Insights into Static Solute Strengthening and Dynamic Strain Aging Behavior of Hexagonal Close Packed Mg Alloys

The addition of solutes to hexagonal close packed (HCP) Mg has been extensively studied due to interests in the broad applicability of strong and lightweight alloys. However, the use of solute strengthening can pose challenges when manufacturing processes or operating conditions force the material into a range of temperatures and strain rates at which the alloys exhibit a negative strain rate sensitivity, a phenomenon which has come to be known as dynamic strain aging (DSA).

In order to develop a complete understanding of these complex phenomena, first the static (low temperature) strengthening effects of a number of important alloying additions to Mg were explored: Dy, Sc, Y, and Zn. At low temperatures, Basinski et al. postulated the principle of “stress equivalence”, which stems from work on thermally activated plasticity of solid-solution-strengthened single crystal FCC and HCP alloys. Low (77 K – ambient) temperature mechanical tests of single crystals of Mg–Zn, Mg–Y, and Mg–Dy alloys were recently performed at Hokkaido University, Japan. Similarly, polycrystalline samples of Mg–Sc and Mg–Y alloys were tested at McMaster University, Canada. These data were reanalyzed within the theoretical framework of thermally activated plasticity. The analysis sought to test the applicability of the concept of “stress equivalence” outside the context of single crystal tests alone. This required the use of an effective Taylor factor acquired from a polycrystal plasticity model which accounts for the significant impacts that crystallographic texture and strain path can have on the deformation mechanism activity within Mg alloys. The results show that the principle of stress equivalence does hold, which means that predictions of the low temperature thermally activated tensile response of textured, polycrystalline Mg alloys can be made provided the flow stress at a single strain rate and temperature is known.

At higher temperatures, the solute become mobile and the diffusional interactions between the solute and the stress fields of dislocations are held responsible for DSA and, in some cases, serrated flow. It has been widely observed that serrated flow does not ensue until reaching a critical strain, suggesting a key role of forest dislocation interactions. The second portion of this thesis seeks a deeper understanding of the role of forest dislocation interactions in DSA. Different reactions between mobile and forest dislocations can occur, including the formation of Lomer-Cottrell locks, Hirth locks, glissile junctions, and collinear annihilation. A code has been written based upon an anisotropic line tension model of dislocation interactions, which may be used to predict zero stress junction lengths as well as junction strengths (the stresses required for a mobile dislocation to escape) in the presence of solute which may bind the forest dislocation. The code has been benchmarked using theoretical predictions for FCC crystals which are available in the literature. It is now possible to extend these predictions to explore the distinctions which exist between FCC and HCP crystals since there are no locking configurations between the most common dislocations in the latter, only glissile junctions and collinear annihilation.