THERMAL CYCLING EFFECT ON THE PREMARTENSITIC AND MARTENSITIC TRANSITION IN A TI RICH NITI ALLOY

The study was carried on a Ti-rich NiTi shape memory alloy. The electr ical resistance and the elastic modulus were determined versus tempera ture. The aim of this work is to study the thermal cycling effect on t ile transformation features as the temperatures or the phase successio n as well as the structural modifications. The major results of this w ork concern the R-phase occurrence and its evolution. We have shown th at the premartensitic phase could exist in a narrow range of temperatu re on cooling and is revealed only after a few cycles. In fact, its pr esence and stability is directly linked to the diminishing of Ms. The main condition for the austenite to be transform in a rhombohedral str ucture is that Ms becomes low enough to be below Tr. Interrupted cooli ng runs also show that contrarily to the austenite-martensite transiti on, the austenite-R-phase transition doesn't exhibit any hysteretic be haviour.