As shown in Figure 2a, XRD analysis indicates that when the Sn content is 0.5% and 2%, only weak diffraction peaks of the Mg₂Sn phases are present in the alloys. This is because the solid solubility exhibits significant temperature sensitivity, with the maximum solubility sharply decreasing from 14.85% at the eutectic point of 561.2°C to 0.45% at 200°C. During rapid cooling, due to the limited diffusion of solute elements, a substantial amount of Sn fails to complete the precipitation process and ultimately remains in a supersaturated solid solution within the solidified alloy matrix, without forming precipitates. When the Sn content exceeds 4%, a significant number of diffraction peaks of the Mg₂Sn phase can be detected in the XRD patterns, and their intensity markedly increases with higher Sn content. This phenomenon originates from solute segregation during solidification, which leads to Sn enrichment in local regions reaching the eutectic composition, thereby promoting the formation of the Mg₂Sn phase. With further increases in Sn content, the precipitation amount of this intermetallic compound shows a clear growth trend. Figure 2b displays the binding energies of α-Mg and Mg₂Sn phase clusters calculated using valence electron theory. The results reveal that the binding energy of the α-Mg matrix is much lower than that of the Mg₂Sn phase. This is because α-Mg contains only weak metallic bonds with delocalized electrons, resulting in low bond energy, which directly leads to the lower binding energy of the α-Mg matrix. In contrast, the Mg₂Sn phase exhibits both covalent and ionic bonds. From the perspective of bond energy analysis: on one hand, the sp³ hybridization of Sn and the s-p orbitals of Mg form strong directional bonds; on the other hand, Sn acquires electrons from Mg, enhancing electrostatic attraction and thereby increasing the binding energy (Yuan et al., 2025). From a structural perspective: the unique cubic anti-fluorite structure of Mg₂Sn is denser than the hexagonal close-packed structure of Mg, with higher atomic coordination numbers, collectively enhancing stability (Fu et al., 2024).

To investigate the morphology and distribution characteristics of secondary phases in the alloys, both high-magnification and low-magnification SEM images were acquired, and the compositional analysis of secondary phases was performed using EDS spectroscopy. The homogenized Mg-Sn alloys primarily consisted of an α-Mg matrix with needle-like and network-shaped Mg₂Sn phases precipitated from the matrix, as shown in Figure 3 and Table 2. For the extruded alloys, the area fractions of phases were calculated using Image-Pro Plus software, with results presented in Figure 4 and Tables 3 and 4. The analytical results revealed that the Mg-Sn alloys were mainly composed of an α-Mg matrix and block-shaped/short rod-shaped Mg₂Sn phases. When the Sn content ranged from 0.5% to 4%, most secondary phases appeared fragmented and were distributed semi-continuously along the extrusion direction in a streamlined pattern within the matrix, exhibiting relatively uniform distribution with an average size of 0.85–0.97 μm. At 6% Sn content, the Mg₂Sn phases showed noticeable aggregation, being discretely distributed along grain boundaries as discontinuous lamellar eutectics while maintaining overall uniformity and fine characteristics. A small portion of secondary phases were crushed into gray-white lamellar structures aligned with extrusion bands during the extrusion process. When the Sn content reached 8%, the Mg₂Sn phases demonstrated increased aggregation and coarsening, forming semi-network structures at grain boundaries. During the high-temperature rapid backward extrusion process, the supersaturated Sn atoms in the homogenized alloy matrix underwent diffusion and redistribution, ultimately precipitating from the matrix in semi-network configurations.

Magnesium alloys undergoing hot extrusion plastic deformation develop deformation textures, whose presence significantly influences the mechanical properties of the alloys. Therefore, to further investigate the evolution of texture and grain orientation in the extruded alloys, EBSD technology was employed to analyze their microstructures. Figure 5 presents the grain orientation distribution maps and pole figures of the longitudinal sections of the alloys. The average grain sizes of the extruded alloys were determined through statistical calculations, as listed in Table 5. The alloy microstructures were homogeneous with relatively uniform grain sizes, predominantly exhibiting equiaxed morphology. Additionally, a small number of twins were observed in the microstructures of all five extruded alloys.

The (0001) basal texture distribution in the extruded rods primarily exhibits symmetry along the extrusion direction, with the (11 2 ¯ 0) and (10 1 ¯ 0) crystal planes showing more concentrated orientation distributions compared to the (0001) plane. As the Sn content increases, the macroscopic texture distribution of each crystal plane does not change significantly, but the texture intensity alters. Compared to the texture intensity of Mg-0.5Sn (7.35), those of Mg-2Sn, Mg-4Sn, Mg-8Sn and Mg-6Sn increase to 7.39, 7.62, 12.73 and 12.81, respectively. This indicates that increasing Sn content enhances the texture intensity.

