Shape restoration effect associated with order-disorder transformation in equiatomic AuCu and AuCu-Ga alloys

Shape restoration effects associated with order-disorder transformation in equiatomic AuCu and Au_0.47Cu_0.47Ga_0.06 alloys were examined by means of surface roughness measurements of sheet specimens and the change in the distance between tips of U-shaped specimens. The specimen surface became irregular and a macroscopic shape change occurred after an ageing heat treatment. The shape change was attributed to the heterogeneity in the deformation which arose from the stress release by twinning at the free surface. After disordering of the age-hardened specimen, they regained completely to its original shape. When the specimen was aged without any constraint, spontaneous deformation occurred and did not result in any given shape. However, when the external stress was applied during ordering heat treatment, a given shape was obtained by the formation of favorably oriented ordered domains and twin lamellae. It is suggested that once the direction of deformation was decided, growth of the existing variants continued.