X Sauvage

A Dédé

A Cabello Muñoz

B Huneau

Precipitate stability and recrystallisation in the weld nuggets of friction stir welded Al-Mg-Si and Al-Mg-Sc alloys

Keywords: friction stir welding, aluminium alloys, recrystallisation, precipitates, three-dimensional atom probe -----

Two different precipitate hardening aluminium alloys processed by friction stir welding were investigated. The microstructure and the hardness of the as delivered materials were compared to that of the weld nugget. Transmission electron microscopy observations combined with three-dimensional atom probe analyses clearly show that β'' precipitates dissolved in the nugget of the Al-Mg-Si giving rise to some supersaturated solid solution. It is shown that the dramatic softening of the weld could be partly recovered by post-welding ageing treatments. In the Al-Mg-Sc alloy, Al 3 Sc precipitate size and density are unchanged in the nugget comparing to the base metal. These precipitates strongly reduce the boundary mobility of recrystallised grains, leading to a grain size in the nugget much smaller than in the Al-Mg-Si alloy. Both coherent and incoherent precipitates were detected. This feature may indicate that a combination of continuous and discontinuous recrystallisation occurred in the weld nugget.

Introduction

The Friction Stir Welding (FSW) was introduced for the first time in 1991 by The Welding Institute (TWI) [1]. It is nowadays a well known welding technique and it has been widely investigated for numerous alloys (for a review, see reference [2]). This is a solid state process based on plastic deformation. A rotating tool made of a pin and a shoulder, travels along the abutting edges of plates that have to be joined. The joint is produced thanks to the plastic flow of the material induced by the friction between the tool and the work piece. The heat generated by the friction and the plastic deformation significantly softens the material and promotes an efficient stirring. In the weld zone, the original microstructure of the work piece is affected both by heating and plastic deformation. Three different regions are usually reported [2]: the Heat Affected Zone (HAZ) where the material has undergone a thermal cycle, the Thermo-Mechanically Affected Zone (TMAZ) where the material has been plastically deformed under the heat flux and the nugget where the plastic deformation and the temperature were the higher leading to some dynamic recrystallisation of the microstructure. Most of FSW published data so far are related to aluminium alloys. The first reason is that the FSW technique is well adapted for these alloys, and especially for the 2XXX and 7XXX series that exhibit inappropriate solidification microstructures if fusion welded [2]. The second reason is that aluminium alloys are relatively soft and the wear of the tool is not a critical issue for applications. However, most of high strength aluminium alloys are obtained thanks to a fine dispersion of nanoscaled precipitates that is usually heat sensitive. Although the heating cycle is rather short during FSW (typically less than one minute [3]), the temperature peak is sufficiently high (up to 0.8 the melting temperature [3]) to induce some transformation of meta-stable precipitates, coarsening or even their full decomposition in the weld nugget [4][5][6][START_REF] Genevois | Doisneau-cottignies[END_REF][8][9][10]. Both the rotation speed and the feed rate of the tool are some key parameters, because they respectively affect the peak temperature and the heating time. These microstructural evolutions control the mechanical properties of the weld and usually give rise to a drop in hardness and sometime to complex gradients [5][6][START_REF] Genevois | Doisneau-cottignies[END_REF]10]. While the transformations of the microstructure in the HAZ and in the TMAZ are often well understood, the situation in the nugget is more complex. This region is indeed subjected to an extremely intense plastic flow at a high temperature leading to some dynamic recrystallisation (DRX). Since precipitates have some strong interactions with dislocations and grain boundaries, they may affect the driving force (dislocation density) and the kinetics (defects mobility) of recrystallisation. The aim of the present study was to compare the influence of "easy to dissolve precipitates" versus "temperature stable precipitates" on microstructural evolutions and mechanical properties of the FSW weld nugget. Two alloys welded in close conditions were investigated: an Al-Mg-Si alloy containing a high density of β'' meta-stable nanoscaled precipitates and an Al-Mg-Sc alloy containing a high density of nanoscaled Al 3 Sc precipitates. Microstructures in the weld nugget were characterised by Transmission Electron Microscopy (TEM) and Three-dimensional Atom Probe (3D-AP). A special emphasis was given on the precipitate evolution, the redistribution of alloying elements, the recrystallisation mechanisms, and the relationships between hardness and microstructural features.

Experimental

Two different aluminum alloys were investigated in the present study. The first alloy, labeled Al-Mg-Si, is a commercial 6061 AA with the following composition (supplier specification in wt.%): 0.8-1.2 Mg, 0.4-0.8 Si, 0.7 Fe, 0.15-0.4 Cu, 0.15 Mn, 0.25 Zn, 0.15 Ti, Al balance. Sheets (thickness 3mm) of this alloy were delivered in the T6 state (solution treated 1h at 803K, water quenched and aged 12h at 433K) and friction stir welded with a hardened steel threaded head pin (rotation speed 400 rpm and travel rate 2 mm.s -1 ). The second alloy, labeled Al-Mg-Sc, has the following nominal composition (wt.%): 4.58 Mg, 0.28 Si, 0.26 Sc, 0.26 Cu, 0.1 Fe, 0.08 Mn, 0.09 Zr, Al balance. This material was delivered in 4mm thick sheets and friction stir welded with a hardened steel threaded head pin (rotation speed 500 rpm and travel rate 1 mm.s -1 ). Vickers microhardness profiles were measured across the welds with a BUEHLER Micromet 2003 machine, using a 200gf load (1.96 N). Measurements were performed on polished samples in the cross section of the sheets. The microstructure of the as received aluminum alloys and of the weld nuggets were characterized by Transmission Electron Microscopy (TEM) and Three-dimensional Atom Probe (3D-AP). TEM samples were prepared in the cross section of the sheets, so that the electron beam was parallel to the pin travelling direction. Discs were cut out with a drilling machine and mechanically thinned down to 100μm. The electron transparency was achieved by electropolishing (TENUPOL 3®) using a 1/3 HNO 3 and 2/3 methanol solution at -30°C at a voltage of 13V. TEM images and selected area electron diffraction patterns (SAED) were performed with a JEOL 2000FX microscope operating at 200kV. 3D-AP samples were also prepared by electropolishing rods (cross section 0.3x0.3mm) that were previously cut with a diamond saw. A standard solution of 5% perchloric acid in 2-butoxyethanol was used at room temperature with a voltage of 15V. 3D-AP analyses were performed at 40K in UHV conditions, with a CAMECA tomographic atom probe detection system (TAP) [12], a pulse repetition rate of 2 kHz and a pulse fraction of 16%.