To investigate the influence mechanisms of grain boundary angles and twin boundaries on mechanical properties, we systematically analyzed the misorientation angle distributions of all five alloys, as shown in Figure 6. The results demonstrate that all five alloys exhibit similar distribution trends, primarily consisting of a large proportion of low-angle grain boundaries (LAGBs, 2°–15°) and a small fraction of high-angle grain boundaries (HAGBs, ≥15°). This phenomenon can be attributed to the continuous increase in strain leading to intensified intragranular deformation, which significantly elevates the intragranular orientation gradient and consequently promotes the formation of HAGBs. Twin boundary characterization reveals that when defining boundaries with misorientation angles of 86° ± 5° as twin boundaries according to crystallographic standards, all five alloys display distinct characteristic peaks at ∼86°. This finding confirms that substantial twinning deformation occurred during the deformation process, forming typical twin structures in all alloys. From the distribution perspective, LAGBs dominate in all five alloys, which aligns with the evolutionary pattern of grain refinement and increasing orientation gradients during plastic deformation. Simultaneously, the presence of characteristic peaks at ∼86° further indicates that twinning deformation plays a significant role in the plastic deformation process.

Figure 7 presents the recrystallization microstructure distribution characteristics of extruded Mg-xSn alloys with varying Sn contents (x = 0.5, 2, 4, 6, 8 wt%). Through electron backscatter diffraction (EBSD) analysis, the microstructures were systematically differentiated into dynamically recrystallized grains (blue), sub-grains (yellow), and deformed grains (red) using color coding, revealing the influence of Sn content on dynamic recrystallization behavior. The results demonstrate that as Sn content increases, the volume fraction of dynamically recrystallized grains exhibits a nonlinear trend of initial decrease followed by increase, while the distributions of subgrains and deformed grains show opposite evolutionary characteristics. This microstructural evolution originates from the dynamic equilibrium mechanism between stored energy release and interface reorganization: deformed grains, as initial high-energy-state structures, gradually evolve into sub-grains through dislocation rearrangement, ultimately transforming into dynamically recrystallized grains under appropriate thermodynamic conditions to achieve microstructural homogenization. Notably, when Sn content falls within the 2–4 wt% range, the alloys display pronounced recrystallization suppression, likely attributable to the drag effect of Sn solute atoms on grain boundary migration. However, when Sn content increases to 6–8 wt%, the recrystallization degree significantly improves, suggesting that higher Sn concentrations may promote recrystallization by altering stacking fault energy or dislocation motion modes.

The Schmid factor (SF), as a critical parameter characterizing the activation difficulty of crystal slip systems, quantitatively reflects the resolved shear stress efficiency of specific slip systems under applied stress (Zhang et al., 2014). In this study, EBSD technology was systematically employed to determine the SF distributions of basal <a>, prismatic <c>, and pyramidal <c+a> slip systems during tensile deformation along the extrusion direction in Mg-xSn (x = 0.5–8 wt%) alloys, revealing the influence of Sn content on deformation mechanisms, as illustrated in Figure 8. Under room-temperature deformation conditions, magnesium alloys exhibit significant differences in critical resolved shear stress (CRSS) among various slip systems: basal slip possesses the lowest CRSS and, combined with its high slip plane density, serves as the primary carrier of plastic deformation (Zhai et al., 2024). Meanwhile, twinning deformation, as another important deformation mechanism with relatively low activation stress (2–3 MPa), provides additional strain accommodation paths (Li et al., 2024). Experimental results demonstrate that as Sn content increases from 0.5 wt% to 8 wt%, the basal SF value monotonically decreases from 0.221 to 0.181, while the prismatic SF increases from 0.434 to 0.449, and the pyramidal SF remains stable within 0.389–0.409. This trend indicates that Sn alloying significantly alters the orientation characteristics of magnesium alloys, specifically manifested as: the geometric softening effect of basal slip continuously weakens with increasing Sn content, while the potential deformation capacity of non-basal slip systems gradually enhances. Detailed analysis of the deformation mechanism transition reveals that when basal SF values drop below 0.2 (Mg-4Sn to Mg-8Sn alloys), the activation difficulty of basal slip significantly increases. This phenomenon primarily originates from the elevated CRSS caused by the pinning effect of Sn solute atoms on basal dislocations. Notably, although prismatic and pyramidal slips exhibit higher SF values (>0.43), their actual contributions at room temperature remain limited due to the intrinsically high CRSS characteristics of non-basal slips in HCP structures. Under these circumstances, {1,012}<1011> tensile twinning (SF ≈ 0.49), with its lower activation stress, likely becomes the dominant mechanism for accommodating c-axis strain, a conclusion that aligns well with previous reports on Sn-induced twinning promotion (Zhou, 2024). In low-Sn-content alloys (Mg-0.5Sn), the basal SF value of 0.221 approaches the typical activation threshold for basal slip in HCP metals (SF ≈ 0.2), indicating basal slip remains the primary deformation mode. However, with increasing Sn content, the synergistic effect between decreasing basal SF and increasing non-basal SF values ultimately leads to a transition in deformation mechanisms from basal-slip-dominant to a coordinated combination of twinning and non-basal slip. In summary, Mg-Sn alloys exhibit pronounced composition-dependent deformation mechanisms: basal slip dominates at low Sn contents (≤2 wt%), while a coordinated deformation mode combining twinning and non-basal slip emerges at high Sn contents (≥6 wt%).

