DETERMINATION OF HEAT OF TRANSFORMATION IN A COLD-ROLLED MARTENSITIC TINI ALLOY

We studied the heat of transformation, DELTAH, in martensitic transformations in a cold-rolled equiatomic TiNi alloy with differential scanning calorimetry (DSC), X-ray diffraction (XRD), and microhardness measurements. Results of our experiment indicate that the martensite stabilization and stress-induced parent (SIP) B2 phase are introduced when the TiNi martensite is cold rolled at room temperature. The SIP formation seems to be related to the lattice softening phenomenon occurring in the martensite, while the DELTAH value of the first reverse martensitic transformation decreases enormously for the cold-rolled equiatomic TiNi alloy. We are proposing possible explanations for these results: (1) the occurrence of SIP, which reduces the transformable martensite volume; (2) the release of accumulated elastic energy induced by the cold rolling; and (3) the recovery of defects induced by cold rolling and release of the heat of recovery. We also found that the retained dislocations can depress the martensitic transformation temperatures and induce the R-phase transformation after the occurrence of the first reverse martensitic transformation.