Effect of basal precipitates on extension twinning and pyramidal slip: A micro-mechanical and electron microscopy study of a Mg-Al binary alloy

Understanding the effects of basal precipitates on plasticity is of scientific interest and practical importance in the development of high-strength magnesium alloys. In this study, compression experiments are carried out on solution-treated and aged Mg-9wt.% Al binary alloy single-crystal micropillars cut from large-grained polycrystalline samples. The samples are compressed in two different orientations (<10-10> and <0001>) to examine the effects of basal-precipitates on extension twinning and pyramidal slip, respectively. In the solution-treated, precipitate-free, <10-10> samples, the propagation and thickening of single twins dominates, while in the aged <10-10> micropillars with Mg17Al12 basal-precipitates, multiple twins are observed suggesting a substantial increase in the stress required to propagate and thicken twins compared to nucleating twins. In addition, these basal precipitates are observed to be more effective than c-axis rod precipitates in Mg-Zn alloy in impeding twin growth for the studied volume fractions. In <0001> samples, the plate-like, Mg17Al12 precipitates are found to provide enormous strengthening under compression, compared to solution-treated samples, by hindering the motion of pyramidal dislocations. Further still, our comparative analysis suggests that pyramidal-slip/precipitate interactions, whether via bowing between or cutting precipitates, differ from one Mg alloy to another depending on the geometry and alignment of their precipitates. Our findings offer insights into how manipulating precipitation in the design and processing of Mg alloys can lead to superior mechanical properties. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.