Exploring stress equivalence for solid solution strengthened Mg alloy polycrystals

Abstract

The principle of “stress equivalence” proposed by Basinski et al. offers insight regarding the nature of thermally activated plasticity of solid solution alloys of FCC and HCP (Mg) crystal structures at low temperatures. More recently, Leyson and Curtin have developed a theoretical framework which builds upon the Labusch solute strengthening model. The current work analyzes some recent results from the literature (for single crystal Mg–Zn, Mg–Y, and Mg–Dy) which permit a further examination of the concept of “stress equivalence.” Then, using an effective Taylor factor obtained from polycrystal plasticity modeling of textured Mg, the results of cryogenic mechanical tests of polycrystalline Mg–Sc and Mg–Y alloys are analyzed to test their adherence to the principle of “stress equivalence.” The analysis accounts for the truly long-range strengthening effect of grain boundaries through established, temperature dependent Hall-Petch relationships, but makes no other assumption of an “athermal stress” that is frequently mentioned in the literature. The results emphasize the broad applicability of Basinski's principle of stress equivalence, by showing that the low temperature thermally activated tensile response of textured, polycrystalline Mg alloys can be predicted once the flow stress at a single rate and temperature is known.