S. Eucken, T. Duerig
Aug 1, 1989
Abstract Underaged Ti-50.85at.% Ni wire was pseudoelastically prestrained between 2 and 8%. A second deformation to the same strain was found to occur at stresses some 60 MPa lower than the flow stresses measured during the prestraining. Deformation beyond the original prestrain required increasing the stress to the original levels. This created two distinct plateaus in the stress-strain behaviour. Related shifts in the transformation temperatures were observed during cooling and heating of the wire under constant load. Ageing the specimens at 160°C (433 K) after the first cycle erases the effect completely. The results give no evidence that either changes in the dislocation structure or sheared precipitates are responsible for this phenomenon. It is suggested that small martensite nuclei persist above Af after unloading which favour the formation of convenient martensite orientations during subsequent deformation. These martensite orientations also form during stress-free cooling of the wires causing a strong two way shape memory effect.