10 min. All samples were weighed with the Sartorius ME215P balance with ± 20 µg accuracy.

Results and discussions

Oxidation kinetics

The cyclic oxidation tests were conducted at 900 °C under laboratory air. They were performed to evaluate the effect of manufacturing processes, surface roughness and heat treatment on the oxidation resistance of the alloy 718 produced by LBM, EBM and wrought process. Results are shown in Figure 1. In contrast to the isothermal and cyclic oxidation tests conducted at 850 °C in a previous paper [START_REF] Sanviemvongsak | High temperature oxidation of IN 718 manufactured by laser beam melting and electron beam melting: Effect of surface topography[END_REF][START_REF] Sanviemvongsak | Oxydation et corrosion à haute température de superalliages à base de nickel issus de la fabrication additive[END_REF] the cyclic oxidation tests at 900 °C show large differences between these three 718 alloys in terms of mass variation (Δm/s). As seen in Figure 1(a), the LBM samples, whether ground or raw, treated or untreated, show the worst resistance to cyclic oxidation compared to the as-built EBM sample and the heat-treated wrought specimen. As shown in Figure 1(b), the LBM samples with raw surfaces present the highest mass gain; this result is due to a higher surface area as shown previously through a quantitative evaluation [START_REF] Sanviemvongsak | High temperature oxidation of IN 718 manufactured by laser beam melting and electron beam melting: Effect of surface topography[END_REF]. The spallation rate of the oxide layer on LBM samples is very fast after 600 to 900 cycles, compared to 1600 cycles for the EBM sample. On the other hand, the oxide scale on the wrought sample spalled but slower than the AM samples. The low negative slope of the oxidation kinetics of the wrought sample shows that its oxide scale is more adherent than that of the AM samples. The mass gain of all samples, reached a maximum value between 0.7 and 1.0 mg.cm -2 before the spallation of the oxide layer becomes predominant. This kind of mass gain corresponds to a thickness of the oxide scale less than 5 µm. The results on the LBM specimens show weak but consistent effects of ground surfaces and heat treatments. These two parameters postpone the accelerated spallation of the LBM samples by about 100 cycles and even 400 cycles when the samples were ground and heattreated. Cyclic oxidation kinetics were quantified using the "p-k p " kinetic model in its Monte Carlo version. This numerical model, initially elaborated in 1998 [START_REF] Monceau | Monte-Carlo simulation code of cyclic oxidation kinetics[END_REF] was used in several works (e.g. [START_REF] Sureau | Numerical simulation of cyclic oxidation kinetics with automatic fitting of experimental data[END_REF]) and was reprogrammed in Python language for this study [START_REF] Monceau | Python code for Monte Carlo simulation of Cyclic Oxidation Kinetics[END_REF]. This model is similar to the random spall site model version of NASA's COSP [START_REF] Lowell | COSP: a computer model of cyclic oxidation[END_REF]. In this model, the surface is divided in hundreds or thousands of cells. In this version, the oxide growth and spallation on a particular cell do not depend on the neighboring cells. At each time step, corresponding to the duration of a high temperature dwell, the oxide scale growth is calculated as a function of the current oxide scale thickness and of the parabolic constant k p .

Then, the oxide scale spalls with a probability p during cooling or heating. In this version of the model, p and k p do not depend on the local oxide thickness, but they depend on time.

Indeed, the pairs of p-k p values were determined for all oxidised samples by adjusting the It is remarkable to note in Figure 3, that all p-k p values of AM alloys follow the same "path", they certainly degrade in the same way, but not with the same speed. During the reaction, the final metal consumption of AM alloys is 10 times faster than the wrought alloy. The isoconsumption of chromium began at around 4 mg.cm -2 and reached 50 mg.cm -2 for 1000 cycles, which corresponds approximately to a consumption of metal of 60-µm thick for 1000 cycles. Furthermore, it would take a much longer test time at 900 °C to see if the wrought sample follows the same degradation route as the AM samples.