Results

Hardness measurements through the weld

Microhardness profiles were recorded in the cross section of the sheets in the middle of the weld for both FSW aluminium alloys (Fig. 1). The hardness in the base material is higher in the Al-Mg-Si alloy than in the Al-Mg-Sc alloy (115 HV ±5 versus 105 HV ±5). However, the hardness of the Al-Mg-Si alloy is much more depressed in the welding zone than the Al-Mg-Sc (ΔHV = 33 ±5 versus 13 ±5 in the weld nugget). Both profiles look fairly symmetric while Murr, Liu and co-authors reported a "double valley" profile for a similar Al-Mg-Si alloy [10,11] and while Lapasset reported an asymmetric profile for a similar Al-Mg-Sc alloy [13]. These discrepancies may be attributed to different welding parameters.

Microstructures of the Al-Mg-Si alloy 3.2.1 Al-Mg-Si alloy -as delivered state

The Al-Mg-Si alloy was friction stir welded in the T6 state, which corresponds to the peak of hardness of this age hardenable alloy. Such hardening results from the fine dispersion of β'' needle shaped precipitates (C-centred monoclinic) that nucleate and grow along the <001> direction of the fcc lattice of the aluminium matrix [14,15]. In the as delivered Al-Mg-Si alloy, the grain size was measured by optical microscopy (data not shown here) and is in a range of 10 to 80 μm. The low magnification TEM bright field image (Fig. 2(a)) exhibits some elongated dispersoids with a size in a range of 0.1 to 0.5 μm. They are typical aluminium intermetallic phases containing Fe and Mn (measured by Energy Dispersive X-ray Spectroscopy, data not shown here). The high magnification TEM bright field image in a <001> zone axis (Fig. 2(b)) shows needle shaped β'' precipitates aligned along the three <100>Al directions. Their length is in a range of 10 to 50 nm. Such precipitates were analysed by 3D-AP and are exhibited in a 3D volume in the Fig. 3(a). For more clarity, the data set was filtered to display only atoms located in regions containing at least 7 at.% magnesium. The procedure used is the following: around each atom in the reconstructed volume, the Mg concentration is measured in a spherical volume (2 nm diameter). If this value is lower than the 7at.% threshold value, the considered atom is removed from the volume. In the volume displayed in the Fig. 3(a), two needle shaped precipitates are arrowed. They are perpendicular to each other because they are aligned along two different <100>Al directions. The few other Mg rich zones are most probably GP zones that form at the early stage of precipitation during ageing [15]. The composition of these precipitates was estimated thanks to composition profiles such as the one displayed in the Fig. 3(b). They contain 17at.% Mg (± 5%), 13 at.% Si (± 5%) and Al balance. Taking into account the thickness of the sampling volume used to compute the profile, it shows that the diameter of the β'' needles is only in a range of 1 to 2 nm. These data are in agreement with previously published data on a similar alloy [15].

Al-Mg-Si alloy -weld nugget microstructure

In the weld nugget, the grain size of the Al-Mg-Si alloy is in a range of 10 to 20 μm (measured by optical microscopy, data not shown here), indicating that dynamic recrystallisation occurred during the FSW process as expected [8]. The low magnification TEM bright field image clearly shows that large dispersoids are still distributed within the grains (Fig. 4(a)). Their shape and size did not significantly change comparing to the bulk material. The original needle shaped β'' precipitates do not appear on the high magnification bright field image in a <001>Al zone axis (Fig. 4(b)). However, some ellipsoidal shaped precipitates with a mean size of about 10 nm are clearly exhibited. They are not randomly distributed within the matrix but aligned along lines, indicating that presumably some discontinuous precipitation occurred on dislocation cores in the weld nugget during the FSW process. Three-dimensional atom probe analyses were performed in the weld nugget of the Al-Mg-Si alloy. Two kinds of data set were collected. In some region, the distribution of Mg and Si looks homogeneous within the analysed volume (Fig. 5(a)) and the Mg and Si concentration is close to the nominal composition. However, the comparison of the frequency distribution of these alloying elements with a Bernoulli distribution (Fig. 5(b)) indicates that they are not randomly distributed and there is most probably some clustering [16] (statistical χ 2 test, probability of a random distribution is less than 1% for Mg and Si [START_REF] Miller | Atom probe field ion microscopy[END_REF]). In other regions, instead of super saturated solid solutions, Mg and Si rich clusters were found (Fig. 5 (c)). They are bigger than the β'' needles of the as received material and not homogeneously distributed within the matrix, thus they most probably correspond to β' precipitates [8] like the ones observed by TEM (Fig. 4(b)). A composition profile computed across such a precipitate (Fig. 5(d)) shows that they contain much more Si and Mg than β'' needles with a Mg over Si ratio of about 2 (40 ± 10 at.% Mg and 20 ± 10 at.% Si). In conclusion, in the weld nugget of the FSW Al-Mg-Si, some dynamic recrystallisation occurred while the original β'' needle shaped precipitates disappeared. The concomitant formation of super saturated solid solutions and of larger precipitates heterogeneously distributed (presumably β') will be discussed in the next section.

