Microstructure evolution of an AZ91D magnesium alloy subjected to intense plastic straining

In the present study, repetitive upsetting-extrusion (RUE) process was employed to induce severe plastic deformation (SPD) for an AZ91D magnesium alloy at temperatures ranging from 285 °C to 380 °C. Finite element method (FEM) and technical experiment were performed to reveal the non-uniform behavior during the RUE process. The results from the finite element analysis showed that effective strain was unevenly distributed in one-cycle strained sample and the non-uniformity would exist permanently with increasing deformation cycles. The microstructure evolution of the RUE processed alloys was characterized by optical microscopy, which clarified the microstructural development from heterogeneous and coarse as-cast structure, heterogeneous and refined structure to final homogeneous and fine-grained structure of the 340 °C and eight-cycle processed sample. In addition, a critical strain was suggested from the metallographic observation results. It was proposed that α-Mg grain structure was sufficiently refined by imposing accumulated strain beyond the critical value and the invariability of grain sizes was illustrated. A homogeneous structure with mean grain size of ∼3.04 μm was obtained after 285 °C and three-cycle RUE process, corresponding to an accumulated true strain of about 3.75. Furthermore, the fragmentation and dispersed distribution of the β-Mg₁₇Al₁₂ phase induced by shearing was confirmed by referring to scanning electron microscope (SEM) examination. Moreover, refining mechanisms responsible for grain structure were discussed in detail with reference to the transmission electron microscope (TEM) analysis results.