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

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 528; no. 1; pp. 208 - 219
• Main Authors: Patel, H.A., Chen, D.L., Bhole, S.D., Sadayappan, K.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier B.V 25-11-2010; Elsevier
• Subjects: Amplitudes; Applied sciences; Casting alloys; Cyclic deformation; Die casting; Die castings; Exact sciences and technology; Fatigue; Foundry practice; Fractures; Magnesium alloy; Magnesium base alloys; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Pseudoelastic modulus; Semi-solid processing; Strain; Strain ratio; Thixomolding; Twinning; Cyclic deformation; Twinning; Strain ratio; Thixomolding; Magnesium alloy; Pseudoelastic modulus; Thermal stress; Fatigue life; Wrought alloy; Mechanical twin; Low cycle fatigue; Fatigue crack; Crack propagation; Hardening; Magnesium base alloys; Superelasticity; Thixoforming; Cyclic load; Mechanical hysteresis; Hysteresis; Crack initiation
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2010.09.016

▶ 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.
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.