Deformation of NiTiCu shape memory single crystals in compression

Single-crystal orientations of NiTi10Cu alloys were studied under increment al, cyclic compression conditions to establish the pseudoelastic and shape memory response of this class of alloys. This material exhibits a two-step transformation involving cubic to orthorhombic: martensite (B2 --> B19) fol lowed by orthorhombic to monoclinic martensite (B19 --> B19'). The transfor mation parameters (shear magnitudes and directions for habit and twin plane s) were determined associated with the B2 --> B19 transformation. The growt h of monoclinic martensite correspondent variant pairs (CVPs) emanating fro m the orthorhombic structure was also analyzed. The transformation strain f or the B2 --> B19 case was orientation dependent and lower than the B19 --> B19' transformation in compression for all orientations except those near the [001] pole. The experimental results show that the critical transformat ion stress is orientation dependent and is in the range 30 to 58 MPa. Orien tations that exhibit lower transformation stress (or high resolved shear st ress factors, [100] and [012]) produce higher recoverable strains las high as 4 pet), while other orientations ([011], [111], and [123]) with lower re solved shear stress factors result in recoverable strains less than 3 pet. At higher strains, inelastic deformation develops, limiting recoverability. The recoverable strains are lower than the theoretical values for two main reasons: the transformation is curtailed first by austenite slip and subse quently by martensite slip, and the orthorhombic structure does not fully t ransform to the monoclinic martensite.