Microsctructures of the Al-Mg-Sc alloy 3.3.1 Al-Mg-Sc alloy -as delivered state

The addition of Sc in aluminium alloys gives rise to the precipitation of the ordered L1 2 Al 3 Sc phase. These precipitates are coherent if their size is below 20 nm and then become semi-coherent during further coarsening [START_REF] Jones | [END_REF]. Despite the relatively low volume fraction of Al 3 Sc resulting from the low solubility of Sc even at high temperature, these precipitates provide a significant strengthening effect [19]. Al 3 Sc precipitates are resistant to coarsening [20] giving rise to some good thermal stability of the microstructure up to 200°C. They are also very effective in stabilizing deformation substructure and in pinning grain boundaries during recrystallisation [21,22]. This stabilization effect of deformation substructures is clearly exhibited in the low magnification TEM bright field image of the as delivered Al-Mg-Sc alloy of the present study (Fig. 6 (a) and (b)). These pictures show elongated low angle boundaries resulting from the cold rolling process (rolling direction is horizontal). The thickness of the domains is in a range of 0.1 to 0.5 μm with a typical length in a range of 1 to 5 μm. To exhibit the small coherent Al 3 Sc precipitates, the TEM foil was orientated in (002)Al zone axis. This way the elastic strain resulting from the small lattice misfit of the precipitates gives rise to the typical coffee bean contrast [21,[START_REF] Hirsch | Electron microscopy of thin crystals[END_REF]. The Al 3 Sc precipitates were also imaged in the dark field mode using the (110)Al 3 Sc superlattice reflection (Fig. 6(c)). Their diameter is in a range of 5 to 20 nm.

Al-Mg-Sc alloy -weld nugget microstructure

The microstructure of the nugget is very different from that of the bulk. Original elongated domains have indeed disappeared and some equiaxed grains are exhibited in the low magnification TEM bright field image (Fig. 7(a)). The grain size is in a range of 1 to 3 μm and the SAED pattern taken with an aperture of 5μm (D1) shows that it consists mostly of high angle grain boundaries. Thus, as reported in the literature for similar alloys [13,[START_REF] Cabello Muñoz | [END_REF], dynamic recrystallisation obviously occurred in the nugget during the FSW process. The SAED pattern taken in a <001>Al zone axis clearly exhibits the superlattice reflections of the Al 3 Sc phase (Fig. 7 (a) -D2), indicating that such ordered and coherent precipitates are still present within some grains. However, in some other grains, incoherent Al 3 Sc precipitates were also detected. Such precipitates are shown in the bright field image in the Fig. 7 (b) and in the dark field image (Fig. 7 (c)) that was taken by selecting the (200) fcc lattice reflection labelled in the corresponding SAED pattern (D3). It is interesting to note that all these precipitates share a common orientation and their size is similar to the original one (Fig. 6 (c)). To check whether some precipitates were dissolved giving rise to some increase of the Sc content in solid solution, three-dimensional atom probe analyses were performed. The data treatment and the 3D reconstruction were done following the procedure described by Forbord and co-authors [25]. A scandium rich precipitate (about 10 nm diameter) was intercepted (Fig. 8 (a)). The concentration profile recorded across this precipitate shows that the solubility of Mg in Al 3 Sc is much lower than in the Al matrix. The measured composition of the precipitate is 20±5 at.% Sc, 3±1 at.% Zr and 0.2±0.1 at.% Mg (Al balance). The substitution of Sc by Zr in such precipitate is well known and has been reported by other authors [19,21]. One should note that the large gradients exhibited at the interface is an artefact resulting from the convolution of the spherical shape of the precipitate with the sampling box that was used to compute the profile. Then, it is worth to note that the scandium concentration in the matrix was below the detection limit of the instrument (200 ppm) indicating that Al 3 Sc precipitates did not dissolved in the weld nugget during the FSW process. These observations are consistent with the numerical estimation of Lapasset and co-authors [13].

