Study of the Phase Transformations in Ni-Ti Based Shape Memory Alloys

Abstract

Structural changes with cooling and heating of Ni-Ti alloys involve three phases : austenite, R-phase and martensite. The corresponding transformation sequences are often studied by techniques such as differential scanning calorimetry and internal friction. Structural transformation between any two phases is identified in these measurements by a peak in the studied property versus temperature plot. On this basis, the evidence of two transformations, austenite -+ R-phase and R-phase -, martensite, have been established in the literature. In spite of exhibiting these two transitions on cooling, the reverse transformation from martensite, on heating, can be associated with one or two peaks. The exact interpretation of this behaviour thus poses an open question. This is attended to in this communication and the possible transformation sequences will be discussed with qualitative model. The data for the present investigation have been obtained on NisoTiso and Ni47Ti5~Co3alloys.