Grain refinement mechanism and improved mechanical properties in Mg–Sn alloy with trace Y addition

Journal of Alloys and Compounds xxx (xxxx) xxx

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Grain refinement mechanism and improved mechanical properties in MgeSn alloy with trace Y addition X.Y. Qian a, Y. Zeng a, *, B. Jiang b, c, Q.R. Yang a, Y.J. Wan a, G.F. Quan a, F.S. Pan b, c a Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Material Science and Engineering, Southwest Jiaotong University, Chengdu, 610031, China b National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing, 400044, China c Chongqing Academy of Science and Technology, Chongqing, 401123, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 February 2019 Received in revised form 10 November 2019 Accepted 20 November 2019 Available online xxx

Mg-0.5Sn-(0, 0.3 wt%) Y alloys were obtained by casting with a metal mould, then extruded to acquire the as-extruded sheets. The microstructures, and mechanical properties of the alloys were studied. The results indicated that Sn3Y5 particles generated after the addition of Y into the MgeSn alloys. This contributed to the presence of fine grains in the extruded Mg-0.5Sn-0.3Y sheets, the size of which decreased from ~16 mm to ~4 mm, accompanied by a significant improvement in the mechanical properties. Specifically, with Y addition, the ultimate tensile strength and elongation along the extrusion direction increased by 21% and 191%, respectively. The crystallographic matching relationships between Sn3Y5 and Mg were established via an edge-to-edge matching model. The calculated results illustrated that Sn3Y5 had small crystallographic misorientation with a-Mg, and served as the sites of heterogeneous nucleation for the Mg matrix, which led to grain refinement of Mg-0.5Sn-0.3Y sheets. Meanwhile, the diffraction patterns obtained by transmission electron microscopy along [0001]Mg // [1123]Sn3Y5 well agreed with the simulation results and validated the prediction. Furthermore, the macro-texture and work hardening behaviour of the extruded sheets implied that in the tensile test, the as-extruded Mg0.5Sn-0.3Y sheets underwent both basal and prismatic slips, while only the basal slip was observed in Mg-0.5Sn. As more slip systems were activated and more mobile dislocations could be formed, this in combination with the fine-grain strengthening effect, resulted in the large tensile elongation and high tensile strength of the Mg-0.5Sn-0.3Y sheets. © 2019 Elsevier B.V. All rights reserved.

Keywords: Magnesium alloys Grain refinement mechanism Mechanical properties Intermetallics

1. Introduction MgeSn-based alloys are promising for using in transportation equipment not only in terms of energy conservation, but also due to their excellent creep resistance and precipitated strengthening effect [1e4]. However, the coarse Mg2Sn eutectic phases tend to be located at the grain boundaries of binary MgeSn alloys with high Sn content, which promotes the improvement in strength but cause decrease in ductility [5e7]. In recent years, it has been reported that adding trace rare earth elements, like Gd [8e12], Y [13e15], and Ce [9,16,17], among others, into Mg alloys can improve both the ductility and strength because of the solid-solution strengthening and texture weakening. In

* Corresponding author. E-mail address: [email protected] (Y. Zeng).

particular, adding Y into Mg resulted in a remarkable improvement € bes et al. [13] found that 3 wt% Y of ductility [13,18e20]. Sandlo addition improved the elongation of cold-rolled pure Mg from ~4% to ~24%. In addition, Zhou et al. [15] investigated the microstructure evolution and deformation modes of the extruded Mge3Y alloy via a tensile test. The extruded Mge3Y alloy displayed an excellent elongation of ~33% due to the activation of various deformation modes, including non-basal and basal slips, contraction and extension twinning, and the interactions between twin and slip. Moreover, Zhao et al. [21] reported that the as-extruded Mg-1.0% Sn-3.5%Y (at.%) alloy with a grain size of 12 mm exhibited a higher ultimate tensile strength (UTS) of 305 MPa. Micro-alloying Y addition into a MgeSn alloy caused an increased ductility of 32.7% due to the activation of prismatic slip, according to the investigations of Wang et al. [22]. However, there has been little research on the microstructures and mechanical behaviours of MgeSneY series alloys.

https://doi.org/10.1016/j.jallcom.2019.153122 0925-8388/© 2019 Elsevier B.V. All rights reserved.

