Low cycle fatigue properties of an extruded AZ31 magnesium alloy

Lightweight magnesium alloys are being increasingly used in automotive and other transportation industries to achieve energy efficiency and environmental protection. Design of magnesium components requires low cycle fatigue (LCF) behavior since these applications are often subjected to cyclic loading and/or thermal stresses. The objective of this investigation was to study the cyclic deformation behavior and LCF life of a large solid extruded section of AZ31 magnesium alloy. It was observed that the alloy was cyclically stable at lower strain amplitudes and exhibited cyclic hardening characteristics at higher strain amplitudes, with a cyclic hardening exponent of about 2.6 times higher than the monotonic strain hardening exponent. A relationship between the plastic strain amplitude (Δεp2) and the number of cycles (N), Δεp2=α+βlog(N),was observed. With increasing total strain amplitude both plastic strain amplitude and mean stress increased and the fatigue lifetime decreased. Bauschinger effect was pronounced at higher strain amplitudes, resulting in asymmetric hysteresis loops due to twinning in compression during unloading and subsequent detwinning in tension during loading. Modulus during cyclic deformation was constant at the low strain amplitude, but it decreased with increasing strain amplitudes and increased with increasing number of cycles at the high strain amplitudes due to the presence of pseudoelastic behavior. Fatigue parameters following the Coffin-Manson and Basquin’s equations were evaluated. Fatigue crack initiation was observed to occur from the specimen surface and crack propagation was characterized by striation-like features coupled with secondary cracks.