To improve fuel economy and reduce greenhouse gas emissions, magnesium alloys were being considered for automotive and aerospace applications because of their high strength-to-weight ratio. The structural applications of magnesium components require understanding of low-cycle fatigue behavior, since cyclic or thermal stresses were frequently encountered in many structural applications. The objective of this study was to examine low-cycle fatigue behavior and the role of twinning on cyclic deformation characteristics in a semi-solid processed (thixomoulded) AZ91D magnesium alloy. The semi-solid thixomolded alloy showed a significantly higher fatigue life than its die cast counterpart especially at the lower strain amplitudes. The alloy was cyclically stable at lower strain amplitudes (0.1 to 0.2%); exhibited cyclic softening followed by cyclic hardening for the remaining life at intermediate strain amplitudes (0.3 to 0.8%) and cyclic hardening at higher strain amplitudes (1.0 to 1.2%). As the total strain amplitude increased, the stress amplitude and plastic strain amplitude increased, while the pseudo-elastic modulus decreased. Unlike the wrought magnesium alloys reported in the literature, the hysteresis loops of the thixomolded AZ91D alloy were symmetrical in tension and compression. However, a strong non-linear or pseudo-elastic stress–strain characteristic remained in both ascending and descending phases especially at higher strain amplitudes due to the occurrence of twinning and de-twinning during cyclic deformation. Two types of twins (wider lenticular extension twins and narrower banded contraction twins) were observed in some favorably oriented larger primary α-Mg grains near the fracture surface. Fatigue life increased with decreasing strain ratio, and partial mean stress relaxation occurred mainly in the initial 10 to 20% of fatigue life. Fatigue crack initiation was observed to occur from the specimen surface or near-surface defects, and crack propagation was characterized by striation-like features.

Cyclic Deformation and Twinning in a Semi-Solid Processed AZ91D Magnesium Alloy. H.A.Patel, D.L.Chen, S.D.Bhole, K.Sadayappan: Materials Science and Engineering A, 2010, 528[1], 208-19