Discussion

The weld nugget microstructures and the related properties of both aluminium alloys are deeply transformed by the large level of deformation and the strong increase of the temperature during the FSW process. For both alloys, dynamic recrystallisation occurred, however β'' and Al 3 Sc particles are not equally affected which gives rise to some very different microstructures. While β'' precipitates are completely transformed in the Al-Mg-Si alloy, Al 3 Sc particles resist against coarsening and decomposition in the Al-Mg-Sc alloy. Thus, in this later material the recrystallised grain size is much smaller because of the pinning effect of these particles [START_REF] Jones | [END_REF]. These features lead to some little change of the hardness: 92 ± 5 in the nugget versus 105 ± 5 in the base material (Fig. 1). Indeed, the hardening contribution of precipitates [19] is probably unchanged while the hardening contribution of boundaries (grain boundaries and dislocation cell boundaries) is only slightly depressed because of the fine recrystallised structure (Fig. 6(a) versus Fig. 7 (a)). In the Al-Mg-Si alloy, the situation is very different because β'' precipitates are obviously dissolved in the nugget (Fig. 4 and5). This feature gives rise to some supersaturated solid solutions in the weld nugget (Fig. 5(a)). However, the hardness in this region is higher than that of the solution treated alloy (table 1). This feature could be attributed to a higher dislocation density. Indeed, this parameter seems to have a strong influence because the hardness of the solution treated alloy processed by cold rolling (48% reduction) was measured up to 110 ± 5 HV (table 1). Assuming that most of Mg and Si were in solid solution in the weld nugget, post-welding ageing treatments were performed to try to fully recover the hardness. After 12h at 175°C (corresponding to the T6 treatment of the as delivered Al-Mg-Si alloy), the base metal hardness is stable while in the nugget it increases up to about 100 ± 5 HV (Fig. 9). Thus, contrary to Sato and co-authors who successfully recovered the original hardness in a 6063 aluminium alloy by post FSW ageing [9], the hardness of the as delivered material (115 ± 5 HV) is not reached in the present case. This feature may be attributed to the heterogeneously distributed precipitates that were observed in weld nugget (Fig. 4(b) and Fig. 5(c)). Since these quite large precipitates contain a significant amount of Mg and Si (Fig. 5(d)), they depress indeed the β'' precipitate volume fraction achievable during the post welding ageing treatment. Therefore the original hardness of the T6 treated Al-Mg-Si alloy is not recovered in the weld nugget. This point was confirmed by ageing the cold rolled alloy (during 12h at 175°C also): the balance between recovery softening and precipitation hardening results indeed in a higher hardness than in the as delivered material (table 1). Two mechanisms leading to the formation of these heterogeneously distributed precipitates in the weld nugget could be considered. The first one could be based on heterogeneous precipitation. In the weld nugget, due to the dramatic increase of the temperature, β'' precipitates dissolve. One should note that dislocations shearing and cutting precipitates may promote this decomposition. Then, once the FSW pin has moved away, the temperature decreases which increases the driving force for precipitation. In the deformed material, there is a high density of dislocations and since the nucleation barrier is lowered on such defects, heterogeneous precipitation is favoured. However, while the temperature is decreasing, the atomic mobility becomes lower and lower and the equilibrium volume fraction of precipitate is not reached. One should note that a similar two stages mechanism involving full decomposition of precipitates followed by re-precipitation was proposed by Cabibbo and co-authors for a friction stir welded 6056 aluminium alloy [26]. However, another scenario based on the interaction between coarsening precipitates and dislocations may be proposed. In the weld nugget, the strong increase of the temperature affects both the solubility of Mg and Si and the atomic mobility. The solubility of Mg and Si increases in the Al matrix, and this leads to a high driving force for the decomposition of β'' needle. However, considering the work of Frigaard and co-authors on a 6082 alloy processed by FSW (T6 state, 1500rpm, 5mm/s), the temperature in the weld nugget is in a range of 350 to 480 °C during only two seconds [3]. During this short time, few precipitates may survive and then coarsen during the cooling time [27].

By pinning some dislocations their coarsening rate would be enhanced thanks to pipe diffusion [28] and finally, in the weld nugget these "large" precipitates appear aligned along feeding dislocations in TEM images (Fig. 4).

In the Al-Mg-Si alloy, β'' precipitates are not temperature resistant and rapidly dissolve.

It is reasonable to assume that the dynamic recrystallisation follows this decomposition stage. The grain boundary mobility is probably not significantly affected by the low amount of Mg and Si in solid solution which gives rise to a recrystallised grain size in a range of 10 to 20μm. In the Al-Mg-Sc alloy, Al 3 Sc precipitates are much more stable (Fig. 7 and8) and could effectively pin grain boundaries [START_REF] Jones | [END_REF]. This feature gives rise to a small recrystallised grain size in a range of only 1 to 3 μm. Aluminium alloys usually undergo discontinuous dynamic recrystallisation (DDRX) because of their high rate of recovery due to the high stacking fault energy [29]. However several authors have suggested that continuous dynamic recrystallisation (CDRX) may occur during the FSW process [2,10]. Comparing the microstructure of the weld nugget (Fig. 7) and of the as delivered Al-Mg-Sc alloy (Fig. 6), it seems unlikely that the recrystallised grains result from the rotation of the original elongated subgrains. It is important to note also that some incoherent Al 3 Sc particles sharing a common orientation were detected in a recristallised grain (Fig. 7 (b) and (c)). Such a situation is only compatible with a DDRX mechanism: during the growth of a recrystallised nucleus, Al 3 Sc particles are absorbed and do not rotate so that they keep the original misorientation between the nucleus and the parent grain. Jones and co-authors have reported a very different mechanism to account for the passage of grain boundaries through Al 3 Sc precipitates [START_REF] Jones | [END_REF], but one should note that in the present alloy the precipitate diameter is one order of magnitude smaller. Anyway, in the weld nugget, some coherent Al 3 Sc particles were also detected (Fig. 7 (a)) and there are only two possible scenarios to account for such a situation. Either in some specific situations Al 3 Sc particles can accommodate the misorientation through a cooperative rotation to minimize the interfacial energy, or locally some CDRX also occurs. This later assumption would mean that the recrystallised structure of the weld nugget results from a combination of CDRX and DDRX.

Conclusions

The effect of the friction stir welding process on the microstructure of two precipitate hardening aluminum alloys has been evaluated and the following conclusions can be drawn:

i) In the Al-Mg-Si alloy, β'' precipitates are unstable against the temperature increase in the weld nugget during the FSW process. These precipitates dissolve giving rise to some Mg and Si supersaturated solid solutions. Locally, β' precipitates aligned along dislocations were also observed. This feature is attributed whether to some heterogeneous precipitation or the coarsening of β'' precipitates that survived the decomposition stage.

ii) The decomposition of β'' precipitates gives rise to a dramatic loss of hardness in the weld nugget. However, the original hardness could be partly recovered thanks to postwelding ageing treatments. This hardness recovery is attributed to the decomposition of the Mg and Si supersaturated solid solutions and the re-precipitation of β'' precipitates.

iii) In the Al-Mg-Sc alloy, Al 3 Sc precipitates are very stable against the temperature increase in the weld nugget during the FSW process. They do not dissolve and no significant coarsening was observed. The loss of hardness in the nugget is therefore much smaller than in the Al-Mg-Si alloy.

iv) The Al 3 Sc precipitates strongly interact with the grain boundaries of recrystallised grains in the nugget and inhibit grain growth. This feature gives rise to a much smaller grain size in the weld nugget of the Al-Mg-Sc alloy than in the Al-Mg-Si alloy.

v) Both coherent and incoherent Al 3 Sc precipitates were detected in the weld nugget.

