Microplasticity-based rationalization of the room temperature yield asymmetry in conventional polycrystalline Mg alloys

The aim of the present work is to assess the activity of the dominant deformation slip systems in the commercial cast alloy Mg-9%wt. Zn (AZ91) in the as-rolled, solution treated and peak-aged conditions. It is well known that the microstructure of this alloy is formed by alpha-Mg grains, beta-Mg17Al12 phases which precipitate mostly as plates parallel to the basal plane, and a smaller fraction of Al8Mn5 particles with a more equiaxed geometry. Earlier works have shown as well that even after peak aging treatments, very limited hardening can be achieved. Here, EBSD-assisted slip trace analysis is utilized to determine qualitatively the relative activities of basal, prismatic and pyramidal systems in the three microstructures under scrutiny following tensile deformation to a given strain level at ambient temperature and quasi-static rates. The incidence of the different systems is then related to the alloy grain size, texture, and precipitation state. Finally, the macromechanical behavior of the AZ91 alloy and, in particular, the strength, the ductility and the yield stress asymmetry are rationalized based on the measured slip activities. It is shown, in particular, that basal slip is the dominant deformation mechanism under in-plane tension at room temperature in all the microstructures investigated and that the yield stress asymmetry appears to be strongly related to the different stress levels required to activate basal slip (in tension) and twinning (in compression). Recommendations for alloy design are given on the light of these findings. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.