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Meanwhile, grain refinement is beneficial to improve the comprehensive mechanical properties of Mg alloys. Hu et al. [23] investigated the microstructure of as-extruded MgeSneZneMn alloys before and after the addition of Y. An extra MgSnY phase was observed due to the addition of Y, and resulted in refinement of the grains by 20%. In addition, the shear-extruded process can also obtain fine grains. After shear-extrusion, the grain size of Mg5.0Sn-1.5Y-0.1Zr alloys decreased from ~90 mm to below 6 mm owing to the particle stimulated nucleation of the Mg2Sn and MgSnY particles [24]. Nevertheless, the detailed mechanism of grain refinement in MgeSneY alloys is still unclear. The edge-toedge matching (E2EM) model, reported by Qiu and Zhang et al. [25,26], was utilised to provide reliable predictions for the orientation relationships (ORs) between parent and product phases. The E2EM model is based on assumption [27] that both the morphology and ORs between two fully or partially coherent phases are governed by minimisation of the interface strain energy. The crystallographic examination applying this model has successfully explained the grain refinement mechanism of Al2Y, Mg24Y5, and LiMgAl2 in Mg alloys [28]. Therefore, our present work aims to clarify whether trace Y addition can lead to a significant improvement in the grain refinement and mechanical properties of as-cast and as-extruded MgeSn alloys. The grain refinement mechanism of Sn3Y5 was examined via calculations of the crystallography matching relationship between Sn3Y5 and the Mg matrix. Furthermore, the reason for the improved mechanical properties with Y addition was analysed in detail, combining the macro-texture and work hardening behaviour. 2. Experimental procedures Raw materials including commercially pure Mg (99.9%), high purity tin (99.99%), and Mge18Y master alloy were used in this study. All compositions represent weight percent throughout this paper unless stated otherwise. The charging was placed in a mild steel crucible with a protective atmosphere composed of CO2 and SF6 (CO2: SF6 ¼ 100:1). Next, the charging completely melted at approximately 730  C in an electric resistance furnace and was isothermally held for 15 min. Then, a preheated mild steel mould was filled with the melt under air cooling, and cylindrical cast ingots of diameter 90 mm were acquired. Subsequently, the cast ingots underwent homogenisation at 400  C for 10 h and were cooled in air, followed by milling to remove the oxidised layers. Finally, the as-cast alloys were extruded with an extrusion speed of 20 mm s1 at 400  C, and the sheets with a thickness of 2 mm and a width of 60 mm were acquired. Inductively coupled plasma-atomic emission spectroscopy, as an effective method, was used to examine the chemical components of the as-cast alloys, and the actual compositions were Mg0.55Sn-0.27Mn and Mg-0.48Sn-0.33Y-0.27Mn, respectively. It should be specifically explained that MgeSn and MgeSneY were our target alloys and trace Mn was applied to eliminate Fe in the casting process, as introduced from the iron crucible, as well as applied in the studies of AZ31 alloys [29,30]. After etching with a picronitric acid solution (5 mL acetic acid, 25 mL ethyl alcohol, and 1.5 g picric acid), the microstructures were analysed by conventional optical microscopy. Via the differential scanning calorimetry (DSC) with the NETZSCH STA 449C system, the solidification path of the MgeSneY alloys was measured and the cooling curve was obtained. For the DSC measurement, the experimental specimens were heated from 20  C to 700  C, at a rate of 5  C/min. Image-Pro Plus 6.0, regarded as a quantitative metallographic method, was used to measure the grain size and size distributions in the central area of the transverse section. The grain-size distributions of the as-

extruded alloys were measured from more than three metallographic images, with 1000 grains counted. The identification of precipitates was performed by secondary scanning electron microscope (SEM, TESCAN VEGA II LMU) accompanied by energydispersive spectroscopy (EDS, INCA Energy, Oxford Ins). The macro-texture of the as-extruded alloys was investigated by X-ray diffraction (XRD, RIGAKU D/MAX 2500PC) with Cu-Ka radiation. Then, the precise morphology, component and atomic-resolution scanning transmission electron microscopy (STEM) observations of the samples were performed by FEI Tecnai G2 20 at the voltage of 200 keV, which was fitted with an energy dispersive spectrometer and a cold field emission gun, with a correction of Cs. Before the measurement, the specimens were mechanically ground to 60 mm, then were punched with 3 mm diameter discs. After, an ion beam thinner was used to expand the thin area. Furthermore, uniaxial tension tests were conducted to investigate the mechanical properties of the as-extruded MgeSneY sheets at room temperature (RT). Tensile samples with 12 mm in gauge length and 6 mm in gauge width were cut from three directions, including the transverse direction (TD), extrusion direction (ED), and 45 from the ED of as-extruded sheets, and were tested with an initial crosshead speed of 3 mm/min. 3. Results 3.1. Microstructure evolution Fig. 1 illustrates the optical microstructures of the as-cast, ashomogenised and as-extruded Mg-0.5Sn and Mg-0.5Sn-0.3Y alloys. Both the as-cast Mg-0.5Sn and Mg-0.5Sn-0.3Y show coarse columnar grains. For the as-cast Mg-0.5Sn alloys, columnar grains with the average length of ~12.0  103 mm and width of ~3.1  103 mm, and equiaxed grains with an average size of ~3.2  103 mm exist in the centre of the ingots. For the as-cast Mg-0.5Sn-0.3Y alloys, these equiaxed grains are replaced by larger columnar grains with the average length of ~22.9  103 mm and width of ~3.3  103 mm. Homogenisation treatment was used to relieve the casting stress and reduce the grain boundaries segregation [31]. It is found that the central equiaxed grains in the Mg-0.5Sn alloys slightly grow from ~3.2  103 mm to ~4.5  103 mm after homogenisation. Excluding this, the size of the columnar grains is nearly invariant in both alloys after homogenisation, as seen from Fig. 1(c) and (d). However, after extrusion, both the grains of Mg-0.5Sn and Mg0.5Sn-0.3Y refine because of the dynamic recrystallisation (DRX) during the extrusion process [32]. Furthermore, compared to the as-extruded Mg-0.5Sn sheets, Mg-0.5Sn-0.3Y exhibits much finer grains and more uniform microstructures. Based on the statistical analysis of the grain-size distributions shown in Fig. 1(e) and (f), the Mg-0.5Sn-0.3Y sheets possess a recrystallised microstructure with an average grain size of ~4 mm, while that of Mg-0.5Sn is ~16 mm. Additionally, it is clear that the Mg-0.5Sn sheets present a relatively wide grain-size distribution, while the Mg-0.5Sn-0.3Y sheets display a more homogeneous distribution of the grain size. This might be because that the addition of Y leads to the grain refinement and microstructural homogenisation of the as-extruded Mg0.5Sn alloys. In order to explain the detailed mechanism of grain refinement with the addition of Y in the Mg-0.5Sn alloys, the secondary phases were investigated. Fig. 2 exhibits the SEM images of the as-cast and as-extruded Mg-0.5Sn and Mg-0.5Sn-0.3Y alloys. As demonstrated in Fig. 2(a), few secondary phases exist in Mg-0.5Sn owing to the high solubility of Sn in Mg [33]. Meanwhile, with 0.3 wt% Y addition, some short rod-shaped and granular secondary phases appear. Similarly, some granular and dispersed secondary phases are generated in the as-extruded Mg-0.5Sn-0.3Y sheets, as shown in

