Cyclic deformation and twinning in a semi-solid processed AZ91D magnesium alloy

Research highlights: → Semi-solid processed Mg alloy has a higher fatigue life than its die cast counterpart. → A strong non-linear or pseudoelastic stress-strain characteristic appears. → Pseudoelastic modulus decreases with increasing total strain amplitude. → Wider lenticular extension twins and narrower banded contraction twins are observed. - Abstract: To improve fuel economy and reduce greenhouse gas emissions, magnesium alloys are 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 (LCF) behavior, since cyclic or thermal stresses are frequently encountered in many structural applications. The objective of this study was to examine LCF behavior and the role of twinning on cyclic deformation characteristics in a semi-solid processed (thixomolded) 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 (0.1-0.2%); exhibited cyclic softening followed by cyclic hardening for the remaining life at intermediate strain amplitudes (0.3-0.8%) and cyclic hardening at higher strain amplitudes (1.0-1.2%). As the total strain amplitude increased, the stress amplitude and plastic strain amplitude increased, while the pseudoelastic 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 pseudoelastic stress-strain characteristic remained in both ascending and descending phases especially at higher strain amplitudes due to the occurrence of twinning and detwinning 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 10-20% 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.