Effect of hydrostatic pressures on thermoelastic martensitic transformations in aged Ti-Ni and ausaged Fe-Ni-Co-Ti shape memory alloys

Abstract

The effect of hydrostatic pressures on thermoelastic martensitic transformations in aged Ti-51 at%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 ↔ R transformations (defined as M (R) and A (R) in the present study) for two kinds of Ti-Ni alloys aged for short and long periods were independent of pressure. M and A temperatures of the successive R ⇔ B19′ 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 M and A1 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 ⇔ R and R ⇔ B19′ marrensitic 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 ⇔ α′ 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.