Effect of solutes on strength and ductility of Mg alloys

Acta Materialia 180 (2019) 218e230

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Effect of solutes on strength and ductility of Mg alloys
rez-Prado a, 1, C.M. Cepeda-Jime
nez a, *, 1 D.F. Shi a, b, M.T. Pe a

IMDEA Materials Institute, C/ Eric Kandel, 2, 28906, Getafe, Madrid, Spain Department of Materials Science, Polytechnic University of Madrid/Universidad Polit
ecnica de Madrid, E.T.S. de Ingenieros de Caminos, 28040, Madrid, Spain

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 August 2019 Received in revised form 6 September 2019 Accepted 11 September 2019 Available online 13 September 2019

This work investigates the origin of the simultaneous increase in strength and ductility that takes place in Mg polycrystals alloyed with Al and Zn solutes. With that purpose, twelve polycrystalline binary Mg eZn and MgeAl alloys, with up to 2 wt % of alloying additions and average grain sizes comprised between 3 and 42 mm, were prepared by casting, hot rolling and annealing and were tested at room temperature and quasi-static strain rates. Electron backscattered diffraction-assisted slip trace analysis was then utilized to characterize slip activity, and the latter was related to the grain size, to the texture, and to the topology of the grain boundary network. Basal slip was found to be the dominant deformation mechanism in all the binary alloys, irrespective of composition and grain size. Alloying additions were observed to have little influence on texture development but acted as strong modifiers of the topology of the grain boundary network developed during processing. In particular, they reduced the connectivity of grains that are well oriented for basal slip, preventing intergranular slip localization and, in turn, leading to considerable strengthening of basal slip. Solutes act also as enhancers of diffuse slip within individual grains. It is proposed that the simultaneous increase in strength and ductility of Mg alloys by the addition of solutes must be understood as a multiscale phenomenon resulting from the coupling of solutedislocation interactions at the atomic scale with alterations of the topology of the grain boundary network at the mesoscale. © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Magnesium Basal slip Slip trace analysis Solid solution Ductility

1. Introduction Magnesium alloys, as one of the lightest structural materials, continue to receive significant attention for weight-saving applications, particularly in the automotive, electronics and aerospace industries [1e4]. However, the widespread industrial application of these materials remains limited due to their low strength and to their intrinsic poor room temperature ductility, which restricts applications in damage-tolerant structural components [5e8]. This shortcoming has been extensively attributed [9e12] to the hexagonal close-packed structure of Mg, associated with a wide disparity in the critical resolved shear stress (CRSS) values of different slip systems, which leads often to the dominance of soft basal slip and/or twinning in detriment of hard, non-basal deformation modes. Strengthening basal slip has proven extremely difficult, as it is capable of shearing fine precipitates [13].

* Corresponding author.
nez). E-mail address: [email protected] (C.M. Cepeda-Jime 1 Equal contribution. https://doi.org/10.1016/j.actamat.2019.09.018 1359-6454/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Furthermore, according to the von Mises criterion for strain compatibility [14,15], in a randomly oriented polycrystal, the lack of at least five independent slip systems that can accommodate strain along the crystal c-axis leads to strong anisotropy and to early failure at low strains through the formation of localized shear bands under external stresses [6]. Several strategies have been put forward to improve formability and fracture toughness of Mg alloys, which have yielded different degrees of success in achieving high ductility. These procedures include basal texture weakening [12,16e20], grain refinement [21,22], and solid solution alloying [6,23e25]. Some experimental studies have pointed out that weaker basal textures lead to a more homogenous deformation and to enhanced ductility of Mg alloys [6,26,27]. However, the approaches utilized for texture weakening are mostly based on adding rare earth (RE) elements, and thus involve high costs due to the scarcity of these strategic raw materials. Grain refinement, in turn, often requires very complicated processing paths, which are unlikely to be implemented by industry either. However, ductility can be also enhanced by alloying with non-RE elements. For example, Zr, Ca, and Sr are quite efficient

