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 th ermodynamic properties of the near-equiatomic TiNi alloys is analyzed. Special attention is paid to the estimation of T-0 temperature from e xperimental M(s) and A(f) temperatures. The properties of the TiNi low -temperature phase (B19') are evaluated from selected experimental dat a by using a two-sublattice model. The Ti-Ni phase diagram including t he B19' phase is then calculated. It reveals that the equiatomic TiNi parent phase (B2) remains stable from high temperatures until 370 K, a nd then the B19' phase becomes thermodynamically stable as a linear co mpound under 370 K. Thermodynamic quantities such as the T-0 temperatu re and transformation enthalpy are also calculated and compared with e xperimental data. Further, the M(s) temperature is predicted and compa red with data from different sources.