Ausforming of NiTi

Lattice defects are introduced into stable austenite (beta) by hot rolling up to 70% (in one pass) in a temperature range between 900 > T-AF > 340 deg rees C (ausforming (AF)). The changes in microstructure, thermal transforma tion behavior, as well as normal and anomalous properties in stress-, strai n-, and temperature-space were studied. Martensitic transformation temperat ures are lowered with decreasing deformation temperature and increasing amo unt of deformation. A two-stage reaction is induced at T-AF less than or eq ual to 400 degrees C Deformation twins are the predominant microstructural feature, in addition to dislocations at T-AF less than or equal to 700 degr ees C. Highly elongated grains indicate that long-range recrystallization i s absent at all ausforming temperatures. Semicoherent pecipitation in addit ion to high dislocation densities are likely to be responsible for the prem artensitic anomalies of thermal transformation for T-AF less than or equal to 400 degrees C. Ausforming leads to a considerable increase in convention al strength properties (yield stress, tensile strength, elongation) without loss of transformability and, consequently, shape memory or pseudo-elastic ity. (C) 1999 Kluwer Academic Publishers.