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Fig. 1. Macro photographs of (aeb) as-cast Mg-0.5Sn and Mg-0.5Sn-0.3Y, (ced) as-homogenised Mg-0.5Sn and Mg-0.5Sn-0.3Y; Optical microstructures of (eef) as-extruded Mg0.5Sn and Mg-0.5Sn-0.3Y; Grain-size distributions of (geh) as-extruded Mg-0.5Sn and Mg-0.5Sn-0.3Y. Additionally, the left refers to the Mg-0.5Sn binary alloys, while the right side presents the Mg-0.5Sn-0.3Y ternary alloys.

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Fig. 2. SEM images of (aeb) as-cast Mg-0.5Sn and Mg-0.5Sn-0.3Y, and (c) as-extruded Mg-0.5Sn-0.3Y sheets; (d) particle size distributions in the as-cast Mg-0.5Sn-0.3Y alloys.

Fig. 2(c). Based on the MgeY binary alloy phase diagram [33], it is known that Y solubility in Mg at RT (298 K) is ~0.4%. Indeed, Sn can react with Y and Mg, and this results in the solubility of Y in Mg being lower than the desired value. Besides, Fig. 2(d) shows the statistical size distributions of the intermetallics in the as-cast Mg0.5Sn-0.3Y alloys (at least 500 particles were measured). This indicates that the average size of intermetallics grains in the Mg0.5Sn-0.3Y alloys is approximately 3.5 mm. Additionally, the volume fraction of the intermetallics grains is almost 0.34%, which is obtained by using Image-Pro Plus 6.0. Next, the phase analysis was conducted by XRD and the types of intermetallics were identified by EDS. Fig. 3 illustrates the XRD patterns of the Mg-0.5Sn and Mg-0.5Sn-0.3Y alloys. It can be found that Mg-0.5Sn-0.3Y alloys are mainly composed of a-Mg and Sn3Y5. The present Sn3Y5 phase has the standard powder XRD file number of 18e1392 in the PDF crystallographic database. Furthermore, according to the analysis of the EDS results in Fig. 4, the atomic ratio of Y and Sn of the bright intermetallics grains, shown in the left SEM image, is close to 5:3. Therefore, it can be concluded that most grains of the secondary phases in Mg-0.5Sn-0.3Y are Sn3Y5. To further verify the suggestion of the secondary phases formed and to provide an explicit explanation, the transmission electron microscopy (TEM) analysis of Mg-0.5Sn-0.3Y was performed, as illustrated in Fig. 5. Fig. 5(a) shows the electron diffraction pattern from Sn3Y5 along the zone axis [0001]Mg// [1123]Sn3Y5. Additionally, the TEM-EDS results indicate that the atomic ratio of Y and Sn of the particle phases is close to 5:3, as shown in the red dotted circle. From the above SEM images, it could be found that the secondary