Discussions regarding the adhesion of the oxide scale

Effect of the heat-treatment

The cyclic oxidation kinetics at 900 °C presented in Figure 1 showed that the homogenisation heat-treatment (solution treatment and ageing) delayed the catastrophic scaling of the LBM samples by about 100 cycles. The effect is not excessive, but this result was observed on ground samples and those with rough surfaces. According to literature, the homogenisation heat-treatment applied to the alloy 718 produced by AM, leads to a microstructure close to that of the wrought heat-treated alloy. This treatment removes traces of melting pools and microsegregation areas [START_REF] Zhang | Effect of standard heat treatment on the microstructure and mechanical properties of selective laser melting manufactured Inconel 718 superalloy[END_REF]. Calandri et al. [START_REF] Calandri | Solution Treatment Study of Inconel 718 Produced by SLM Additive Technique in View of the Oxidation Resistance[END_REF] showed the benefit of an optimized heattreatment on the redistribution of Nb in the alloy 718 produced by LBM, but they didn't show its effect on the oxidation resistance. Moreover, McDowell et al. showed that an alloy with smaller grains and therefore a higher density of diffusion short-circuit paths has a higher diffusion flux of protective element (Cr) to the surface than an alloy with larger grains [START_REF] Mcdowell | Alloy-grain-size dependence of the effectiveness of silica coatings as oxidation barriers on stainless steel[END_REF].

Therefore, alloys with small grains, such as wrought alloys, would form a protective oxide layer quicker and experience lower mass gains. However, EBSD analyses conducted on oxidised samples presented in Figure 4 and Table 2, show that the ground EBM alloy has larger grains than the LBM and wrought alloys, yet it has the lowest mass gain and presents a better cyclic oxidation resistance than the LBM alloy.

In addition, it has been shown previously [START_REF] Sanviemvongsak | High temperature oxidation of IN 718 manufactured by laser beam melting and electron beam melting: Effect of surface topography[END_REF][START_REF] Sanviemvongsak | Intergranular oxidation of Nibase alloy 718 with a focus on additive manufacturing[END_REF] that even if the AM samples were studied in their as-built forms and compared to the heat-treated wrought sample, the three alloys -EBM, LBM and wrought -had similar oxidation resistances in terms of mass change after 3000 h in air at 850 °C. Their microstructure differences had only a small effect on the morphology of the oxide at the grain boundaries [START_REF] Sanviemvongsak | Intergranular oxidation of Nibase alloy 718 with a focus on additive manufacturing[END_REF]. In the present study, cyclic condition at 900 °C shows that the solution heat-treatment conducted on LBM samples has a small positive effect on the adhesion of the oxide scale, which may be attributed to the homogenization of the microstructure.

Effect of surface defects

Surface defects in AM samples may also be a source of catastrophic spallation. Indeed, sintered powder particles or "spatter" type defects were observed on specimens with rough surfaces in Figure 5(b) and (d). These surface asperities can be removed more easily, particularly if the interface between the sintered solid particle and the alloy is susceptible to oxidation. It has been shown previously [START_REF] Sanviemvongsak | High temperature oxidation of IN 718 manufactured by laser beam melting and electron beam melting: Effect of surface topography[END_REF] that this interface may exhibit rutile-type titanium oxide (TiO 2 ). The growth of this oxide at the particle/surface interface can then cause a greater loss of material. In addition, Evans et al. have shown that asperities generated by the surface topography can lead to deformation of the alloy at the metal/oxide interface during thermal cycling, leading to the initiation of fatigue cracking in the oxide layer [START_REF] Evans | Effect of interface undulations on the thermal fatigue of thin films and scales on metal substrates[END_REF]. Higher surface roughness leads to more areas under compressive stress concentration. However, the oxidation kinetics in Figure 1, shows that the oxide scales of the ground LBM specimens are susceptible to spallation as catastrophic as the spallation rate of the oxide scale developed on the rough-surface specimens, but about 100 cycles later.

Effect of the chemical composition and the chemical segregation at the oxide/metal interface

