Anelastic deformation during cyclic loading-unloading of die-cast magnesium alloys

摘要: Magnesium (Mg) alloys have a complex progression of deformation mechanisms due to their hexagonal closed-packed crystal structure. This type of crystal structure does not provide sufficient independent basal slip systems to satisfy the von Mises-Taylor criterion [1] and the non-basal slip mechanisms (< a> prismatic and< c+ a> pyramidal) only activate at higher stress levels at room temperature [2]. Consequently, twinning plays an essential role in the deformation of Mg and its alloys [3, 4]. Twinning in Mg alloys is unstable in the deformed state and twinning can partially revert during unloading [5, 6], giving rise to large hysteresis loops (ie large anelastic strain) in the stress-strain curve as observed in pure Mg and Mg-Zn alloys [7], AM40, AM60 (Mg-6Al-0.3 Mn), AZ91 (Mg-9Al-0.6 Zn) and AE44 [4] under cyclic loading. The study of anelasticity is important as it influences several properties including yield strength [8, 9], fatigue strength [10], apparent stiffness [7, 11] and sound dampening [6].This study focuses on high-pressure die-cast alloys. Die-cast alloys account for the large majority of Mg alloy usage and anelasticity contributes significantly to deformation due to the fine grain size. Experiments showed an increase in anelasticity in fine-grained AZ91 due to formation of fine and unstable twins, which are more prone to revert upon unloading [11]. Apart from grain size, solute is reported to influence anelasticity, although solute effects are less predictable. A monotonic decrease in anelasticity was observed with increasing Zn [7] and Gd [12] contents, but a similar trend was not observed in Mg-Al alloys; both Mg-0.5 Al and Mg-2Al alloys showed …