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in turning Mg ductile at relatively low concentrations [28]. Similarly, Al and Zn, the two most common solutes in Mg alloys [3,5,29e34], which are significantly more cost effective than rare earths [35,36] are known to be moderate ductility enhancers [3,30,37]. The intrinsic mechanisms by which the addition of solutes leads to a ductility improvement are still controversial. Several studies have argued that solutes modify the basal stacking fault energy (SFE) by alloying [1,3,38]. Quantum-mechanical first principles calculations have shown that some solutes, such as Li, Mn, Pd, Pt, Ru, Ir, increase the I2 SFE and thus lead to improved cross-slip probability of basal dislocations onto prismatic planes [5]. Other studies combining transmission electron microscopy (TEM) and ab initio calculations [26], suggest that the addition of Y and Y-like solutes leads to I1 stacking faults with lower SFE, which constitute preferred nucleation sites for dislocations [25]. Recent works based on density functional theory (DFT) calculations, however, question the importance of the basal SFE on ductility, claiming that the latter is instead controlled by the ease of cross-slip of the screw segments of dislocations [3,28]. In particular, reportedly, high ductility is achieved when the cross slip rate is higher than the rate of the thermally activated transitions of the edge dislocation components to basal-oriented segments, which are sessile in nature (pyramidal-to-basal (PB) transition). Since the energy barriers for the PB transitions are small, they occur relatively frequently, thereby severely reducing the mobility of dislocations, contributing to their high critical resolved shear stress (CRSS), and limiting the capability of the material to accommodate c-axis plastic strain. According to this model, solute additions such as Y, Nd, Zr, Ca, and Sr are excellent promoters of cross slip, thereby reducing the difference in the CRSS of this mechanism with respect to that of softer deformation modes such as basal slip or twinning, while Al and Zn have somewhat moderate effects. Studies analyzing the effect of solute elements on the CRSS of basal and non-basal slip systems have also rendered contradictory results. For instance, Blake and Caceres [39] attribute the moderate ductility enhancement in binary MgeZn solid solutions with respect to that of pure Mg to solute softening of prismatic slip. Akhtar and Teghtsoonian [40] also report prismatic slip softening by Zn and Al additions, while other studies observed solute strengthening of both prismatic [41e43] and pyramidal systems [42]. Jang et al. [30], on the contrary, found that Zn additions have a moderate effect on the CRSS of prismatic and pyramidal systems, while they strengthen significantly basal slip. The influence of solutes on the anisotropy of CRSS values and on the strength and ductility of magnesium alloys remains largely under debate. Electron backscatter diffraction (EBSD)-assisted slip trace analysis provides direct and statistically sound evidence of slip activity and allows relating the local texture and the nature of grain boundaries (GBs) to the activation of different deformation modes [44]. In recent years this technique has proven very valuable to improve our understanding of controversial macroscopic mechanical phenomena in Mg alloys, including the wide dispersion of Hall-Petch coefficients [45], the twinning to slip transition [46], the yield stress asymmetry [47], and slip transfer across GBs [46]. Slip trace analysis allows early detection of coarse, localized slip bands, which are often precursors of the cracks that ultimately lead to poor ductility. Moreover, it facilitates identification of the slip systems that are more prone to undergo localization. However, this technique has, to date, not been utilized to investigate the origin of the limited ductility of magnesium alloys. The aim of the present study is to contribute to a better understanding of the microscopic mechanisms responsible for the simultaneous increase in strength and ductility taking place in

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polycrystalline Mg with Zn and Al solute additions. To address this complex issue, EBSD-assisted slip trace analysis is utilized to characterize slip activity at room temperature in binary MgeZn and MgeAl alloys with different amounts of solutes. The slip activity is then related to the grain size, to the texture, and to the topology of the GB network for each binary alloy. This study provides a new perspective on the origin of the poor ductility of magnesium alloys as well as new design guidelines for high toughness. 2. Experimental procedure Four solid solution MgeZn and MgeAl binary alloy ingots with solute contents of 1 and 2 wt% were prepared by casting in an induction furnace under Ar atmosphere from 99.95% pure Mg, Al, and Zn. The as-cast alloys were solutionized at 450  C for 12 h and quenched in cool water in order to obtain supersaturated solid solutions. Next, slabs with approximately 10 mm in thickness and 30 mm in width were machined out of the four binary alloy ingots and were hot rolled at 250  C to a final thickness of ~3 mm. The rolling schedule included three passes, each of 50% reduction, and 10 min inter-pass annealing treatments. The rolled alloys were then heat treated in order to generate polycrystals with three different average grain sizes for each composition and all with similarly strong basal textures. The heat treatment conditions ranged between 200 and 350  C for 5e120 min for the MgeZn alloys and between 200 and 450  C for 10e120 min for the MgeAl alloys. Thus, a total of 12 materials (two MgeZn and two MgeAl alloys, each with three, comparable, grain sizes) were produced for the present study. The grain sizes and textures after processing and annealing were designed to be similar to those developed in a previous study in pure Mg in order to facilitate the assessment of the effect the Al and Zn solutes on ductility [45]. The microstructure and the crystallographic microtexture of all materials were examined by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) using a field emission gun microscope (Helios NanoLab 600i, FEI) equipped with an HKL EBSD system, a CCD camera, as well as both the Aztec and the Channel 5.0 data acquisition and analysis software packages. EBSD measurements were conducted using a step size comprised between 0.3 and 2 mm depending on the initial grain size, at an accelerating voltage of 15 kV, and with a beam current of 2.7 nA. The average grain size values were determined by the linear intercept method from inverse pole figure EBSD maps in the normal direction (ND) to the rolling plane, using only GBs with misorientation angles greater than 15 . Sample preparation for EBSD included mechanical polishing with diamond pastes of increasingly finer particles, and a final finishing using a colloidal silica slurry. The macrotexture of the processed samples was measured by the Schulz reflection method using an Empyrean Panalytical X-ray diffractometer with a Cu-Ka radiation source and parallel beam operated at 45 kV and 40 mA. The 2q angles ranged from 25 to 65 with a step of 0.262 and an acquisition time of 2 s. The surface area examined was ~1 cm2. The (0001), (10-10), (10e11), (10e12), (10e13), and (11e20) pole figures were measured. X-ray diffraction data were corrected for background and defocusing using the software X'Pert HighScore Plus. From the incomplete measured pole figures, the orientation distribution function (ODF) and the complete calculated pole figures were constructed using the MTEX MATLAB code [48]. Dog-bone tensile samples with a transversal section of 3  2.5 mm2 and a gage length of 10 mm were electrodischargemachined out of the rolled and annealed sheets, with the tensile direction parallel to the rolling direction (RD). Uniaxial tensile tests were then carried out on an Instron 3384 universal testing machine at room temperature and at initial strain rate of 103 s1. Two tests

