The inhibition of deformation twinning by precipitates in a magnesium-zinc alloy

Abstract

Deformation twinning can be inhibited in a Mg-5·1 wt. % Zn alloy by the presence of effective precipitates. This is true even in single crystals favourably oriented for twinning. This resistance to twinning precludes those premature fractures which frequently nucleate at twins. At 4·2°k twinning is highly favoured but effective precipitation postpones it until twice the glide strain observed in quenched crystals with no precipitates. Twin related fracture at 4·2°k is resisted in the fully hardened condition until three times the glide strain for fracture in the quenched condition. In polycrystals, grain size and texture as well as precipitation have profound effects on the deformation mechanism as a function of temperature. The flow stress in the quenched ease can decrease with decreasing temperature when grain size is large and the texture is appropriate for twinning. This behaviour is similar to that of quenched single crystals when twinning predominates. Conversely, the flow stress increases with decreasing temperature when the grain size is small and the texture is inappropriate for twinning. The latter behaviour is characteristic of slip controlled behaviour. Effective precipitation also encourages slip controlled behaviour. A Petch analysis showed very close correlation between the friction stress resisting twin growth in quenched polyerystals and the normal yield stress of single crystals undergoing twinning.