Grain Boundary Controlled Superplasticity of FSP AZ91 Mg Alloy
摘要
Commercial as-cast AZ91 Mg alloy with a dendritic microstructure and an average grain size of 195 μm was subjected to multipass friction stir processing (FSP). The processing modified the dendrites to a microstructure containing fine grains with an average grain size of 5.8 μm. In addition to refinement, FSP led to the generation of twins in the microstructure. To understand the effect of twins on superplastic deformation, tensile samples with gauge length along the processing direction were subjected to deformation at strain rates varying from 10−4 to 10−3 s−1 and temperatures ranging from 473 to 573 K. A maximum elongation of over 400% was observed for a sample tested at a strain rate of 4 × 10−4 s−1 and a temperature of 573 K. The grain boundary sliding (GBS) mechanism dominated the superplastic deformation with dislocation activity as an accommodation mechanism. It is shown that the interaction of dislocations with twins led to a reduction in the twin boundaries due to the loss of coherency. The prior twin boundary is also shown to form a curvature by dislocation climb and absorption. In the superplastic regime, a reduction in twins contributes to an increase in the fraction of boundaries where grain boundary sliding can occur.