Microstructure evolution during high strain rate tensile deformation of a fine-grained AZ91 magnesium alloy

摘要

A fine-grained AZ91 magnesium alloy prepared by submerged friction stir processing is subjected to high temperature tensile test at 623 K and 2 × 10~ (-2)s~(-1) to intermediate strains of 270%, 510%, 750% and failure strain of 990%, and microstructure evolution of the experimental material during tensile test is investigated. The initial grain size is about 1.2 μm. Microstructures within the gauge region are much finer than those of grip region, and the grain aspect ratios remain approximately 1.0 in the whole superplastic deformation. With the tensile strains increasing, the average size of p-Mg_(17)Al_(12) particles increases, and the density of the β-Mg_(17)Al_(12) particles decreases. Due to the pinning effect of p-Mg_(17)Al_(12) particles and the occurrence of DRX, the fine microstructures are maintained in the whole superplastic deformation process. Grain boundary sliding is the main deformation mechanism, and cavities are formed in the triple junctions of grains and around the second phase particles during deformation. The excellent high strain rate superplasticity of the AZ91 magnesium alloy is mainly attributed to its initial fine microstructure and good thermal stability.