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per composition and grain size were performed to failure with the aim of characterizing the mechanical response (yield stress (s0.2), work-hardening, ultimate tensile stress (UTS), and ductility). Additional tests were stopped at a plastic strain of ~10% in order to carefully analyze the active deformation mechanisms (slip and twin activity), as described below. Slip activity was measured by EBSD-assisted slip trace analysis in the following way. First, selected large areas at the center of the gage length of the tensile samples were mapped by EBSD before testing. After a tensile strain of ~10%, grains with visible slip traces within those regions were located by SEM and their orientation was determined by post-mortem EBSD examination. Subsequently, the Euler angles of each of the selected grains were input into a MATLAB code [49] which provided as output a visual representation of all the possible plane traces for the orientation under analysis. Finally, each trace was assigned to the slip system with the best match. In addition, the MATLAB code also provides the Schmid Factor of the corresponding slip system. An effort was made to consider more than one hundred traces in each sample, as the analysis accuracy relies on having statistically relevant information [44,46,50]. The activity of the different slip systems was then related to the frequency of the corresponding observed traces. In all the alloys investigated, only one set of parallel traces were detected in each grain. Accordingly, each set of traces was counted as one single trace. Additionally, the area fraction of twins was calculated from the inverse pole figure EBSD maps of the deformed samples. Twins become clearly evident in these maps, as they give rise to dramatic rotations of the crystal lattice (the rotation angles depend on the particular twin system that is active), resulting in the appearance of lamellae within the original grains. Twin traces, which are parallel to the long axis of the corresponding lamellae, were analyzed using the methodology described above for slip trace identification. This study allowed determining the active twin variants as well as their corresponding Schmid factors [49]. 3. Results

(SFbasal), assuming a tensile stress along RD, corresponding to each binary alloy. The SFbasal values corresponding to the twelve polycrystals investigated are shown in Figs. 1 and 2. It can be seen that in all alloys SFbasal decreases with increasing grain size. The difference between the SFbasal corresponding to small grained and large grained polycrystals (SFbasal small - SFbasal large) amounts to 0.041 (Mg-1wt.%Zn), 0.042 (Mg-2wt.%Zn), 0.034 (Mg-1wt.%Al), and 0.023 (Mg-2wt.%Al). Fig. 3 compares the effect of grain size on the correlated (neighbor to neighbor) misorientation distribution histograms for each alloy composition. All distributions have a maximum frequency of boundaries with misorientation angles (q) close to 30 [45]. Irrespective of composition, the changes in the distributions with grain size are relatively small. Fig. 4 compares the correlated misorientation distribution histograms of the binary alloys to those of pure Mg polycrystals with comparable grain sizes [45]. It can be clearly seen that the fraction of boundaries with q < 30 is considerably smaller in the solid solution alloys than in pure Mg, irrespective of the specific alloying species and of the alloy concentration, especially in the fine grained polycrystals. The highest fraction of boundaries with q < 30 is present in the pure Mg polycystals with small average grain sizes (Fig. 4a). Fig. 5 compares the correlated and uncorrelated (texture derived) distribution histograms corresponding to MgeZn and MgeAl alloys and to pure Mg polycrystals with small (Fig. 5a), medium (Fig. 5b), and large (Fig. 5c) grains. The excess of boundaries with q < 30 in the correlated distributions suggests the presence of orientation clustering, i.e., the grouping of grains with similar orientations. This effect is most noticeable in the pure Mg polycrystals. Altogether, the above misorientation data reveal that solid solution leads to an decrease of the fraction of boundaries with q < 30 and to a smaller tendency towards clustering of grains with similar orientations. In summary, the above results demonstrate that the addition of Zn and Al solutes to pure Mg leads to hot rolled and annealed microstructures with similar basal fiber textures but with significant differences in the topology of the grain boundary network, particularly in polycrystals with small and medium average grain sizes.

