THE MARTENSITIC-TRANSFORMATION IN TI-RICH TINI SHAPE-MEMORY ALLOYS

The martensitic (Ms) transformation temperatures and their DELTAH values of Ti51Ni49 and Ti50.5Ni49.5, alloys are higher than those of equiatomic or Ni-rich TiNi alloys. The Ti-rich TiNi alloys exhibit good shape recovery in spite of a great deal of second phase Ti2Ni or Ti4Ni2O existing around B2 grain boundaries. The nearly identical transformation temperatures indicate that the absorbed oxygen in Ti-rich TiNi alloys may react with Ti2Ni particles, instead of the TiNi matrix, to form Ti4Ni2O. Martensite stabilization can be induced by cold rolling at room temperature. Thermal cycling can depress the transformation temperatures significantly, especially in the initial 20 cycles. The R-phase transformation can be promoted by both cold rolling and thermal cycling in Ti-rich TiNi alloys due to introduced dislocations depressing the Ms temperature. The strengthening effects of cold rolling and thermal cycling on the Ms temperature of Ti-rich TiNi alloys are found to follow the expression Ms = To - KDELTAsigma(y). The K values are affected by different strengthening processes and related to the as-annealed transformation temperatures. The higher the as-annealed Ms (or As), the larger the K value.