<Original Papers>Two-Step Martensitic Transformation and Yield Strength in TiNi_<0.8>Cu_<0.2>

X-ray diffraction and DSC measurement revealed that TiNi_Cu_ undergoes two-step phase transformation during cooling. The first transforms from the B2 structure to an orthorhombic martensitic phase at Ms\u27=332K and then to a monoclinic martensitic phase at Ms=280K. The corresponding heats of transformations are 0.93kJ/mol and 0.29kJ/mol, respectively. The yield stress of TiNi_Cu_ was about one fourth stronger than that of Ti-50.3 mol% Ni at a parent state. On the other hand it was about one third softer than that one at a martensitic state. According to the stress-strain curves for TiNi_Cu_, two-step types with easy glide region appeared immediately up to the Ms temperature and over a temperature range where the orthorhombic martensitic phase was thermally stable. The formers were according to stress-induced transformations from B2 type parent phase to orthorhombic martensitic phase and from orthorhombic martensitic phase to monoclinic martensitic phase in succesion. The latters were due to reorientation of thermally stable martensitic phase with orthorhombic system and to stress-induced transformation from orthorhombic to monoclinic martensitic phase