Incoherent precipitates indicate that DDRX occurred, and coherent precipitates indicate that whether Al 3 Sc precipitates rotate while crossed by a moving grain boundary or CDRX also occurred.

Figure 1 :

 1 Figure 1 : Microhardness profiles across the weld joints of the AlMgSi alloy and of the AlMgSc alloy. (Retreating side on the left, advancing side on the right).

Figure 2 :

 2 Figure 2: Bright field TEM images of the bulk microstructure of the AlMgSi alloy. (a) Low magnification image showing grain boundaries and large dispersoids. (b) high magnification image showing nanoscaled needle shaped precipitates aligned along <001> directions.

Figure 3 :

 3 Figure 3: Three-dimensional atom probe data set collected in the bulk AlMgSi alloy (15x15x40nm 3 ). (a) Filtered data : only Al, Mg and Si atoms within regions containing at least 7at.% Mg are displayed to exhibit two orthogonal needle shaped precipitates (iso-position procedure). (b) Composition profile computed across a needle shaped precipitate (sampling box thickness is 1 nm).

Figure 4

 4 Figure 4 Bright field TEM images of the weld nugget microstructure of the AlMgSi alloy. (a) Low magnification image showing grain boundaries and large dispersoids. (b) high magnification image showing heterogeneous precipitation of ellipsoid shaped precipitates.

Figure 5 (

 5 Figure 5 (a) Three-dimensional atom probe data set collected in the weld nugget of the AlMgSi alloy (10x10x78 nm 3 ) ; 3D distribution of Mg and Si atoms within the volume (0.75 ± 0.05 at.% Mg, 0.74 ± 0.05 at.% Si) (b) Frequency distribution of Mg and Si (bars) compared to a Bernoulli random distribution (lines). From the χ 2 statistical test [17], the probability of a Mg or a Si random distribution is less than 1%. (c) Another three-dimensional atom probe data set collected in the weld nugget of the AlMgSi alloy (8x8x60 nm 3 ) ; 3D distribution of Mg and Si atoms within the volume (0.80 ± 0.05 at.% Mg, 0.55 ± 0.05 at.% Si) (d) Composition profile computed across a precipitate (sampling box thickness is 1 nm).

Figure 6 :

 6 Figure 6: TEM images of the bulk microstructure of the AlMgSc alloy. (a) Low magnification bright field image showing the typical microstructure of the rolled sheet (rolling direction is horizontal) and corresponding SAED pattern (aperture diameter 5μm). (b) High magnification bright field image (SAED, aperture diameter 0.4 μm) and (c) corresponding dark field image obtained by selecting the (200) lattice reflection and showing nanoscaled precipitates.

Figure 7 :

 7 Figure 7: TEM images of the weld nugget microstructure of the AlMgSc alloy. (a) Low magnification image showing recrystallised grains and two SAED patterns: D1 was obtained with an aperture diameter of 5 mm, D2 was obtained with an aperture of 2μm in the darkly imaged grain in the middle of the image. (b) High magnification bright field image and corresponding SAED pattern (c) dark field image obtained by selecting the (200) lattice reflection and showing diffracting nanoscaled precipitates.

Figure 8 :

 8 Figure 8: Three-dimensional atom probe data set collected in the weld nugget of the AlMgSc alloy (20x20x6 nm 3 ). (a) 3D distribution of Mg and Sc atoms showing partly a spherical Sc rich precipitate. (b) Mg, Sc and Zr concentration profiles computed across the precipitate (sampling box thickness is 1 nm).

Figure 9 :

 9 Figure 9: Microhardness profiles across the weld joint of the AlMgSi alloy subjected to post-welding ageing treatment at 175°C (Retreating side on the left, advancing side on the right).

Table 1 :

 1 Microhardness of the Al-Mg-Si alloy; Homogenised: solution treated during 1h at 535°C, water quenched and natural ageing at room temperature during 20 days; 48% CR: 48% thickness reduction by cold rolling of the homogenised state; 48% CR + ageing : artificial ageing after cold rolling during 12h at 175°C

		As delivered	Weld nugget	Homogenised	48% CR	48%CR + ageing
	HV	115 ±5	82 ±5	71 ±5	110 ±5	124 ±5


Dominique Castex

Sacha Kacki

Demographic Patterns Distinctive of Epidemic Cemeteries in Archaeological Samples

The analysis of biological parameters such as age and sex is particularly relevant to the interpretation of ancient skeletal assemblages related to abrupt mortality crises, and more particularly epidemics. In such a context, the mechanisms of selection within a population or part of a population differ according to the pathogen involved. They may also vary depending on the period and location in which the population lived. Here, we illustrate the specificity of plague mortality through the study of several European burial sites contemporary with the first and second plague pandemics. The paleodemographic patterns obtained for different plague outbreaks from the 6th to the 16th centuries reveal some constant features over time and space as well as some differences that suggest a possible evolution in the epidemiological characteristics of the disease.

Some ancient burial grounds are valuable testimonies of past epidemics. For both funerary archaeologists and paleobiologists, such archaeological sites offer a remarkable research framework to reveal unfamiliar aspects of these historical events, especially those that occurred during periods for which very few or no written sources exist. The multiplicity of congresses, articles, and syntheses on this topic in recent years illustrates the interdisciplinary research that has progressively emerged over the past two decades.