3.1. Effect of solutes on the microstructure 3.2. Effect of solutes on the mechanical behavior The polycrystalline structures corresponding to the binary MgeZn and MgeAl alloys fabricated for the present study are illustrated in Fig. 1 (MgeZn) and Fig. 2 (MgeAl) by means of several EBSD inverse pole figure (IPF) maps in the ND. High angle boundaries (>15 ) are colored in black. The IPF EBSD maps reveal that the microstructures of the 12 polycrystals are homogenous and contain equiaxed grains. This is consistent with the occurrence of recrystallization during processing. The average grain size values corresponding to each sample are included as insets in the corresponding IPF EBSD maps. Figs. 1 and 2 confirm that, for each alloy composition, polycrystals with “small” (d ~3e6 mm), “medium” (d ~10e14 mm), and “large” (d ~34e42 mm) average grain size values were produced. For simplicity, hereafter only the terms “small”, “medium” and “large”, and not the specific d values, will be utilized. These grain sizes are similar to those developed in a previous study in pure Mg [45]. The XRD macrotextures corresponding to each binary alloy are also represented as insets in Fig. 1 (MgeZn) and Fig. 2 (MgeAl) by means of the (0001) direct pole figures. All alloys exhibit the typical basal fiber texture, where the <0001> poles are tilted a few degrees away from ND towards RD [51]. Irrespective of composition, the intensity of the basal fiber texture increases slightly with increasing grain size due to preferential growth of basal oriented grains during post-processing annealing. The variation of the texture with grain size can be quantified by calculating the Schmid factor for basal slip

Fig. 6 depicts the tensile stress-strain curves of the MgeZn (Fig. 6a) and MgeAl alloys (Fig. 6b) deformed until failure at room temperature and at an initial strain rate of 103 s1. In Fig. 7 the yield strength (s0.2), the UTS, and the ductility of all the investigated alloys are compared with those of pure Mg [45] for each grain size range. This figure reveals that the solid solution MgeZn and MgeAl alloys are considerably stronger than pure Mg polycrystals with comparable grain sizes, although the strengthening due to solutes becomes less pronounced in the samples with coarser grains. Fig. 7a shows, additionally, that alloys within the same grain size range have similar s0.2, irrespective of composition, and that, as expected, the s0.2 decreases markedly with increasing grain size. The average s0.2 values for binary alloys with small, medium, and large grain sizes are, respectively, 178, 126, and 106 MPa. UTS data corresponding to different alloys also exhibit a clear inverse variation with grain size (Fig. 7b). The average UTS values corresponding to binary alloys with small, medium, and large grain sizes are, respectively, 261, 231, and 217 MPa. Finally, Fig. 7c reveals that the addition of solutes leads to a considerable increase in ductility, which is also more pronounced in the polycrystals with the smallest grain sizes. Although ductility data corresponding to different alloy compositions within each grain size regime are quite scattered due to defects and heterogeneities resulting from the thermomechanical processing, on average

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Fig. 1. EBSD IPF maps in the ND and XRD {0001} pole figures corresponding to MgeZn alloys with different grain size: a, c, e) Mg-1wt.% Zn; b, d, f) Mg-2wt.% Zn.

Fig. 2. EBSD IPF maps in the ND and XRD {0001} pole figures corresponding to MgeAl alloys with different grain size: a, c, e) Mg-1wt.% Al; b, d, f) Mg-2wt.% Al.

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Fig. 3. Correlated misorientation distribution histograms corresponding to MgeZn and MgeAl binary alloys with different average grain sizes.

ductility in the alloys decreases with increasing grain size from 18% (small), to 17% (medium) to 15% (large). More importantly, these values are significantly higher than those measured in pure Mg polycrystals with similar grain sizes, which amounted to 6% (small), 9% (medium), and 9% (large) [45]. In absolute terms, the largest plastic strains were measured in the Mg-1 wt% Zn alloy with small grain size (25%) and in the Mg-2wt%Zn alloy with large grain size (21%). Fig. 8 depicts the work hardening behavior of the MgeZn (Fig. 8a) and MgeAl (Fig. 8b) binary alloys under study, which was measured from the curves plotted in Fig. 6. In particular, the variation of q(s-s0.2) versus (s-s0.2), where q ¼ ds/dε, is the WH rate, and s and ε are the macroscopic true stress and true plastic strain, respectively, is shown. All alloys exhibit a near-parabolic WH behavior, with a clear decrease in the rate of dislocation storage with decreasing grain size, which is consistent with earlier observations in pure Mg [45] and in other Mg alloys [52e55]. The influence of grain size on WH in pure Mg and Mg alloys is still a subject of debate and opposing views have been published on this matter. The mentioned decrease in WH with grain refinement for Mg and other hcp alloys cannot be rationalized using the current WH models which, on the contrary, predict an increase in WH in finer microstructures due to extra storage of dislocations [54]. The different reasons for this effect that have been put forward include a decrease in the twinning activity with decreasing grain size, an increase in the GB sliding contribution, or enhanced dynamic recovery in fine-grained samples. While it is true that twinning becomes less frequent with decreasing grain size as will be seen below, due to the strong rolling texture present in the current alloys, twinning acts predominantly as an accommodation mechanism and, thus, the twinning activity is low even in the coarsest

