Formation of deformation twins in f.c.c. crystals

It is proposed that the dislocation reaction 12〈011〉 + 12〈101〉 → 3 × 16〈112〉 may govern the nucleation of three-layer twins in f.c.c. crystals. Conceivably a macroscopic twin may evolve when these three-layer embryonic twins, distributed at different levels within a localized slipped region, grow into each other. Using transmission electron microscopy, the circumstantial evidence in support of the model has been developed by correlating the crystallographic features of twins, faults and associated slip in cobalt-9.5 wt. % iron alloy deformed at 77°K. The observed correlations are totally consistent with the suggested reaction. They cannot, however, be explained in terms of the dislocation reaction considered by Venables and Cohen and Weertman. Our “glide” sources for twins are compared with Venables' “glide” and “prismatic” sources. On the basis of both models, it is possible to satisfactorily account for the orientation dependence of the twinning behavior observed by Blewitt, Coltman and Redman, and Suzuki and Barren. However, the crystallography of twins observed by Chin, Hosford and Mendorf during constrained deformation of the [110] (110) oriented cobalt-8 wt. % iron crystal cannot be rationalized on the basis of Venables' and Cohen-Weertman's approach, although it can be explained in a satisfactory manner in terms of our model.