Figure 9 presents the engineering stress-strain curves of five extruded Mg-Sn alloy rods, with their mechanical properties summarized in Table 6. The results demonstrate that all alloys exhibit considerable ductility, with elongation values exceeding 14%. As Sn content increases, the elongation shows an inverted N-shaped trend of initial decrease, subsequent increase, and final reduction, with Mg-4Sn alloy achieving the maximum elongation (17.5%). Microstructural analysis suggests this enhanced ductility likely correlates with its relatively high proportion of low-angle grain boundaries (LAGBs) and degree of dynamic recrystallization (DRX). Regarding strength characteristics, increasing Sn content improves mechanical performance. When Sn content reaches 8 wt%, the Mg-8Sn alloy demonstrates optimal strength properties, with yield strength (YS) and ultimate tensile strength (UTS) reaching 236 MPa and 144 MPa, respectively. Compared to the low-Sn-content Mg-0.5Sn alloy, the Mg-8Sn alloy shows remarkable strength enhancement. Microstructural comparison reveals that the Mg-8Sn alloy possesses finer grain size and higher pole density in its deformation texture. Therefore, we conclude that the superior strength of Mg-8Sn alloy primarily results from the combined effects of grain refinement strengthening and texture strengthening.

Through correlative analysis of fracture morphology and mechanical properties in Mg-Sn alloys, this study reveals the intrinsic relationship between fracture mechanisms and microstructural evolution. Fractographic analysis demonstrates composition-dependent fracture characteristics. As observed in Figure 10 SEM images, low-Sn-content alloys (Mg-0.5Sn) exhibit typical ductile fracture features characterized by dimples with non-uniform sizes. Combined with EDS results in Table 7, the second-phase particles in Mg-4Sn alloy dimples are identified as Mg₂Sn phase. This observation aligns with the mechanical properties (Rm = 206 MPa, ε = 16.8%), indicating a fracture process involving: initial interfacial decohesion between second-phase particles and matrix forming microvoids, subsequent void growth during plastic deformation, and final fracture through void coalescence. When Sn content increases to 2 wt% (Mg-2Sn), the alloy demonstrates improved tensile strength (221 MPa) with 14.3% elongation. The fracture surface maintains ductile characteristics but shows more uniform dimple size distribution, reflecting improved Mg₂Sn phase dispersion. This microstructural feature enables simultaneous strength enhancement and preserved ductility. Notably, Mg-4Sn alloy exhibits optimal strength-ductility balance (Rm = 227 MPa, ε = 17.5%). Its fracture surface displays homogeneous fine dimples, suggesting that approximately 4 wt% Mg₂Sn phase effectively promotes dislocation accumulation for strengthening without significantly compromising plasticity. This synergy likely relates to optimized dynamic recrystallization. With further Sn increase to 6-8 wt% (Mg-6Sn and Mg-8Sn), while tensile strengths reach 232 and 236 MPa respectively, elongations stabilize around 16%. The fracture morphology shows transitional characteristics: retained dimple structures coexist with cleavage features, forming typical mixed-mode fracture. This transition indicates gradual evolution from purely ductile to quasi-cleavage fracture with increasing Mg₂Sn content. Particularly in Mg-8Sn alloy, fine DRX grains and strong deformation texture contribute to excellent strength, while excessive second phases enhance local embrittlement tendency.

From the perspective of microstructural evolution during fracture: low-Sn alloys (0.5–2 wt%) primarily rely on dislocation slip and interfacial decohesion to accommodate deformation; medium-Sn alloys (4 wt%) achieve strength-ductility synergy through optimized DRX degree and second-phase distribution; while high-Sn alloys (6–8 wt%), despite exhibiting easier crack propagation along phase boundaries due to abundant Mg₂Sn phases, still maintain certain plasticity through grain refinement strengthening effects.