phases, which existed in the Mg-0.5Sn-0.3Y alloys, maintained a similar morphology. Thus, combined with the TEM analysis and the SEM results, strong proof is provided for demonstrating that the secondary phases of Mg-0.5Sn-0.3Y are dominated by Sn3Y5. 3.2. Mechanical properties Fig. 6 shows the true stress-strain tensile curves of the asextruded Mg-0.5Sn and Mg-0.5Sn-0.3Y sheets, respectively. Excluding the measurements of strength with the yield strength (YS) and ultimate tensile strength (UTS), the ductility values containing elongation were acquired from the true stress-strain curves. These results are summarised and listed in Table 1. The YS and UTS along the TD of the as-extruded Mg-0.5Sn and Mg-0.5Sn-0.3Y sheets are highest, followed by those along 45 direction and ED. The strength values of the Mg-0.5Sn-0.3Y sheets along the three directions are all higher than those of the Mg-0.5Sn sheets. In particular, after adding Y, the YS and UTS values along the 45 direction increase from 137 MPa to 170 MPa and from 251 MPa to 286 MPa, respectively. As for the ductility, the elongations along the three directions of the Mg-0.5Sn sheets are only ~10%. Additionally, the elongation sharply improves to 28%e30%, and increases by ~191%, 181%, and 229% along the ED, 45 direction, and TD, respectively. Both the strength and ductility of Mg-0.5Sn with Y addition significantly improve. To demonstrate the mechanical behaviour during the tensile test, the hardening rate curves of the as-extruded Mg-0.5Sn and Mg-0.5Sn-0.3Y sheets are obtained via the tensile test data, as seen

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Fig. 3. XRD patterns of (a) Mg-0.5Sn, and (b) Mg-0.5Sn-0.3Y.

Fig. 4. EDS analysis in SEM image of Mg-0.5Sn-0.3Y.

in Fig. 5. The work hardening rate (q) measures further plastic deformation ability at an instant status, as obtained from the slope of the true stress-strain curves. Generally, a high q value for metal materials indicates that further deformation is difficult. It can be expressed as q ¼ ds/dε, where s and ε refer to the instant true strain and stress, respectively [34,35]. As shown in Fig. 7, after adding Y, the work hardening behaviour of the as-extruded Mg-0.5Sn sheets presents three distinct differences. Firstly, two sheets show a presence of different stages during the tensile process. The as-extruded Mg-0.5Sn sheets only display one stage with the decrease of the slope of the work hardening curve, while the Mg-0.5Sn-0.3Y sheets exhibit two stages (stage III: a linear decreasing strain hardening rate with stress, and stage Ⅳ: a linear hardening region with a low hardening rate [36,37]). Secondly, both as-extruded Mg-0.5Sn and Mg-0.5Sn-0.3Y have

different initial q values. The q values along the three directions in the Mg-0.5Sn-0.3Y sheets are much closer than those in Mg-0.5Sn. Thirdly, they have a different decreasing rate at stage III. The Mg0.5Sn-0.3Y sheets possess a larger q decreasing rate at stage III. A detailed analysis of stage III and Ⅳ will be presented in the following discussion section together with the macro texture. 3.3. Texture Fig. 8 shows the (0002) basal and (1010) prismatic pole figures of the extrusion sheets. It appears that the texture of the asextruded Mg-0.5Sn sheets can be characterised by a typical basal texture [38] and the ED is perpendicular to the normal axis and parallel to the (0002) basal planes. In addition, the (1010) pole figure of the Mg-0.5Sn sheets presents a scattering prismatic

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Fig. 5. TEM results of Mg-0.5Sn-0.3Y: (a) diffraction pattern of secondary phase along the zone axis [0001]Mg // [1123]Sn3Y5, (b) morphology of secondary phase; (c) EDS analysis of secondary phase.

Fig. 6. True stress-strain curves of (a) as-extruded Mg-0.5Sn sheets, and (b) as-extruded Mg-0.5Sn-0.3Y sheets.

Table 1 Measured results of YS, UTS and elongation of the as-extruded sheets along the extrusion direction (ED), 45 and transverse direction (TD). Alloy

Mg-0.5Sn Mg-0.5Sn-0.3Y

YS (MPa)

UTS (MPa)

Elongation (%)

ED

45

TD

ED

45

TD

ED

45

TD

130 141

137 170

157 188

239 288

251 286

266 302

10.4 30.3

10.0 28.1

8.5 28.0

is heterogeneous nucleation. According to Greer’s study [42], it is well-accepted that the final phase structure and grain size strongly depend on the conditions of nucleation and growth during the phase transformation. It is worth noting that the GRF is usually applied to quantitatively evaluate the solute effect and is expressed as follows [42]:

GRF ¼

X mi co;i ðki  1Þ

(1)

i

texture, resembling conventional deformed Mg alloys, such as AZ31, and AZ61, among others [38e41]. After adding 0.3 wt% Y, the intensity of the basal peak drops by ~46%, accompanied by a preferred orientation in the (1010) prismatic pole figure along the ED. 4. Discussion 4.1. Grain refinement mechanism of Y addition in Mg-0.5Sn alloy Generally, the grain refinement in metallic materials via alloying is mainly attributed to two factors. One is the growth restriction factor (GRF), caused by solute elements segregation. The other one

where ki refers to the solute distribution coefficient, co,i is the original content of the alloying element, and mi represents the slope of the liquidus in a binary phase diagram. A higher GRF of a solute signifies a stronger restricted impact of grain growth. According to the combination of Equation (1) and the binary phase diagrams of MgeSn and MgeY [33], the per-unit GRF values corresponding to Sn and Y in Mg are 1.47 and 1.70, respectively. Hence, not only Sn, but also Y have weak grain growth restriction impacts. In crystallisation of metals [43], if intermetallics can serve as the potential sites of heterogeneous nucleation during solidification, two rules should be satisfied: (i) the intermetallics should precipitate from the liquid before the matrix; (ii) specific crystallisation

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Fig. 7. Strain hardening rate curves of (a) as-extruded Mg-0.5Sn sheets, and (b) as-extruded Mg-0.5Sn-0.3Y sheets.

