STUDY OF THE PHASE-TRANSFORMATIONS IN NI-TI BASED SHAPE-MEMORY ALLOYS

Structural changes with cooling and heating of Ni-Ti alloys involve th ree phases : austenite, R-phase and martensite. The corresponding tran sformation sequences are often studied by techniques such as different ial scanning calorimetry and internal friction. Structural transformat ion between any two phases is identified in these measurements by a pe ak: in the studied property versus temperature plot. On this basis, th e evidence of two transformations, austenite --> R-phase and R-phase - -> martensite, have been established in the literature. In spite of ex hibiting 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 transform ation sequences will be discussed with qualitative model. The data for the present investigation have been obtained on Ni50Ti50 and Ni47Ti50 Co3 alloys.