Quantitative measurements of strain-induced twinning in Mg, Mg-Al and Mg-Zn solid solutions

Abstract

The area fraction and number density of twins were determined in pre-polished tensile specimens of cast pure Mg, Mg-Al [0.5, 2 and 9 at.% Al] and Mg-Zn [0.8 and 2.3 at.% Zn] solid solutions, for different applied tensile strains. Profuse twinning developed rapidly in pure Mg, while it occurred very gradually in the Mg-Zn alloys, with Mg-Al showing an intermediate behavior. In the more dilute Mg-Al alloys the area fraction and the number density decreased with the concentration, increasing again for the 9 Al, reaching a level above that of pure Mg at large strains. In Mg-Zn both parameters decreased monotonically with the Zn concentration up to the solubility limit (2.5 at.%). For given alloy and solute content, the number density increased (the twins became smaller) for finer grain sizes in all three materials. It is argued that the availability of nucleating sites created by the microplasticity preceding profuse twinning, which is greater for the alloys due to the delayed onset of twinning, or for fine grained polycrystals due to the increased specific grain boundary area, accounts for the increased number density with the solute concentration or reduced grain size. Solute specific effects are rationalized using concepts of solid solution softening of the prismatic planes in all alloys, and the stronger solid solution hardening due to short range order in the concentrated Mg-Zn alloys.