Fig. 9. DSC analysis of the as-cast Mg-0.5Sn-0.3Y alloys.

Fig. 8. Corresponding pole figures of (a) as-extruded Mg-0.5Sn sheets, and (b) asextruded Mg-0.5Sn-0.3Y sheets.

relationships should exist between the intermetallics and matrix. The SEM and XRD analysis showed that the secondary phase in Mg-0.5Sn-0.3Y was Sn3Y5, and that it was distributed inside the grains having a short-rod shape. The cooling curve, explaining the solidification route, was analysed to establish the precipitation of the specific phases during solidification of the Mg-0.5Sn-0.3Y alloys. The DSC curve in Fig. 9 indicates that there are two distinct exothermic peaks. One starts at 647  C and reaches a peak value at 642  C, while the other one starts at 545  C and reaches a peak value at 524  C. The theoretical crystallisation temperature of Mg is 650  C, and is out of the observed exothermic temperature ranges. Actually, a supercooling was necessary when nucleation and growth occurred in the melt, resulting in a lower actual phase transition temperature compared with the theoretical one [44]. Thus, the first peak in the DSC curve should be the exothermic peak of the a-Mg crystallisation. The second peak is for the Sn3Y5 phase. Under this circumstance, the solidification path of the Mg-0.5Sn0.3Y alloys can be depicted as: Liquid /（647  C）a-Mg/ （545  C）Sn3Y5. This indicates that Sn3Y5 precipitates later than

the Mg matrix; hence, Sn3Y5 cannot act as the sites of heterogeneous nucleation for the matrix during the solidification. Therefore, it can be deduced that the weak solute effect and ineffective nucleation site leads to the coarse columnar grains in the as-cast Mg-0.5Sn and Mg-0.5Sn-0.3Y alloys. Additionally, as illustrated in Fig. 1 (a) and (b), the as-cast Mg-0.5Sn alloys consisted of surrounded columnar grains and central equiaxed grains, while Mg0.5Sn-0.3Y only had columnar grains. The inversed grain refinement of the as-cast MgeSn alloys with Y addition might be because [45,46]: (i) the segregation effects of Sn and Y solutes were weakened by the reaction between them; (ii) the generated Sn3Y5 particles, which precipitated later than the Mg matrix, could neither serve as the nucleation sites nor obstruct the grain boundary migration. Comparing the as-extruded Mg-0.5Sn and Mg-0.5Sn-0.3Y alloys with the as-cast alloys, the grains were refined obviously, as exhibited in Fig. 1. As is well-known, this refinement can be attributed to the activation of DRX during the process of extrusion. Nevertheless, the grains of the as-extruded Mg-0.5Sn-0.3Y were fined by 70% than those in Mg-0.5Sn. The classical theory [35,36] suggested that the particles in metals could produce additional strain around the matrix during hot deformation. Thus, a fine dense

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substructure would appear and the nucleation of DRX was stimulated. The E2EM model [25,26] explained that the intermetallics, which had suitable ORs with the matrix, would be prone to be the effective sites of nucleation for the matrix in the solidification and DRX processes. The suitable ORs between the matrix and particles mean that there should be at least one couple of matching planes and matching rows between them. The matching planes refer to the close-packed planes with interplanar spacings (d-values), signifying a lower mismatch (fd) of below 10% (the strict critical threshold is 6%). The matching rows refer to the close-packed atomic rows with interatomic spacing misfit (fr) of less than 10% in the corresponding pair of rows. Besides this, the matching rows should be involved in the matching planes. Thus, the critical value was applied to determine the matching directions and matching planes of Sn3Y5 and a-Mg. This could be used to further explain the grain refinement mechanism of the asextruded Mg-0.5Sn alloys with Y addition. Therefore, in the Sn3Y5 and a-Mg system, the d-value mismatch of the matching planes and the r-value misfit along that direction ought to be calculated first. According to the crystallographic database and results of the powder XRD [47,48], the crystal structures of Sn3Y5 and a-Mg are hexagonal close-packed (HCP). The standard crystallographic data are listed in Table 2. The three close-packed planes of Sn3Y5 are the f2131gSn3 Y5 , f1122gSn3 Y5 and f2133gSn3 Y5 planes, while those of aeMg are the f1011gMg , f1010gMg and f0002gMg planes. Table 3 shows the fd values between Sn3Y5 and the a-Mg matrix, calculated by the cited formula [28]. It is easily found that there are four pairs of fd values below 6%, indicating that four pairs of potential matching planes might exist in Sn3Y5 and a-Mg. These are: f0002gMg / f2131gSn3 Y5 , f0002gMg / f1122gSn3 Y5 , f1010gMg / f2131gSn3 Y5 and f1010gMg / f1122gSn3 Y5 . As a typical HCP structure, the atomic configurations with the corresponding close-packed atom rows of the a-Mg of the f0002gMg and f1010gMg planes can be depicted, as shown in Fig. 10(a) and (b). In addition, the atomic configurations of the f2131gSn3 Y5 and f1122gSn3 Y5 planes for Sn3Y5 are obtained according to the standard crystallography data in Table 3, as shown in Fig. 10(c) and (d). Similarly, the fr values below 10% between Sn3Y5 and the a-Mg matrix are shown in Table 4. Therefore, the crystallographic matching relationship between Sn3Y5 and the a-Mg matrix can be predicted as:

.. . < 1129 > SSn3Y5 ; < 0001 > SMg < 0001 > SMg . . . f2131gSn3Y5 ; < 0001 > SMg  < 2116 > SSn3Y5 ; f1010gMg . .  < 1123 > SSn3Y5 ; f1010gMg f1122gSn3 Y5 : In fact, the matching atom rows maintained a parallel relationship and the matching planes were distinguished. The aforementioned ORs should be further refined by the Dg parallelism criteria. The simulated spot diffraction pattern along the zone axis [0001]Mg // [1123]Sn3Y5 is obtained according to the Dg theory, as shown in Fig. 11. More than three parallel Dg values (parallel to the habit plane) appeared in the overlapping diffraction spots, indicating that a well-matched relationship between the intermetallics

and matrix. Therefore, one of the refined ORs between Sn3Y5 and aMg and the relevant habit planes can be acquired as:

½0001Mg

..

½112 3 Sn3Y5

    ð101 0ÞMg  14:98 from 112 2 Sn3Y5

Analogously, according to the second pair of matching rows and the corresponding pair of matching planes, other one ORs between Sn3Y5 and a-Mg can also be predicted as:

½0001Mg

.. ½1 1 29Sn3Y5

    ð101 0ÞMg  0:07 from 213 1 Sn3Y5

and½0001Mg 

.. ½2 116Sn3Y5 

ð101 0ÞMg 1:67 fromð213 1ÞSn3Y5 The predictions of the ORs between Sn3Y5 and a-Mg imply the crystallographic possibility of the formation of a partially coherent interface between Sn3Y5 and aeMg. Therefore, Sn3Y5 particles can serve as effective sites of heterogeneous nucleation for a-Mg during the extrusion process, resulting in refinement of grains of Mg0.5Sn-0.3Y by ~70%, as compared to the as-extruded Mg-0.5Sn sheets which does not contain Sn3Y5. Fig. 11 shows the simulated results of the diffraction patterns of the Mg matrix and Sn3Y5 along the zone axis [0001]Mg // [1123]Sn3Y5. To fully validate the prediction from E2EM model, STEM measurements of one relatively large Sn3Y5 particle were performed, which correspond to the selected region in Fig. 5. Fig. 12(a) presents the electron diffraction pattern and shows that the incident beam of the Mg matrix orientates along [0001]Mg and the Sn3Y5 alignes along [1123]Sn3Y5. In addition, a plane-matching relationship between Mg matrix is (1010)Mg 18.84 from (1122)Sn3Y5. Further detailsinterface matching between the a-Mg and Sn3Y5 are demonstrated in Fig. 12(c). It is evident that at the interface, two close-packed directions and two close-packed planes, referring to [0001]Mg and [1123]Sn3Y5, (1010)Mg and (1122)Sn3Y5, respectively, are nearly maintain the parallel with each other. Combined with the results of the electron diffraction pattern and interfacial structure, a specific OR between the Mg matrix and Sn3Y5 can be acquired, as: [0001]Mg // [1123]Sn3Y5, (1010)Mg 18.84 from (1122)Sn3Y5. Therefore, this confirms that the TEM results well agree with the prediction of the E2EM model and the simulated diffraction patterns, indicating the reliability of the simulation. More importantly, the fairly low values of the mismatch and misfit indicate that the Sn3Y5 particles most likely serve as the effective sites of heterogeneous nucleation for the matrix and highly promote grain refinement of the Mg-0.5Sn-0.3Y alloys. 4.2. Effect of Y addition on the mechanical behaviours of the asextruded Mg-0.5Sn sheets It can be found that the YS and UTS values of the fine-grained

Table 2 Crystallographic data of Sn3Y5 phase [47]. Space group

Crystal system

Pearson

Lattice constant

Atomic coordinate

P 63/mcm

Hexagonal

hP16

a ¼ b ¼ 8.902; c ¼ 6.536

Sn (0.61 0 0.25); Y1 (0.33 0.67 0); Y2 (0.25 0 0.25)

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Table 3 Interplanar spacing mismatch (fd) (%) along the close-packed planes between a-Mg and Sn3Y5. Some matching close-packed planes with high fd values are ignored. Plane pairs

f1011gMg /f2131gSn3 Y5

f1011gMg /f1122gSn3 Y5

f0002gMg /f2131gSn3 Y5

f0002gMg /f1122gSn3 Y5

f1010gMg /f2131gSn3 Y5

f1010gMg /f1122gSn3 Y5

Mismatch (%)

8.08

7.26

1.73

0.96

2.65

5.33

Fig. 10. Atomic configuration of the matrix and intermetallics on the (a) f0002gMg , (b) f1011gMg , (c) f2131gSn3 Y5 , (d) f1122gSn3 Y5 .