The chemical segregation, particularly of sulfur, at the interface between metal and oxide scale is known as one of the main causes of spallation of the oxide scales [START_REF] Smialek | Effect of sulfur removal on Al2O3 scale adhesion[END_REF][START_REF] Meier | The effects of reactive element additions and sulfur removal on the adherence of alumina to Ni-and Fe-base alloys[END_REF]. In our case, the oxide scale spallation mainly occurred at the metal/oxide interface as shown with an example in Figure 6. This result was found for the three alloys. Moreover, all samples present internal and intergranular oxidation. As discussed in a previous paper [START_REF] Sanviemvongsak | Intergranular oxidation of Nibase alloy 718 with a focus on additive manufacturing[END_REF], the mass gain corresponding to this internal/intergranular oxidation is small compared to one corresponding to the external oxide scale. This preferential intergranular oxidation was found thicker and sometimes deeper on the AM alloys compared to the wrought one. This difference was mainly attributed to alloy grain sizes, in which case a grain could act as element reservoir [START_REF] Sanviemvongsak | Intergranular oxidation of Nibase alloy 718 with a focus on additive manufacturing[END_REF]. The concentration of minor elements in the alloys of the present study was therefore measured. Results presented in Figure 7 show that the initial Al and Ti content is too high to quantify these elements accurately by GDMS analysis. However, the analyses were still carried out and the values were found to be in good agreement with the WDS analyses in Table 3. The wrought sample has more Co than the AM samples but this low content does not explain the differences in oxidation resistance. The EBM sample shows 0.21 wt.% of Si compared to 0.04 and 0.08 wt.% for the LBM and wrought samples. According to the work of Douglass and Armijo on a Ni-20Cr alloy, 3 wt.% Si is required to form a SiO 2 layer that can act as a diffusion barrier to Cr [START_REF] Douglass | The effect of silicon and manganese on the oxidation mechanism of Ni-20 Cr[END_REF]. However, these authors also found that by forming SiO 2 particles at the metal/oxide interface, the oxide layer could easily spall as found by Romedenne et al. on the AM-Hastelloy X alloys [START_REF] Romedenne | High temperature air oxidation behavior of Hastelloy X processed by Electron Beam Melting (EBM) and Selective Laser Melting (SLM)[END_REF] . But in this last work, the role of sulfur has not been discussed, the AM alloys presented 20 to 24 ppm wt. of sulfur against 0.6 content of Si, it presents the worst cyclic oxidation resistance. Moreover, the EBM sample presents the highest content of Si and presents an intermediate oxidation resistance.

Therefore, the Si content alone does not explain the results. The elements Y, La and Ce are reactive elements, they are beneficial to the cyclic oxidation behaviour of chromia former alloys, but they must be present in larger quantities, typically 0.1-0.5% [START_REF] Pieraggi | Chromia Scale Growth in Alloy Oxidation and the Reactive Element Effect[END_REF][START_REF] Chevalier | Mechanisms involved by reactive elements upon high temperature chromia scale growth[END_REF]. The Re could have a beneficial effect like Pt [START_REF] Tien | Mechanism of oxide adherence on Fe-25Cr-4Al (Y or Sc) Alloy[END_REF], but it would certainly need several percent. Here, the concentration in Re is likely too low to explain the differences in cyclic oxidation kinetics.

Boron and phosphorus are known to segregate at grain boundaries of alloy 718. A content of boundaries of the wrought alloy and would limit the growth of oxides along them [START_REF] Sun | Effects of phosphorus on the δ-Ni3Nb phase precipitation and the stress rupture properties in alloy 718[END_REF]. The can segregate strongly at the metal/oxide interface with very low concentrations in the bulk.

an alumina-forming superalloy [START_REF] Smialek | Effect of sulfur removal on Al2O3 scale adhesion[END_REF]. The AM samples present 30 times higher contents of sulfur compared to the wrought alloy. This may be the main cause of the fast spallation of their oxide scale. In addition, Mn is known to trap sulfur in the matrix by forming MnS [START_REF] Robinson | Liquation cracking during the welding of austenitic stainless steels and nickel alloys[END_REF].

