CRYSTALLOGRAPHY OF BETA-2-]GAMMA-'2 MARTENSITIC-TRANSFORMATION IN AU-47.5 AT-PERCENT CD AND AU-47.5 AT-PERCENT CD-CU ALLOYS

The crystallography of the beta-2 --> gamma'-2 stress-induced and thermally-induced martensitic transformations in Au-47.5 at .% Cd and Au-47.5 at .% Cd-Cu alloys were reexamined by simultaneous measurements of the habit plane, twinning plane and orientation relationships. Although the lattice invariant shear was essentially {111}gamma'-2 Type I twinning, <121>gamma'-2 Type II twinning was also found in one case. No difference was found in the crystallography of stress-induced and thermally-induced transformations. Two non-equivalent solutions of the phenomenological crystallographic theory, Type I (+) and (-) solutions, were in excellent agreement with experimental observations. The results for the Type I (+) solution were essentially the same as those previously reported by Lieberman et al., but the agreement between theory and experiment was much better than theirs. Furthermore, (-) solution for Type I twinning and (+) solution for Type II twinning were also found for the first time. Both results show that the phenomenological crystallographic theory applies well in all respects-to the present alloy system.