Table 4 Interatomic spacing misfit (fr) (%) along the closed packed atomic directions between a-Mg and Sn3Y5. Some pairs of the close-packed atomic directions with high fr values are ignored. Direction pairs

< 0001 > SMg / < 1129 > SSn3 Y5

< 0001 > SMg / < 2116 > SSn3 Y5

< 0001 > SMg / < 1123 > SSn3 Y5

Misfit (%)

3.38

0.04

0.94

Mg-0.5Sn-0.3Y sheets (3e4 mm) along the three directions were all higher than those of the coarse-grained Mg-0.5Sn sheets (15e17 mm). This improvement of the strength might be primarily attributed to the grain refinement after adding Y into the MgeSn alloys. It is common knowledge that Mg alloys exhibit strong grain size dependence on stress owing to a lack of slip systems and their large Taylor factor [49,50]. Furthermore, except for the grain refinement, the solid-solution strengthening and precipitation strengthening caused by Y addition might also contribute to the improvement of strength in the Mg-0.5Sn-0.3Y sheets. More importantly, the tensile test indicated that, except for the improved strength, the elongation values along the three directions in the Mg-0.5Sn-0.3Y sheets were all increased by two times as compared to Mg-0.5Sn. Thus, it is necessary to discuss the effect of Y addition on the ductility of the Mg-0.5Sn extruded sheets. As previously indicated, the Y addition into the Mg alloys promoted

the activity of the non-basal dislocation slips, resulting in the weakened texture and improved ductility at RT. In our study, after adding 0.3 wt% Y into Mg-0.5Sn, the proportion of non-basal slip increased during the process of deformation. This was manifested firstly via the macro-texture. The analysis of the texture demonstrated that in the Mg-0.5Sn alloys, the main deformation pattern should be basal slip during the extrusion process. The c-axes of grains and the normal direction (ND) stayed almost parallel in the as-extruded Mg-0.5Sn sheets. When the tensile test was conducted at RT on the sheets, the Schmid factor along the ED and TD would be close to zero and the deformation of these sheets would become more difficult. Meanwhile, the Schmid factor along the 45 direction was relatively large, and this generated a strong anisotropy in the sheets with strong basal texture [51,52]. Fig. 6 and Table 1 showed that the YS value along the TD was much higher than that along 45 .

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Fig. 11. Simulated results of diffraction patterns of Mg matrix and Sn3Y5 along the zone axis. [0001]Mg // [1123]Sn3Y5, exhibiting a set of parallel Dg values. Here, the subscript ‘S’ represents Sn3Y5 and subscript ‘M’ implies the Mg matrix. The dashed line refers to the habit plane.

Fig. 12. TEM results of Mg-0.5Sn-0.3Y alloys: (a) diffraction pattern of [0001]Mg // [1123]Sn3Y5 zone, (b) diffraction pattern of Mg matrix, (c) high resolution image of the interface between the Mg matrix and Sn3Y5. The specific morphology corresponds to the aforementioned TEM images (Fig. 5b).

Interestingly, Y addition weakened the basal texture of Mg-0.5Sn, accompanied by the broadening of the range of the weak orientated texture, indicating that a large amount of the c-axes of grains rotated along the TD. Besides this, in the Mg-0.5Sn-0.3Y extruded sheets, the preferred orientation along the ED appeared in the ð1010Þ pole figure. This might be because many grains rotated along their c-axes so that the ð1010Þ prismatic plane was parallel to the ED-ND plane. Therefore, the weakened basal texture and sequential prismatic texture ensured the improvement of RT elongation of the Mg-0.5Sn-0.3Y sheets. Similarly, the deformation

behaviours during the tensile test in the cold-rolled pure Mg and € bes et al. [53]. It was Mg-3 wt.% Y alloys were studied by Sandlo found that the elongation increased from ~4% to 24% after adding Y into Mg. Next, a SEM-EBSD analysis of the basal slip band revealed that pure Mg primarily deformed by tensile twinning and basal slip, while the MgeY alloy showed a high activity of pyramidal slip, secondary twinning and compression twinning [53]. The different mechanical behaviours of the as-extruded Mg0.5Sn and Mg-0.5Sn-0.3Y sheets could be proved through the work hardening behaviour. Fig. 7 exhibited that the Mg-0.5Sn and Mg-

Please cite this article as: X.Y. Qian et al., Grain refinement mechanism and improved mechanical properties in MgeSn alloy with trace Y addition, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153122