microstructure. Thus, the observed clear decrease in WH with decreasing grain size cannot be attributed to twinning effects. On the other hand, the occurrence of grain-boundary sliding (GBS) in Mg at room temperature is nearly negligible, since only at temperatures above ~250  C this mechanism has been found to become active at the strain rates used in the tensile tests in the present study. Finally, the dynamic recovery at RT is also questionable as cross-slip and climb are thermally activated processes. In a previous work by the present authors [45] it was reported that the origin of the decrease in WH in pure Mg polycrystals with decreasing grain size is related to a change in the dominant deformation mechanism from non-basal to basal-dominated flow with decreasing grain size. Further research would be necessary to explain the origin of the decrease in WH in the solid solution alloys of the present study with decreasing grain size, as this is outside the scope of the present work. In summary, the beneficial effects on strength and ductility resulting from the addition of Zn and Al solutes to pure Mg polycrystals become distinctly more prominent with decreasing grain size to the extent that, despite the comparatively poorer WH capability of the finest polycrystals investigated, they are endowed with overall higher toughness that their coarse grained counterparts.

3.3. Effect of solutes on slip and twin activity The activity of the different slip systems after a tensile strain of 10% at room temperature was evaluated quantitatively in all the binary alloys by EBSD-assisted slip trace analysis and it is summarized in Fig. 9. It can be clearly seen that basal slip is the dominant deformation mechanism in the 12 polycrystalline alloys

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Fig. 4. Comparison of the correlated misorientation distribution histogram corresponding to the binary alloys and to pure Mg, for each grain size range.

under investigation. However, clear differences in slip activity can be noticed in alloys with Zn and Al solutes. In particular, while in MgeZn alloys (Fig. 9a) the added contribution of non-basal slip systems (prismatic plus pyramidal slip) increases gradually from (~5e7%) to ~20% with increasing grain size and alloying content, in all MgeAl alloys (Fig. 9b) this contribution remains constant at 5%. Figs. 10 and 11 show the SF distribution histograms with respect to the global external stress corresponding to grains in which basal, prismatic and pyramidal traces were detected in the MgeZn (Fig. 10) and MgeAl (Fig. 11) alloys. The corresponding average SFbasal values are also included next to each SF distribution. The orientations of the grains in which basal and non-basal slip traces were detected are represented in {0001} pole figures by means of blue and red poles, respectively. It can be seen that, irrespective of composition, basal slip exhibits predominantly a typical “Schmid behavior”, i.e,. basal slip traces are preferentially found in grains with high SF. On the contrary, non-basal systems exhibit a “nonSchmid behavior”, i.e., they become active in grains with very low SF with respect to the applied stress. This suggests that these mechanisms are activated in response to local stress concentrations at GBs [56] due to the need to accommodate strain incompatibilities between neighboring grains with different

orientations. Fig. 12 shows the area fraction of tensile twins calculated from the IPF EBSD maps after a tensile strain of 10% at RT in the binary alloys. Several EBSD IPF maps corresponding to selected alloys after 10% straining are also included in Fig. 12 in order to prove the presence of twins. This figure reveals that tensile twinning occurs in general more frequently with increasing grain size. The average Schmid factor for twinning (SFtwin), calculated taking into account only the grains in which twins were observed, was in all the cases lower than 0.2. Tensile twinning exhibits, therefore, a “nonSchmid” behavior, i.e., it becomes active as an accommodation mechanism in response to local stress concentrations at GBs. A clear correlation between this accommodation mechanism and mechanical properties cannot be properly rationalized. No compression twins were observed. 4. Discussion Our results reveal that basal slip is the dominant deformation mechanism in all the binary alloys investigated, as reported earlier for pure Mg polycrystals with small and medium average grain sizes [45]. It seems therefore reasonable to argue that the considerable increases in strength and ductility originated by the addition

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Fig. 5. Correlated vs uncorrelated misorientation distribution functions corresponding to the binary MgeZn and MgeAl alloys and to pure Mg, with different average grain sizes.

