Thermodynamic study of the low-temperature phase B19′ and the martensitic transformation in near-equiatomic Ti-Ni shape memory alloys

Experimental information on the transformation temperatures and the thermodynamic properties of the near-equiatomic TiNi alloys is analyzed. Special attention is paid to the estimation of T0 temperature from experimental Ms and Af temperatures. The properties of the TiNi low-temperature phase (B19′) are evaluated from selected experimental data by using a two-sublattice model. The Ti-Ni phase diagram including the B19′ phase is then calculated. It reveals that the equiatomic TiNi parent phase (B2) remains stable from high temperatures until 370 K, and then the B19′ phase becomes thermodynamically stable as a linear compound under 370 K. Thermodynamic quantities such as the T0 temperature and transformation enthalpy are also calculated and compared with experimental data. Further, the Ms temperature is predicted and compared with data from different sources.

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