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0.5Sn-0.3Y sheets showed a presence of different work hardening stages, indicating that they had different plastic deformation mechanisms. The as-extruded Mg-0.5Sn presented strong basal texture, subjected to the tensile stress vertical to the c-axes of grains during the tensile test. In this situation, the basal slip was hard to be activated as the Schmid factor of the basal slip was almost zero. Thus, the prismatic slip most possibly occurred in the Mg-0.5Sn sheets during the tensile test process. Nevertheless, the critically resolved shear stress (CRSS) of the prismatic slip was high and the sheets was hard to further deform. This was the reason for the gentle curve slope along all three directions in the Mg-0.5Sn sheets, as shown in Fig. 7(a). For the as-extruded Mg0.5Sn-0.3Y sheets, the weak and dispersed basal texture meant that some non-basal orientated grains existed in the sheets. The Schmid factor of the basal slip in these grains was high during the tensile test along the three directions. Thus, at stage III of the RT deformation, the basal slip was prone to be easily activated. It should be noted that there was a low CRSS of the basal slip so that the sheets could be deformed easily. This contributed to a large curve slope in the Mg-0.5Sn-0.3Y sheets, as indicated in Fig. 7(b). Therefore, the deformation process of the Mg-0.5Sn-0.3Y sheets during the RT tensile test can be deduced, as: At stage III, combined with the tensile force couple, most of the grains underwent the basal slip because of the grain rotation along the direction of the tensile force. Then, the former non-basal orientated grains rotated to the basal plane, and these grains were not available for the basal slip, which pushed the deformation to stage Ⅳ. Similar to stage III of the Mg-0.5Sn sheets, the basal-orientated grains of the Mg-0.5Sn0.3Y sheets only underwent prismatic slip at stage Ⅳ. Further deformation would be hard, assuming that the curve at stage Ⅳ was gentler than that at stage III. Overall, during the RT tensile process, the as-extruded Mg-0.5Sn sheets underwent only prismatic slip, while Mg-0.5Sn-0.3Y had both basal and prismatic slips. The more slip systems were activated, the more mobile dislocations could be provided, and the less deformation resistance would exist, and this resulted in a significant improvement of RT ductility.

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(4) The improved comprehensive mechanical properties of Mg0.5Sn-0.3Y sheets could be attributed to the following reasons. One was the fine-grain strengthening. The other one was the contribution of Y on activating the non-basal slip during the extrusion process. This led to more slip systems in Mg-0.5Sn-0.3Y during the tensile test. The more slip systems of Mg-0.5Sn-0.3Y promoted to form more dislocations and less deformation resistance, and this resulted in a significantly improved ductility at room temperature.

Declaration of interest statement We state an author agreement for this manuscript as following: All authors have participated sufficiently in this work to take public responsibility for this manuscript. All authors have reviewed the final version of the manuscript and approve it for publication. Neither this manuscript nor one with substantially similar content under our authorship has been published or is being considered for publication elsewhere. Author Contribution Statement Xiaoying Qian: Data curation, Writing-Original draft preparation. Ying Zeng: Conceptualization, Methodology, Software, Supervision, Reviewing and Editing, Validation. Bin Jiang: Reviewing and Editing. Qiurong Yang: Reviewing and Editing, Investigation. Yangjie Wan: Reviewing and Editing. Gaofeng Quan: Validation, Reviewing and Editing. Fusheng Pan: Reviewing and Editing. Acknowledgements The authors gratefully acknowledge the Fundamental Research Funds for the Project of Science & Technology Department of Sichuan Province (2018HH0026), China Scholarship Council (201907005018), the Chongqing Science and Technology Commission (cstc2017zdcy-zdzxX0006), National Natural Science Foundation of China (51531002 and U1764253).

5. Conclusions References Mg-0.5Sn and Mg-0.5Sn-0.3Y alloys were prepared by hotextrusion followed by metal mould casting. The grain refinement mechanism was discussed through the E2EM model, and the reason for the improved mechanical properties was analysed in detail. From this study, the following conclusions are drawn: (1) Both the as-cast Mg-0.5Sn and Mg-0.5Sn-0.3Y alloys showed coarse grains due to the similarly weak solute effect of Sn and Y in Mg. Meanwhile, with 0.3 wt% Y addition, the average grain size of the as-extruded Mg-0.5Sn sheets sharply decreased from ~16 mm to ~4 mm. (2) The fine grains of the as-extruded Mg-0.5Sn-0.3Y sheets could be attributed to the Sn3Y5 particles, serving as the sites of heterogeneous nucleation for the matrix during the extrusion process. A fine crystallographic misorientation between Sn3Y5 and a-Mg was established through the E2EM model: [0001]Mg // [1123]Sn3Y5, (1010)Mg 14.98 from (1122)Sn3Y5. Additionally, the great consistency indicated the accuracy of the simulations, as verified by the TEM results. (3) After adding 0.3 wt% Y into the Mg-0.5Sn sheets, both the strength and ductility were significantly improved along the three directions. In particular, the UTS, YS, and elongation values of Mg-0.5Sn-0.3Y increased by ~12%, 20%, and 229% along the TD, respectively.

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Please cite this article as: X.Y. Qian et al., Grain refinement mechanism and improved mechanical properties in MgeSn alloy with trace Y addition, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153122