of Zn and Al solutes, described in section 3.2, and which are particularly pronounced in the fine grained polycrystals, may be related to an increase in the CRSS of basal slip with respect to that of non-basal systems [27]. In the following, the effects of solutes on different potential strengthening mechanisms of basal slip are discussed. 4.1. Influence of solutes on texture strengthening of basal slip The average Schmid Factors of the basal slip system (SFbasal) (assuming a tensile stress along RD) in all the polycrystalline alloys investigated were calculated from the IPF EBSD maps of Figs. 1 and 2 and are included as insets in those figures. For a given grain size range, SFbasal values are similar in all alloys, irrespective of composition. However, for each composition, SFbasal decreases slightly with increasing grain size. The SFbasal corresponding to polycrystals with small, intermediate, and large grain sizes, averaged over the four different compositions under study, are,

respectively, 0.247, 0.229, and 0.212. These values are, furthermore, higher than those corresponding to pure Mg polycrystals with similar grain sizes, which amount, respectively, to 0.208, 0.180, and 0.188 [45], indicating that the addition of Al and Zn solutes leads to moderate softening of basal slip. Therefore texture strengthening must be ruled out as a potential origin of the increase in strength and ductility observed in the MgeZn and MgeAl alloys with respect to pure Mg. 4.2. Influence of solutes on the intergranular localization of basal slip It is well known that slip localization along bands traversing multiple grains leads to softening and early fracture of polycrystalline metals [57,58]. In particular, it was shown recently [46] that in fine grained pure Mg polycrystals, processed by hot rolling and annealing, the development of transgranular bands of basal slip traces during room temperature straining was very pronounced, in

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Fig. 6. Tensile true stress-true plastic strain curves corresponding to the Mg binary alloys deformed at room temperature and at an initial strain rate of 103 s1: a) MgeZn alloys; b) MgeAl alloys.

Fig. 7. Comparison of the mechanical properties corresponding to all the binary alloys and to pure Mg.

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Fig. 8. Work-hardening behavior of the Mg binary alloys: a) MgeZn alloys; b) MgeAl alloys.

Fig. 9. Frequency of slip traces corresponding to basal and non-basal slip systems during tension of the Mg binary alloys along RD at room temperature and at an initial strain rate of 103s1: a) MgeZn alloys; b) MgeAl alloys.

detriment of the mechanical behavior. In the same study, Cepeda
nez et al. [46] showed a higher tendency towards intergranular Jime localization of basal slip in microstructures with a large degree of clustering of grains with SFbasal larger than 0.2. In the latter, basal slip transfer took place across GBs with misorientation angles smaller than a threshold value (qth) of 30 . In the present study, the differences observed between the correlated and uncorrelated misorientation distributions of the solid solution alloys and those of pure Mg (Figs. 4 and 5) reveal that the addition of Zn and Al reduces orientation clustering in polycrystals with comparable grains sizes, and that this effect becomes distinctly more prominent with decreasing grain size. In order to quantify the influence of the alloying elements on the connectivity of grains with SFbasal larger than 0.2 in the twelve alloys investigated, the triple junction distributions of EBSD maps including only such grains [59,60] were analyzed. Triple junctions are usually classified according to the number of GBs with a misorientation smaller than a given qth that merge in them: type J3 junctions consist of 3 GBs with q
performed under those same conditions. Fig. 13 compares the fraction of J2þJ3 junctions (fJ2þJ3) corresponding to the polycrystalline alloys with those of pure Mg polycrystals with similar grain sizes. This analysis further corroborates that the degree of clustering of grains that are well oriented for basal slip is considerably lower in the solid solution alloys than that in pure Mg. Fig. 14 consists of several secondary electron SEM micrographs that illustrate the distribution of slip traces in the surface of deformed polycrystalline pure Mg (Fig. 14a), in Mg-1wt%Zn (Fig. 14b) and in Mg-1wt%Al (Fig. 14c) alloys, all with small average grain sizes. Indeed, while, in pure Mg basal traces localize in clearly defined transgranular bands, in the alloys they are homogeneously distributed throughout the microstructure. Therefore, it seems clear that the addition of Zn and Al solutes leads to changes in the GB network topology that contribute to hinder basal slip transfer and prevent basal slip localization, thus improving ductility (Fig. 7c). It is our contention that the reduced orientation clustering resulting from the addition of solutes contributes to the observed enhancement of yield and maximum strength in the solid solution alloys with respect to pure Mg (Fig. 7a and b). The grain size effect on yielding can be calculated using a Hall-Petch (HP) type relation (sy ¼ s0 þ kd1/2), where s0 is the lattice friction stress for dislocation movement and considered to be similar for all the alloys, and where k, the HP slope, adopts a value of 281 ± 55 MPa mm½,