The detection of an ancient mortality crisis generally relies on archaeological evidence such as mass graves-that is, pits where numerous bodies were buried at the same time [START_REF] Castex | Revealing archaeological features linked to mortality increases[END_REF]. However, retaining the hypothesis of an epidemic crisis also requires some paleobiological investigations. First of all, a careful examination of the skeletal remains must be undertaken to make sure that they do not exhibit any traumatic lesions evocative of violent death. Such stigmata would be expected in the case of mass graves related to war or massacre, so that their absence favors the epidemic hypothesis. However, a paleopathogical study cannot demonstrate an epidemic context because the highly virulent pathogens involved kill individuals before any skeletal lesion can develop. Other methods of analysis are therefore needed to ascertain the precise nature of an epidemic. Besides paleomicrobiological studies, analysis of the age and sex composition of skeletal assemblages can provide some valuable arguments. Indeed, the ageand sex-specific mortality rates within a population or part of a population at a given time and place may differ depending on the pathogen involved. The study of these biological parameters is particularly relevant to a discussion of the cause of an epidemic mortality crisis.

The scope of this article is voluntarily limited to the demographic analysis of burial grounds related to plague epidemics. Through the study of several of these sites, we illustrate the specificities of mortality by age and sex in such a context. We also discuss some demographic differences between several epidemic outbreaks.

MATERIALS AND METHODS

This work is based on a study of seven European burial sites of plague victims (Fig. 1). With the exception of the Hereford site, all of these burial grounds were used exclusively to bury the victims of plague epidemics. We therefore included in the analysis every skeleton recovered, whether from individual burials or multiple burials. For the Hereford cemetery, which was also used out of the epidemic context over a long period, we studied only the skeletons recovered from three multiple burials, which were proved to be the skeletons of the victims of a plague epidemic.

Two of the sites investigated (Sens and Poitiers) are contemporaneous with the first plague pandemic, known as Justinian's plague. This pandemic began in the 6th century and continued intermittently in Europe until the mid-8th century. Three other sites (Dreux, Hereford, and Barcelona) are contemporaneous with the Black Death-that is, the first outbreak of the second plague pandemic in the mid-14th century. Plague then waxed and waned in Europe until the late 18th century, and the last two sites we analyzed (Les Fédons and Dendermonde) are related to resurgences of the disease at the end of the 16th century. For every one of these archaeological burial grounds (Fig. 2), the cause of death is attested to by historical sources (Poitiers, Dendermonde) or by DNAanalysis (Sens, Dreux, Hereford, Barcelona, Les Fédons).

The first step of the study of each site was to estimate the age at death and determine the sex of all the exhumed individuals by using reliable skeletal indicators (for a critical review of current methods and their application, see reference 8). Estimation of the age at death of non-adults was based primarily on the degree of dental maturation [START_REF] Moorrees | a. Age variation of formation stages for ten permanent teeth[END_REF][START_REF] Moorrees | Formation and resorption of three deciduous teeth in children[END_REF][START_REF] Ubelaker | Human Skeletal Remains: Excavation, Analysis, Interpretation[END_REF]. When teeth were absent or fully mineralized, we additionally considered the diaphyseal length of the long bones [START_REF] Maresh | Measurements from roentgenograms[END_REF] and the stage of fusion of secondary ossification enters [START_REF] Scheuer | Developmental Juvenile Osteology[END_REF][START_REF] Coqueugniot | Brief communication: a probabilistic approach to age estimation from infracranial sequences of maturation[END_REF]. Sex determination was carried out only for adult individuals, with reliance primarily on the morphoscopic and morphometric characteristics of the hip bones [START_REF] Bruzek | A method for visual determination of sex, using the human hip bone[END_REF][START_REF] Murail | DSP: a tool for probabilistic sex diagnosis using worldwide variability in hip-bone measurements[END_REF]. In order to reduce the number of individuals whose sex possible, a secondary sex determination based on the extrapelvic dimorphism of each skeletal assemblage [START_REF] Murail | A new approach to sexual diagnosis in past populations. Practical adjustments from Van Vark's procedure[END_REF]. These data were then used to define the composition of the population as well as possible. The mortality profile and the rate of masculinity (i.e., ratio of the number of men to the number of men and women) of each skeletal assemblage were critically discussed within the framework of a comparative analysis, with reliance on historical data and various standard life tables. Data were compared with an archaic/pre-Jennerian mortality pattern, with the variability of mortality and the confidence intervals of the probabilities of death taken into account. The model life tables of Ledermann [START_REF] Ledermann | Nouvelles tables-types de mortalité[END_REF] were used to make the comparisons. Such an approach has an objective distinct from those of traditional paleodemographic analyses, as it does not aim to determine the demographic characteristics of living populations (e.g., life expectancy at birth, average age at death) but rather to detect anomalous demographic patterns that can be interpreted as evidence of burial selection or abnormal mortality [START_REF] Castex | Identification and interpretation of historical cemeteries linked to epidemics[END_REF]. If one assumes that the reference pattern(s) reflect a "natural" demography (i.e., deaths over a long period of time with no majormortality crises), the discrepancies between the reference and the paleodemographic data can reflect change in mortality parameters due to a peculiar event.

Given the poor accuracy of the methods of estimating adult age from the skeleton (8), mortality profiles were analyzed only for immature subjects. As in the model life tables used for comparison, the immature individuals were distributed into 5-year groups (apart from the first two groups, which were 1and 4-year groups) of attained age. The mortality profile of the subjects from each site was then established by calculating the mortality quotients (Table 1) and was compared with the theoretical mortality pattern [START_REF] Ledermann | Nouvelles tables-types de mortalité[END_REF]. The mortality quotient takes the form aQx, where x is the age of entry into an age group and a is the duration in years of the age group. aQx is the ratio of the number of people in an age group who die to the number of people alive at the start of that age group. It thus represents the probability of dying within a precise period of life. It differs from the rate of death, which is the proportion of deaths within an average population. The mortality quotient is more pertinent for comparing a theoretical natural mortality with the mortality profile obtained from an archaeological sample. The life tables of Ledermann [START_REF] Ledermann | Nouvelles tables-types de mortalité[END_REF] allow calculation of the 95% confidence interval of the mortality quotients (illustrated by a range of values in the graphs). The model life tables used in the current study are for a life expectancy at birth ranging from 25 to 35 years; these parameters lie between the 20-and 40-year life expectancy range that characterizes known pre-Jennerian populations [START_REF] Masset | La mortalité préhistorique[END_REF][START_REF] Sellier | Lamise en évidence d'anomalies démographiques et leur interprétation: population, recrutement et pratiques funéraires du tumulus de Courtesoult[END_REF].

