Effect of aluminium ion implantation on shape memory properties of titanium-nickel alloy

Aluminium ion implantation into near equi-atomic titanium-nickel alloy was carried out to clarify the effect of the surface characteristics on the sha pe memory properties in cyclic deformation. Obvious changes in deformation behavior were observed in the ion-implanted specimens. At a low implantatio n dose, the R phase accommodation process followed by the martensite phase deformation proceeded at a lower stress than that of the non-implanted allo y. On the other hand, stress-strain curves of the specimen which has a high implantation dose revealed the characteristics of a mixed structure consis ting of the R phase, the martensite phase and the austenite phase. In the l atter case, the shape memory properties were degraded and almost disappeare d after several cyclic deformations. Two factors were considered to explain these results: First, the residual stress field induced by implanted alumi nium ions at the surface assiseted the R phase and the martensite accommoda tion stress. Second, a shift in transformation temperatures due to preferen tial sputtering of titanium atoms at the surface followed by a composition change. When the implantation dose was low, the first factor seemed to play a major role. When the implantation dose exceeded some critical value, the second factor was important. Thus we assume the mechanism in which the def ormation in the surface region has a trigger effect, transmitting the defor mation toward the inner region of the specimen.