Effect of strain rates on mechanical behavior, microstructure evolution and failure mechanism of extruded-annealed AZ91 magnesium alloy under room-temperature tension

Abstract

Loading rate is usually considered as a significant factor affecting formability of AZ91 magnesium alloy structural components. Herein, uniaxial tensile tests are conducted on extruded-annealed AZ91 alloy under room temperature with strain rates ranging from 1 × 10−4 s−1 to 1 × 10−2 s−1. Mechanical properties, microstructure characteristics, and fracture morphologies tensioned at these different strain rates are investigated. Extruded-annealed AZ91 alloy exhibits a positive strain-rate dependence of strength and a negative strain-rate dependence of elongation. With strain rate increasing, strain rate sensitivity basically decreases while strain-hardening rate increases. At higher strain rate, the larger strength is mainly attributed to the more rapid accumulation of dislocation and the larger dislocation density, and the lower elongation is mostly owing to the more elevated level of inhomogeneous deformation related with dislocations accumulation. As strain rate increases, the area fraction of basal texture increases while that of prismatic and pyramid texture decreases in tensioned samples, which is associated with the dominant room-temperature deformation mode of basal-slip and twinning co-induced lattice rotation. With tension strain rate increasing, there are three kinds of fracture modes including ductile facture with more and deeper dimples at low strain rate, ductile-brittle facture with some dimples and cleavage planes at mediate strain rate and brittle facture with dominant cleavage planes at high strain rate.