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Fig. 10. Schmid factor distribution corresponding to MgeZn binary alloys. Only grains in which slip traces were detected are considered. The orientations of such grains are plotted in the {0001} pole figures (in blue, grains with basal traces; in red, grains with non-basal traces). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 11. Schmid factor distribution corresponding to MgeAl binary alloys. Only grains in which slip traces were detected are considered. The orientations of such grains are plotted in the {0001} pole figures (in blue, grains with basal traces; in red, grains with non-basal traces). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 12. Area fraction of twins measured from the EBSD maps performed after tension up to a strain of 10% in the Mg binary alloys: a) MgeZn alloys; b) MgeAl alloys. EBSD IPF maps in the ND after a strain of ~10% for MgeZn and MgeAl alloys are also included to show the evolution of tensile twins during deformation.

grains that are well oriented for basal slip due to the presence of alloying elements, which contributes to increase ductility and strength by hindering basal slip localization. 4.3. Influence of solutes on the intragranular localization of basal slip

Fig. 13. Connectivity of grains that are well oriented for basal slip (SFbasal > 0.2). Comparison of the frequency of f J2þJ3 triple junctions in the binary alloys and in pure Mg.

measured in the rolled AZ31 alloy in the grain size regime of 13e89 mm [61]. This law predicts an increase of ~80 MPa in the yield stress value when the grain size decreases from ~36 mm to ~5 mm. However, the s0.2 values for all pure Mg polycrystals are very similar (Fig. 7a). This suggests that the high connectivity and enhanced basal slip transfer taking place across GBs with small misorientation angles in pure Mg lead to a decrease in strength. On the contrary, the s0.2 values in the solid solution alloys show a variation with grain size that is more in agreement with a HP law (Fig. 7a). This is consistent with the poorer connectivity between

Fig. 14 also evidences that, while the grain interiors in pure Mg are populated with a small number of thick traces, associated to deep surface steps, a phenomenon known as coarse slip or intragranular slip localization, in the binary alloys slip is diffuse and a large number of fine slip lines are present. It is known that coarse slip is a consequence of the softening of individual planes by the passage of dislocations [62,63], in such a way that the nucleation and movement of further dislocations along such planes becomes progressively easier, i.e., takes place at increasingly lower stresses. Thus, slip concentrates preferentially along such comparatively softer basal planes. It is our contention that the presence of solutes, offering an added resistance to the passage of dislocations, severely limits the mentioned softening, thus contributing to a more homogeneous distribution of dislocations along different basal planes. It seems reasonable to think that the stresses at the intersection of basal planes and grain boundaries depend on the density of dislocations moving along each basal plane, and therefore should be smaller when diffuse slip is present. This, in turn, would reduce the likelihood of basal slip transfer [62,63], as observed in the present study (Fig. 14). 4.4. Outlook on solution strengthening of magnesium by Zn and Al atoms Solid-solution strengthening results from the binding between solute atoms and dislocations [5,64]. The binding energies depend, among others, on the size mismatch between solute and matrix atoms. For example, Zn, with smaller size than Mg, tends to diffuse

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Fig. 14. Secondary electron SEM micrographs illustrating pattern of slip traces in (a) pure Mg and in (b) Mg-1wt.% Zn and (c) Mg-1wt.% Al alloys, all with small grain size.

to the compression region of dislocation core [65]. Recent simulation studies suggest [30,66] that solute atoms such as Al have stronger dislocation binding tendency and solid solution strengthening effect on basal slip planes than on non-basal slip planes, and therefore reduce the anisotropy in the critical resolved shear stress between slip systems, and eventually improve the room temperature ductility. The current work suggests that the effect of solutes on the mechanical behavior of Mg alloys extends beyond solutedislocation interactions at the atomic scale. Our results evidence that, furthermore, at the mesoscale, the addition of Al and Zn solutes leads to changes in the topology of the grain boundary network which result in poorer connectivity between grains that are well oriented for basal slip, thus contributing, on the one hand, to strengthen this mechanism and, on the other hand, to avoid basal slip localization. The simultaneous increase in strength and ductility in Mg binary solid solutions must, therefore, be justified by coupling these phenomena occurring at multiple length scales. 5. Conclusions The aim of this research was to investigate the mechanisms responsible for the simultaneous increase in strength and ductility in polycrystalline binary MgeZn and MgeAl solid solution alloys with respect to pure Mg. With that goal, twelve polycrystalline MgeZn and MgeAl alloys, with up to 2 wt % of alloying additions and average grain sizes comprised between 3 and 42 mm, were prepared by hot rolling and annealing and were tested at room temperature and quasi-static strain rates. EBSD-assisted slip trace analysis was then utilized to characterize slip activity, and the latter was related to the grain size, to the texture, and to the topology of the GB network. The main conclusions of the present study are the following: 1. Basal slip is the dominant deformation mechanism in all the solid solution MgeZn and MgeAl alloys investigated, irrespective of composition and grain size. The activity of non-basal slip and tensile twinning, which operate mostly as accommodation mechanisms in response to local stresses, increases with increasing grain size but remains minor in comparison with that of basal slip. 2. Hot rolled and annealed binary MgeZn and MgeAl alloys have similar basal fiber textures within all the composition ranges investigated. The texture strength increases slightly with increasing grain size, irrespective of the amount of Al and Zn solutes. The binary alloys have moderately weaker textures than pure Mg with comparable grain sizes. 3. The addition of Al and Zn solutes modifies the topology of the GB network that results from processing. On the one hand, it leads to a significant decrease in the volume fraction of boundaries with misorientation angles below 30 , particularly in the fine grained alloys; on the other hand, it results in a