The high Mn contents of the wrought sample and the EBM sample could explain the better cyclic oxidation resistance at 900 °C of these two alloys compared to the LBM alloy. Similar hypothesis of the interplay between sulfur and manganese for a chromia forming alloy was tested previously with stainless steel 304 [START_REF] Pascal | Breakaway oxidation of austenitic stainless steels induced by alloyed sulphur[END_REF]. Sulfur can segregate at the metal/oxide interface and affect the adhesion of the oxide layer, for both alumina former alloys [START_REF] Smialek | The effect of sulfur on Al2O3 scale adhesion[END_REF] and chromia former alloys [START_REF] Lees | On the reasons for the effects of dispersions of stable oxides and additions of reactive elements on the adhesion and growth-mechanisms of chromia and alumina scales-the "sulfur effect[END_REF][START_REF] Grabke | Segregation of sulfur during growth of oxide scales[END_REF]. This result is very well known for alumina-forming alloys [START_REF] Smialek | The effect of sulfur on Al2O3 scale adhesion[END_REF][START_REF] Lees | On the reasons for the effects of dispersions of stable oxides and additions of reactive elements on the adhesion and growth-mechanisms of chromia and alumina scales-the "sulfur effect[END_REF][START_REF] Grabke | Segregation of sulfur during growth of oxide scales[END_REF][START_REF] Hou | Sulfur segregation to Al2O3 interfaces studied by Field Emission-Auger Electron Spectroscopy[END_REF]. With these alloys, even lower amounts of sulfur (between 0.1 and 1 wt ppm) were shown to affect the spallation behaviour. The segregated sulfur is in equilibrium with the sulfur in solution in the γ-Ni phase. This sulfur in solution in the γ-Ni phase is called here the "free" sulfur, to differentiate it from the sulfur "trapped" in sulfides. The quantity of free sulfur in the 718 alloys oxidised at 900°C was determined using thermodynamic calculations. In earlier works, Meier, Pettit and Smialek used the concept of the solubility product to discuss the effect of Hf and Y on the trapping of sulfur [START_REF] Meier | The effects of reactive element additions and sulfur removal on the adherence of alumina to Ni-and Fe-base alloys[END_REF]. In the present work, this approach was extended with calculations considering the global concentrations of the main alloying elements (Ni-Cr-Fe-Al-Ti-Nb-Mo), which were quantified by microprobe analyses, as well as the global concentrations of S and Mn, which were quantified by GDMS, Table 3. These calculations were carried out using the Thermocalc TM software, with the Thermo-Calc NI28 Ni-based alloys database (accessed on February, 14 th 2020) which is under development and now includes S. The results are presented in Table 4.

First, the calculation of the equilibrium phases at 900 °C allows to determine the most stable sulfides as well as the amount of S remaining in solution in γ-Ni. From these results, the quantity of S trapped in the sulphides was determined in each alloy.

the oxide/metal interface. Thermodynamic calculations (using Thermocalc TM software) were carried out considering all the sulfides described in the Thermo-Calc NI28 Ni-based alloys database (accessed on February, 14 th 2020) and set as Condition 1 in Table 4. The Alabandite (Cr,Fe,Mn)S (NaCl, cF8, Fm3m) and Pyrrhotite (Al,Co,Cr,Fe,Mn,Nb,Ti)S type sulfides were considered.

However, calculation results showed that only Pyrrhotite type sulfides are expected to be present. Under these conditions, the amount of S in the γ-Ni phase of the three alloys reaches the surface with a surface coverage level around 10% for LBM and wrought alloys, compared to 6.8% for the EBM alloy. However, if we assume that sulfur is responsible for the catastrophic spalling observed during thermal cycling, these results do not correspond to the mass variation kinetics shown in Figure 1. Indeed, the wrought sample should present less free sulfur in γ-Ni, and therefore the lowest quantity of segregated S at the metal/oxide interface, which is not the case.

Consequently, new calculations were performed, considering only Alabandite (Cr,Fe,Mn)S (NaCl, cF8, Fm3m) sulfides (Condition 2, from Table 4). Under these conditions, results

show quantities of free S in γ-Ni consistent with the kinetic oxidation results. Indeed, the LBM sample spalled faster than the two other samples, and according to the thermodynamic calculations, it is also the one with the highest S segregation (ϴ = 54%). The wrought sample, which spalled much slower than the other two alloys, has the lowest S segregation rate (ϴ = 28%). The EBM sample, which exhibits intermediate oxidation kinetic, has intermediate segregation rate (ϴ = 31%).

In order to validate our hypothesis that S segregation occurs from S in solution in the γ-Ni phase, without affecting the volume fractions in sulfides, the amount of S segregated at the surface was evaluated and compared to the amount of S present in the γ-Ni phase. Assuming a sample thickness of 1 mm and a 50% coverage rate of an S layer of half Ni unit cell parameter, set as 0.352/2 nm thickness. The proportion of S that segregates was found very low (Table 4). Thus, the segregation of S does not lead to the depletion of the S concentration in γ-Ni, and thus does not lead to the dissolution of sulfur-trapping precipitates.

To summarize, 77 to 99.9% of the sulfur present in the alloy is trapped in the sulfides. The remaining quantities are in solution in γ and a very small proportion (≈ 0.02 to 0.05%) is segregated at the metal/oxide interface. In [S] at in solution at equilibrium for a given initial concentration of Mn. This effect is illustrated in Figure 9 which presents the S coverage level at the surface of the γ-phase of alloy 718, as a function of the total concentration of Mn in the alloy. On this figure, it can be seen that the S segregation which is a direct function of the concentration of S in solution in γphase decreases with increasing levels of Mn. It can be seen also that the S segregation level is similar for two very different total concentrations of S in the alloys. These calculations show that Mn controls the level of S segregation at the surface of the gamma phase.

