R-Phase Transformation in Ti_50−xNi_47+xFe₃ Shape Memory Alloys

Phase transformations among the parent (B2), intermediate (I), and rhombohedral (R) phases were systematically investigated in Ti50−xNi47+xFe3 (x = 0.0–1.0) alloys. In the Differential Scanning Calorimetry (DSC) curves of alloys with x = 0.40 to 0.80, broad peaks due to the B2/I phase transformation were detected at about 200 K. The B2/R transformation temperature and entropy change gradually decrease with increasing x, but drastically decline by the appearance of I phase at around x = 0.40. The existence of the R phase at low temperatures was confirmed by in situ XRD measurements for x = 0.00, 0.35, and 0.40. However, by in situ TEM observation, while the R phase exhibits an ordinary twin-boundary microstructure for x = 0.00 and 0.35, it has a fine cluster-like microstructure without twin boundaries for x = 0.40. Thus, the microstructure and phase stability of the R phase is significantly affected by the appearance of the I phase.