Rates of masculinity were calculated only for adults, as there is currently no reliable method of sex determination for sub-adults. Values were compared with a theoretical rate of 50%.

DEMOGRAPHIC CHARACTERISTICS OF THE PLAGUE BURIAL SITES

The results described below point out the demographic characteristics of each site. Comparisons among the sites reveal similarities in mortality profiles specific for plague mortality, as well as some inter-site differences.

Proportion of Immature Individuals

According to the model life tables of Ledermann [START_REF] Ledermann | Nouvelles tables-types de mortalité[END_REF], the proportion of immature individuals in an archaic population theoretically ranges from 45% to 64%, for a life expectancy at birth of between 25 and 35 years. The values in the skeletal assemblages show some variation(Fig. 3). For three of the sites, the proportions recorded are compatible with the one expected in a context of natural mortality. For the four other sites, the values are slightly above the theoretical range. There is a small deficit of subjects who died before 20 years of age.

At first sight, the proportion of non-adults appears more important over time, from the first wave of Justinian's plague in the 6th century to the resurgences of the second pandemic in the 16th century. However, two sites from the 14th century (Hereford and Barcelona) differ from this general pattern. The differences in the proportion of immature individuals for the same time period may reflect geographical variations in the epidemiological characteristics of plague, resulting in a lower mortality rate in the youngest in Barcelona (41.7%) or a higher rate in Dreux (50.7%). However, we cannot exclude a potential bias that would have led to an underestimation of the number of immature individuals in the English and Spanish sites.

Two important biases can be mentioned. The first one concerns the Hereford cemetery. For this site, only individuals from mass graves were taken into account. Several studies have demonstrated that various burial methods were adopted during epidemics and that single graves may coexist with mass graves [START_REF] Bizot | La saison d'une peste (avril-septembre 1590)[END_REF][START_REF] Kacki | Réflexions sur la variété des modalités funéraires en temps d'épidémie. L'exemple de la Peste noire en contextes urbain et rural[END_REF]. Thus, a consideration of only mass graves may yield misleading results. The second bias concerns the plague pit of Barcelona. Several of the 120 recovered skeletons consisted only of lower limb bones and were categorized as probable adults according the bone dimensions. This arbitrary choice may have resulted in an underestimation of the number of adolescents and consequently of the number of immature individuals as a whole.

Based on the data currently available, the lowest representation of non-adults in the most recent plague burial sites deserves to be pointed out but cannot be interpreted as clear evidence of an evolution of plague epidemiological characteristics. Further studies would be needed to reach a conclusion on this question.

Mortality Quotients by Age Groups

The mortality profiles were established by calculating the mortality quotients for all age groups. They were then compared with a theoretical mortality model [START_REF] Ledermann | Nouvelles tables-types de mortalité[END_REF]. Regardless of their chronology, the skeletal assemblages show recurrent anomalies (Fig. 4). In each case, there is a deficit in the number of children younger than 5 years of age. The infantile (younger than 1 year) mortality quotient and the mortality quotient of the 1-to 4-year age group are low in comparison with those characterizing a natural mortality. The most significant deficit affects the first age group. Whatever the site, the mortality quotient values of children younger than 1year of age never exceed 30‰, whereas the theoretical values range from 156.3‰ to 459.8‰. For the two sites linked to Justinian's plague (Sens and Poitiers), infants are totally missing (Fig. 4a).

The relative constancy of the infantile mortality quotient values suggests that this deficit is specific to plague mortality. We cannot ignore that a cultural bias may have contributed to this anomaly, as very young children may have been buried in places other than the plague cemeteries. However, the anomaly is so recurrent that it is more likely that it is a distinguishing demographic characteristic of plague. This particularity should be investigated further in future research.

The mortality quotient of the 1-to 4-year age group is such more variable. Its value should range between 116.9‰ and 607.6‰. It is considerably lower for the two sites contemporaneous with Justinian's plague and for the Dendermonde cemetery, whereas it is close to the lower boundary of the theoretical interval for Barcelona, Hereford, and Les Fédons and has a median value for Dreux. Except for the last site, this mortality quotient never exceeds 147.3‰.

Conversely, the mortality quotients in the 5-to 9-year, 10-to 14-year, and 15-to 19-year age groups are noticeably higher than expected. Moreover, the proportions of individuals in all of these three age groups differ from the proportion for a natural mortality, which is characterized by a minimum number of deaths in the 10-to 14-year age group. These anomalies are obvious on the mortality profiles. The over-representation of older children and adolescents is also clearly demonstrated by the high values of the juvenility index (D5-14/D20+), which systematically exceed the range of 0.1 to 0.3 expected in an archaic mortality pattern [START_REF] Bocquet | Estimateurs en paléodémographie[END_REF]. The juvenility indices are notably similar for plague burial sites from the 6th and 14th centuries, with values between 0.37 and 0.47 (Table 1). This anomaly is much more pronounced for the 16th century sites, which have very high juvenility indices.

Numerous individuals between the ages of 15 and 9 years also died. Although the mortality quotients for this age group vary depending on the sites, the values obtained are systematically close to the upper boundary of the theoretical interval (Hereford, Les Fédons) or far beyond it. This systematic surplus reveals that older adolescents were not spared during plague epidemics. No differences were observed according to the chronology of the epidemics.

