Mechanisms of Hardening in HCP Structures through Dislocation Transmutation and Accommodation Effects by Glide Twinning: Application to Magnesium

Oppedal, A. L. (2010). Mechanisms of Hardening in HCP Structures through Dislocation Transmutation and Accommodation Effects by Glide Twinning: Application to Magnesium. Mississippi State University: Mississippi State University.

At low temperatures, glide twinning activates in HCP structures easier than non-close packed slip necessary to accommodate strain along the c-axis. In contrast to slip, twinning occurs as an accumulation of successive stacking faults that properly report reconstruction of the stacking sequence in a new crystal-reorientated lenticular lamella. These faults are spread by partial dislocations known as twinning dislocations,forcing atoms to switch positions by shear into new crystal planes. As the twinning dislocations thread the faults, the new crystal lamella grows at the expense of the parent. Grain texture changes upon strain, and a strong non-linear trend marks the strain hardening rate. The strain hardening rate changes to a point where it switches sign upon strain. Since activation of these twinning dislocations obey Schmid’s law, twinning could be precluded or exhaustively promoted in sharp textures upon slight changes in loading orientations, so strong anisotropy arises. Moreover, a twinning shear can only reproduce the stacking sequence in one direction, unless the twin mode changes or the c/a ratio crosses a certain ratio. When a twin mode arises with reversed sign, the reorientation is different and more importantly, the strength is different and also the growth rate. Therefore, in addition to strain anisotropy, twin polarity induces a strong asymmetry in textured HCP structures, e.g. wrought HCP metals. This anisotropy/asymmetry is still a barrier to the great economic gain expected from the industrialization of low density, high specific strength and stiffness, HCP Magnesium. This barrier has stimulated efforts to identify the missing links in current scientific knowledge to proper prediction of Magnesium anisotropy. The effect of twinning induced texture change on the mechanical response is of a major concern. Mesoscale modelers still struggle, without success to predict simultaneously twinning and strain hardening rates upon arbitrary loading directions. We propose a new mechanism that relies on admitting dislocation populations of the twin by dislocations transmuted from the parent when they intersect twinning disconnections. These dislocations interact with original dislocations created in the twin to cause hardening able to faithfully capture anisotropy upon any loading orientation and any initial texture.