The Effect of Precipitates on Twinning in Magnesium Alloys

Understanding how to strengthen against twin mediated deformation is critical for controlling the mechanical properties and formability of magnesium. One route to strengthening is through precipitation of shear resistant particles. This paper presents the current understanding of how precipitate particles and twins interact in magnesium, and how this influences strength. Precipitates increase the yield stress for twin dominated deformation but none of the current models reliably predict the strengthening effect. Precipitates have never been observed to completely suppress twinning, but usually lead to an increased number of narrower twins. Precipitates are not effective in increasing the stress for twin nucleation sufficiently to preferentially activate alternative deformation modes, but the effect on the stresses for both twin propagation and growth are significant. In the propagation stage, twin tips overcome precipitates by bowing of multiple twinning dislocations. For thick twins this can probably be assisted by pile up of twinning partials behind the tip. During growth, precipitates increase the stress to propagate new twinning steps, mainly due to increasing the unrelaxed back-stress. Since nucleation, propagation, and growth of twins all have to occur before macroscopic yield, a complete model to predict the strengthening effect must consider all of these stages.