The results of calculations under Condition 2 (alabandite only), show a significant segregation of S at the surface (28 to 54% of an atomic half-monolayer). For comparison, Hou and Moskito measured S contents at the metal/oxide interface of 4 at% or close to half to a bulk [START_REF] Hou | Sulfur segregation to Al2O3 interfaces studied by Field Emission-Auger Electron Spectroscopy[END_REF]. Here, the determined S segregations are certainly sufficient to embrittle the metal/oxide interface and cause premature spallation of the 718 LBM samples. Calculations performed considering only Alabandite-type sulfides are consistent with the oxidation kinetics in Figure 1, nevertheless the reality could be more complicated as discussed below. However, these sulfides are nanometric, they are difficult to observe and their crystallographic structures have not been identified. According to the literature, the alloy 718 can also present carbosulfides like (Ti,Nb) 2 CS type which have the same crystallographic nature as the Pyrrhotite (P63/mmc) type sulphides [START_REF] Vincent | Precipitation around welds in the nickel-base superalloy[END_REF][START_REF] Vernier | Flash oxidation tests to evidence the segregation of boron at the grain boundaries of superalloy Inconel 718[END_REF]. Thus, additional calculations taking into account the carbon content would be necessary to go further. It is also possible that the database for sulfides is not accurate enough or for a nucleation reason, Alabandite type sulfides are present and not Pyrrhotite type sulfides. Smaller scale chemical and microstructural characterizations using transmission electronic microscopy are required to identify the precise nature of sulfides or carbosulfides in the γ matrix of 718 alloys. Moreover, it is also possible to verify experimentally whether S is responsible for these catastrophic spalling differences, by performing cyclic oxidation tests on the three alloys after desulphurization under H 2 or by polishing them repeatedly after a few cycles at high temperature [START_REF] Smialek | Effects of hydrogen annealing, sulfur segregation, and diffusion on the cyclic oxidation resistance of superalloys: a review[END_REF]. In the present paper, results showed that the idea that sulfur segregation could be responsible for catastrophic spallation could be applied to a commercial chromia forming alloy, since thermodynamic calculations showed that the amount of sulfur measured in our alloy is sufficient to segregate in large concentrations at the metal/oxide interface despite the formation of sulphides in the bulk.

Conclusions

Previous results regarding the air isothermal oxidation resistance at 850 °C of several 718 alloys, showed that the wrought alloy was only slightly more resistant to oxidation in terms of mass gain and intergranular oxidation compared to the ones produced by LBM and EBM [START_REF] Sanviemvongsak | High temperature oxidation of IN 718 manufactured by laser beam melting and electron beam melting: Effect of surface topography[END_REF] [24]. However, in the present paper, the cyclic air oxidation tests carried out at 900 °C on the same alloys from the same batches, showed large differences in terms of the rate of oxide layer spallation. This is an important result that may limit the use of 718 alloys produced by AM process. The grinding and the heat-treatment delay the catastrophic spallation of the LBM sample by about 100 cycles. The oxide scale of LBM samples spalled much faster than the one found on EBM and wrought samples. The hypothesis that these differences in oxide layer adhesion could be attributed to the sulfur and manganese content in the alloys was discussed in details in this paper. The LBM and EBM samples show sulfur content between

Figure 1 .

 1 Figure 1. Cyclic oxidation kinetics in laboratory air at 900 °C of 718 alloys produced by LBM, EBM and wrought processes (cycles of 1 h at 900 °C and 15 min cooling down to room temperature).

  model parameters in order to reproduce the experimental curves of mass variation. All curves were fitted using three successive pairs of p-k p values as shown in Figure 2. These successive pairs represent different oxidation regimes. Results are presented in Table 1 and these pairs of p-k p values are compared on the performance map [19, 20], Figure 3.

Figure 2 .

 2 Figure 2. Example of cyclic oxidation adjustment made with "p-k p " model in its Monte-Carlo version, three successive pairs of "p-k p "-values were used to fit the cyclic oxidation kinetics of the ground heattreated LBM-718 sample oxidised in laboratory air at 900 °C.

Figure 3 .