To summarize all these results, the age composition of the skeletal samples from plague burial sites clearly differs from the one expected in a non-epidemic context in two main respects: an underrepresentation of very young infants and a high number of deaths in individuals between the ages of 5 and 14 years. This conclusion confirms those of previous studies by showing a specific mortality profile that mimics the age structure of the living population [START_REF] Castex | Identification and interpretation of historical cemeteries linked to epidemics[END_REF]. The current study, however, reveals how these demographic features vary from site to site and according to epidemic cycles. These new data open perspectives to be explored in future studies.

Adult Sex Distribution

At the level of the entire corpus, the sex proportions do not differ significantly from a theoretical masculinity rate of 50%. There are, however, some peculiarities according to the funerary site considered (Fig. 5). Two skeletal assemblages from the 14th century (Hereford and Dreux) are characterized by a larger proportion of females (masculinity rates of 43.5% and 42.3% respectively), whereas in others the sex ratios are more balanced. The disparity between the masculinity rate and its theoretical value is more important for the sites related to the Black Death than in those related to Justinian's plague and to the resurgences of the second pandemic; however, it must be stressed that the number of individuals whose sex could be determined is relatively small. Thus, there is no strong argument to conclude that there was a preferential female mortality during the 14th century plague epidemics. Given that the differences between the observed and theoretical masculinity rates are never significant, it seems that plague did not kill discriminately with respect to the sex of individuals.

DEMOGRAPHIC SPECIFICITIES OF THE PLAGUE

The study of seven burial sites related to plague revealed many similarities in the age and sex composition of the skeletal samples, which differ in some respects from those expected in a context of natural mortality. The results strongly reinforce those of previous anthropological studies [START_REF] Castex | Identification and interpretation of historical cemeteries linked to epidemics[END_REF][START_REF] Margerison | Paleodemographic comparison of a catastrophic and an attritional death assemblage[END_REF][START_REF] Gowland | Detecting plague: palaeodemographic characterisation of a catastrophic death assemblage[END_REF][START_REF] Signoli | Études anthropologiques des crises démographiques en contexte épidémique: aspects paléo-et biodémographique de la peste en Provence[END_REF]. The opportunity to analyze a large number of plague burial sites from different time periods has allowed a finer analysis of the demographic repercussions of the disease. This study reasonably supports that a demographic signature existed characterizing plague mortality, irrespective of chronological and geographical contexts. Its main characteristics are as follows:

1. mortality quotient for individuals younger than 20 years compatible with that for a natural mortality; 2. deficit of young children, particularly noticeable for individuals younger than 1year; 3. excessive number of individuals between 5 and 14 years of age (high juvenility index); 4. over-representation of the oldest adolescents (15 to 19 years); 5. balanced sex ratio. Despite these characteristics common to the sites, it is noteworthy that there are also some differences between sites. Minor variations may be ascribed to differences in the biological composition of the living populations. Others can be linked to methodological issues resulting, for example, from poor bone preservation or from incomplete recovery of skeletons due to partial excavation of the sites.

In contrast, some differences seem to be linked to the chronology of the funerary sites. The most significant one concerns the number of individuals in the 5-to 14-year age group. The extent to which they are overrepresented varies considerably according to chronology. The anomaly is moderate during Justinian's plague and the Black Death, whereas it is much more pronounced during the resurgences of the second pandemic. The reason for these differences cannot be readily explained. There are currently few sites available for each period. Part of the explanation may perhaps be found in the biological composition of the human groups affected, in which individuals may already have been selected according to age. Another hypothesis is that the demographic impact of plague changed over time as the result of a modification in the virulence of Yersinia pestis and/ or changes in the vulnerability of the human population to the disease.

CONCLUSIONS

By using adapted methodology and tools, paleobiologists can analyze the age and sex composition of an archaeological population and reveal its demographic particularities. Such an approach is extremely valuable in the interpretation of epidemic mortality crises, as it can provide arguments on which to base a discussion of the nature of the disease. As illustrated herein, it is possible to distinguish among different "anomalous demographic patterns" a specific plague mortality profile. The increasing number of burial sites now available makes it possible to explore new lines of research, such as tracking a possible evolution in the demographic signature of plague epidemics over centuries. The interpretation of such slight variations in the mortality profiles, however, will be possible only through the study of many more plague cemeteries all over Europe that are of various chronologies.

Fig. 1 .

 1 Fig. 1. Names and locations of the seven European burial sites used for the demographic analysis.

Fig. 2 .

 2 Fig. 2. Examples of burials with simultaneous deposits (i.e., mass graves): Poitiers, Poitou-Charentes, France (a); Dreux, Eure-et-Loir, France (b); Dendermonde, Belgium (c).

Fig. 3 .

 3 Fig. 3. Proportion of non-adults for each plague site.

Fig. 4 .

 4 Fig. 4. Distribution of ages at death for immature subjects. Comparisons with theoretical values of Ledermann (1969): Justinian's plague, 6th century (a); Black Death, 14 th century (b); resurgences, 16th century (c).

Fig. 5 .

 5 Fig. 5. Adult sex distribution for each plague site. Sex ratio of the Barcelona sample is unknown due to the low number of sexed individuals.

Table 1 .

 1 Demographic data (number of individuals, age, sex, mortality quotients, juvenility index) for each plague site.

ACKNOWLEDGMENTS: This research has been funded by a grant from the Maison des Sciences de l'Homme d'Aquitaine (projet Région Aquitaine) and by a ministerial grant from the French Research National Agency (program of investments for the future, grant ANR-10-LABX-52). The anthropological study of the burials sites referred to in the paper was conducted as part of the research for a Ph.D. thesis (S.K.) and as academic research (D.C.). S.K. is grateful to the institutions that provided access to the skeletal material from Hereford (Bradford Archaeological Research Centre, Bradford, United Kingdom), Barcelona (Museu d'Historia de Barcelona, Spain), Les Fédons (UMR 7268 -ADES, Marseille, France), and Dendermonde (Royal Belgian Institute of Natural Sciences, Brussels, Belgium).