reduced clustering of grains that are well oriented for basal slip (SFbasal > 0.2), leading to a reduced tendency towards intergranular basal slip localization. 4. The addition of Al and Zn solutes prevents intragranular slip localization, thus reducing the likelihood of basal slip transfer across grain boundaries. 5. The simultaneous increase in the macroscopic strength and ductility in polycrystalline binary MgeZn and MgeAl alloys with respect to pure Mg with comparable grain sizes must be understood as a multiscale phenomenon, in which the solid solution strengthening derived from solute-dislocation interactions at the atomic scale, is coupled with modifications in the grain boundary network at the mesoscale. Acknowledgments The research leading to these results has received funding from the Madrid region under programme S2018/NMT-4381-MAT4.0CM project. DFS acknowledges the financial support from the China Scholarship Council (Grant no 201706050154). Dr. Fernando ~ o (CENIM-CSIC, Madrid) is sincerely thanked for alloying the Carren use of the rolling equipment for this research work. The authors also thank Dr. Javier García from the X-ray lab at IMDEA Materials Institute for experimental assistance with the XRD technique. References [1] H. Somekawa, C.A. Schuh, Effect of solid solution elements on nanoindentation hardness, rate dependence, and incipient plasticity in fine grained magnesium alloys, Acta Mater. 59 (2011) 7554e7563. [2] T. Nakata, C. Xu, R. Ajima, K. Shimizu, S. Hanaki, T.T. Sasaki, L. Ma, K. Hono, S. Kamado, Strong and ductile age-hardening Mg-Al-Ca-Mn alloy that can be extruded as fast as aluminium alloys, Acta Mater. 130 (2017) 261e270. [3] B. Yin, Z. Wu, W.A. Curtin, First-principles calculations of stacking fault energies in Mg-Y, Mg-Al, and Mg-Zn alloys and implications for activity, Acta Mater. 136 (2017) 249e261. [4] Z.R. Zeng, Y.M. Zhu, R.L. Liu, S.W. Xu, C.H.J. Davies, J.F. Nie, N. Birbilis, Achieving exceptionally high strength in Mg-3Al-1Zn-0.3Mn extrusions via suppressing intergranular deformation, Acta Mater. 160 (2018) 97e108. [5] J.A. Yasi, L.G. Hector Jr., D.R. Trinkle, First-principles data for solid-solution strengthening of magnesium: from geometry and chemistry to properties, Acta Mater. 58 (2010) 5704e5713. €bes, S. Zaefferer, I. Schestakow, S. Yi, R. Gonzalez-Martinez, On the [6] S. Sandlo role of non-basal deformation mechanisms for the ductility of Mg and Mg-Y alloys, Acta Mater. 59 (2011) 429e439. €bes, M. Friak, S. Korte-Kerzel, Z. Pei, J. Neugebauer, D. Raabe, A rare[7] S. Sandlo earth free magnesium alloy with improved intrinsic ductility, Sci. Rep. 7 (2017) 10458. [8] M. Ghazisaeidi, L.G. Hector Jr., W.A. Curtin, Solute strengthening of twinning dislocations in Mg alloys, Acta Mater. 80 (2014) 278e287. [9] S.R. Agnew, M.H. Yoo, C.N. Tome, Application of texture simulation to understanding mechanical behavior of Mg and solid solution alloys containing Li or Y, Acta Mater. 49 (2001) 4277e4289. [10] S.R. Agnew, J.A. Horton, M.H. Yoo, Transmission electron microscopy investigation of dislocations in Mg and a-solid solution Mg-Li alloys, Metall. Mater. Trans. A 33 (2002) 851e858. [11] Y. Chino, M. Kado, M. Mabuchi, Enhancement of tensile ductility and stretch formability of magnesium by addition of 0.2 wt%(0.035 at%) Ce, Mater. Sci. Eng. A 494 (2008) 343e349. [12] T. Al-Samman, X. Li, Sheet texture modification in magnesium-based alloys by

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