Effect of Hydrostatic Pressures on Thermoelastic Martensitic Transformations in Aged Ti–Ni and Ausaged Fe–Ni–Co–Ti Shape Memory Alloys

The effect of hydrostatic pressures on thermoelastic martensitic transformations in aged Ti-51at%Ni and ausaged Fe-31.9Ni-9.8Co-4.1Ti (at%) shape memory alloys has been investigated by electrical resistivity measurements under hydrostatic pressures up to 1.5 GPa. The results obtained were as follows: Transformation temperatures of the B2\ ightleftarrowsR transformations (defined as Ms(R) and As(R) in the present study) for two kinds of Ti–Ni alloys aged for short and long periods were independent of pressure. Ms and As temperatures of the successive R\ ightleftarrowsB19′ transformations were linearly increased with increasing pressure for the short aged Ti–Ni alloy, but remained constant for the long aged one. On the other hand, the Ms and Af temperatures in the ausaged Fe–Ni–Co–Ti alloy were inversely decreased with increasing pressure. A thermodynamic analysis on the pressure dependence of the thermodynamic equilibrium temperature, which was obtained from the measured transformation temperatures, showed that the effect of hydrostatic pressures on the B2\ ightleftarrowsR and R\ ightleftarrowsB19′ martensitic transformations in the short aged Ti–Ni alloy was successfully explained by a modified Patel and Cohen's equation derived in the previous study, but was not for the γ\ ightleftarrowsα′ transformation in the ausaged Fe–Ni–Co–Ti alloy. The disagreement in the ausaged Fe–Ni–Co–Ti alloy is considered to be caused by the invar nature of the alloy, as in the Fe–Ni and Fe–Ni–C invar alloys previously studied.