Influence of precipitation on tension and compression twinning in Mg-6.5Zn alloy

Depending on the stress state, magnesium alloys can display tensile and/or compression twinning. The present letter exploits bimodal textures in alloy Mg-6.5Zn to examine how age hardening impacts on the two different twin types. We found that tensile twinning is stimulated by precipitates (i.e. higher twin volume fractions are seen at an equivalent plastic strain) whereas compression twinning is suppressed by precipitates (i.e. lower twin volume fractions are seen at an equivalent plastic strain). The difference is rationalized in terms of a lower twin surface energy and a larger twinning dislocation Burgers vector in compression twins. The larger Burgers vector renders compression twinning dislocations more resistant to bowing. The surface energy effect is more subtle. All else constant, twins with lower surface energies can propagate with a lower total content of twinning dislocations. Thus, a higher applied stress is required to attain the Peach-Koehler force needed to bypass precipitates.