Understanding deformation mechanisms and grain-size effects in SIMA-processed AZ91 alloy during high-solid-fraction semi-solid deformation

The deformation behavior of semi-solid magnesium (Mg) alloys is strongly affected by the coexistence of solid grains and intergranular liquid, yet the deformation accommodation mechanism under high-solid-fraction conditions remains insufficiently understood. In this study, the semi-solid compression behavior and grain-size-dependent orientation evolution of strain-induced melt activation (SIMA)-processed AZ91 alloy were investigated by hot compression tests, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and molecular dynamics (MD) simulations. The SIMA-treated alloy exhibited a spheroidized, nearly equiaxed microstructure with a retained extrusion texture. With increasing temperature, the contribution of liquid-assisted interfacial accommodation increased, particularly at 480–490 °C, while the solid grains remained the principal load-bearing framework. EBSD results revealed a clear grain-size-dependent crystallographic response: large grains retained stronger preferred orientations and accumulated higher intragranular orientation gradients, whereas small grains showed weaker textures, lower grain reference orientation deviation (GROD) values, and more diffuse misorientation-axis distributions, consistent with a greater tendency for grain-scale reorientation. MD simulations further showed that shear strain and non-affine squared displacement (NSD) were preferentially concentrated at grain boundaries, interface-adjacent regions, and structurally disordered atomic environments, while small grains exhibited higher rotational capability than large grains. TEM observations and MD defect analysis confirmed that constrained large grains could activate localized intragranular plasticity involving dislocations and stacking-fault-like defects. These results demonstrate that high-solid-fraction semi-solid compression of SIMA-processed AZ91 alloy is governed by a coupled mechanism involving liquid-assisted interfacial accommodation, grain-scale rotation/rearrangement, and localized intragranular plasticity. Grain size strongly influences the relative participation of grain-scale reorientation and intragranular plastic accommodation, providing a grain-size-dependent framework for understanding deformation coordination in semi-solid Mg alloys.