 3 Figure 3. "p-k p " performance map of the alloys 718 produced by LBM, EBM and wrought oxidised under laboratory air at 900 °C (1 h at 900 °C and 15 min cooling).

Figure 4 .

 4 Figure 4. EBSD analyses of grain size distribution of the three alloys after oxidation, observed in the (XY) plan perpendicular to the built direction of AM samples (size of the analysed area under the oxide scale: 1500 × 130 µm²).

Figure 5 .

 5 Figure 5. SEM-SE micrographs of the ground (P600) surface LBM specimen on the (XZ) plane after (a) 500 cycles and (c) 1438 cycles, and of the raw surface of LBM specimen after (b) 500 cycles and (d) 1183 cycles, all samples were exposed to cyclic air oxidation at 900 °C.

Figure 6 .

 6 Figure 6. SEM micrographs of the oxide scale spallation occurring at the metal/oxide interface of the alloy 718 produced by LBM (as-built and ground) after 500 air oxidation cycles at 900 °C. Similar results were found on EBM and wrought alloys.

Figure 7 .

 7 Figure 7. GDMS analyses of minor elements and IGA analyses of the carbon element present in the various alloys 718.

Figure 8 .

 8 Figure 8. Sulfur surface coverage level on the first atomic layer of a nickel surface as a function of the concentration of free S in the volume, calculations conducted at 900 °C.

Figure 9 .

 9 Figure 9. Sulfur coverage level at the surface of the gamma phase of alloy 718 as a function of the concentration of Mn in the alloy. Calculations conducted at 900 °C (Condition 2, see text).

Table 1 .

 1 Pairs of p-k p values determined for cyclic oxidation kinetics oxidised under laboratory air at 900 °C of alloys 718 produced by LBM, EBM and wrought (1 h at 900 °C and 15 min of cooling).

	Nb cycles	k p (mg².cm -4 .s -1 )	p (%)

Table 2 .

 2 Average grain sizes of LBM, EBM and wrought alloys, extracted from the EBSD analyses.

		LBM	EBM	Wrought
	Nb of grains	2248	1409	5379
	Avg. grain sizes	10.3 µm	17.3 µm	9.0 µm
	2σ	10.4 µm	11.1 µm	5.3 µm

Table 3 .

 3 Chemical compositions of the different 718 alloys (EPMA and GDMS) used as input data for Thermocalc TM calculations.

	718 alloys				EPMA-WDS (%)			IGA (ppm)	GDMS (ppm)
			Ni	Cr	Fe	Al	Ti	Nb	Mo	C	Mn	S
		wt.	bal.	20.1	19.2	0.4	1.0	5.0	2.9	280	51	23
	LBM	at.	bal.	22.3	19.9	0.9	1.2	3.1	1.7	1341	53	41
		wt.	bal.	21.3	19.4	0.5	1.1	5.0	2.9	250	380	36
	EBM	at.	bal.	23.1	19.6	1.1	1.3	3.0	1.7	1175	391	63
		wt.	bal.	20.4	18.3	0.4	0.9	4.0	2.7	430	810	0.85
	Wrought	at.	bal.	22.9	19.1	0.9	1.1	2.5	1.6	2078	860	1.55
	3.2.4. Determination of the quantity of sulfur at the metal/oxide interface

Table 4

 4

		, it
	is noticeable
	that	the
	concentration
	of sulfur in
	solution in the
	γ-phase	is
	similar for the
	three alloys,
	much higher than the S concentration, the solubility product (K S = [Mn].[S]) sets the value of

Considered phases (Thermocalc TM ) Samples Obtained phases (fraction vol.) (Thermocalc TM ) Total amount of S (GDMS) (at. fraction) S in γ (at. fraction) (Thermocalc TM ) Percentage of S trapped in sulphides Segregation rate ϴ eq. (2) Percentage of S segregated at the surface

	Conditions 2 :	γ (FCC_L12#1) ;	δ (Ni3TA_D0A) ;	(Cr,Fe,Mn)S	(ALABANDITE)	LBM EBM Wrought	γ (FCC_L12#1): γ (FCC_L12#1): γ (FCC_L12#1):	8.74 × 10 -1 8.67 × 10 -1 9.11 × 10 -1	(Ni3TA_D0A): δ (Ni3TA_D0A): δ δ (Ni3TA_D0A):	1.26 × 10 -1 1.33 × 10 -1 8.92 × 10 -2	ALABANDITE: ALABANDITE: ALABANDITE:	1.25 × 10 -4 1.96 × 10 -4 4.65 × 10 -6	4.16 × 10 -5 6.51 × 10 -5 1.53 × 10 -6	1.16 × 10 -6 4.39 × 10 -7 3.81 × 10 -7	97.5 99.4 77.2	5.4 × 10 -1 3.1 × 10 -1 2.8 × 10 -1	0.011 0.017 0.018
	Conditions 1 :	γ (FCC_L12#1) ;	δ (Ni3TA_D0A) ;	(Al,Co,Cr,Fe,Mn,Nb,Ti)S (PYRRHOTITE)	(Cr,Fe,Mn)S (ALABANDITE)	LBM EBM Wrought	γ (FCC_L12#1): γ (FCC_L12#1): γ (FCC_L12#1):	8.73 × 10 -1 8.67 × 10 -1 9.11 × 10 -1	δ (Ni3TA_D0A): δ (Ni3TA_D0A): δ (Ni3TA_D0A):	1.27 × 10 -1 1.33 × 10 -1 8.92 × 10 -2	PYRRHOTITE: PYRRHOTITE: PYRRHOTITE:	1.11 × 10 -4 1.73 × 10 -4 3.95 × 10 -6	4.16 × 10 -5 6.51 × 10 -5 1.53 × 10 -6	1.09 × 10 -7 7.23 × 10 -8 1.17 × 10 -7	99.8 99.9 93.0	1.0 × 10 -1 6.9 × 10 -2 1.1× 10 -1	0.022 0.022 0.021

Table 4 .

 4 Determination of phase volume fractions and the quantity of sulfur in γ-Ni According to Sun et al., cast 718 alloy may exhibit sulfides rich in Nb, Ti and Ni [41].

	using Thermo-Calc NI28 Ni-based alloys database

Acknowledgements

The authors thank Arnaud Proietti and Sophie Gouy, both from the Raimond Castaing Microanalysis Centre (Toulouse, France). We thank them for their analyses and constructive discussions for the EBSD results and EPMA-WDS results. We also thank Boris Albouy and Jérémy Berot-Lartigue, from Eurofins EAG (Toulouse, France), for their IGA and GDMS analysis.

Subsequently, equilibrium segregation calculations were performed based on the amount of free sulfur in solution in the γ-Ni phase. For this calculation, the segregation of S at grain boundaries was not considered, assuming that the grain boundaries behave like the surface. Furthermore, the enthalpy of S segregation was considered the same in the γ-phase of the alloy 718 and in pure Ni. Moreover, the hypothesis that S segregates at the metal/oxide interface as it segregates at a free surface was made. Therefore, the calculated segregation values are certainly overestimated, but these calculations allow comparison of the three alloys.

The phenomenon of equilibrium segregation can be approximated to chemisorption with a lowering of the surface energy. It can therefore be calculated with the Langmuir-McLean relationship for a binary system. The surface coverage level ϴ corresponding to the segregation of S at the surface is defined as the concentration of S at the surface divided by the maximum concentration at saturation of the surface atomic layer. It is assumed here that 50% of the surface sites are occupied by the S at saturation (ϴ = 1) [START_REF] Miyahara | Sulfur segregation on nickel[END_REF]. The surface coverage level is determined from the Langmuir-McLean adsorption parameter (β p ) presented in Equations ( 1) and (2), with the concentration of free S in the nickel volume and ΔG' the free enthalpy of sulfur segregation (-135 kJ.mol-1) [START_REF] Miyahara | Sulfur segregation on nickel[END_REF].

(2)

From Equations ( 1) and (2), the surface coverage level ϴ was determined based on the atomic fraction of S in solution in γ-Ni. These calculations allow determination of the minimum sulfur concentration at which S starts to segregate significantly at the surface.

sufficient to have a significant amount (1%) of S on the surface, and to have a possible consequence on the adhesion of the oxide layer. The AM alloys contain much more S, only a very small proportion of the S in alloy 718 needs to be in solution in the gamma phase in order to have a significant segregation and to generate oxide spallation. In our case, the AM by the thermodynamic calculations using a new Calphad-type database, would limit the segregation of the latter at the metal/oxide interface. However, the presence of these sulfides and possibly the presence of carbosulfides, both remain to be confirmed by high resolution microscopies and nano-scale chemical analyses.

Declaration of 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.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: