Kinetic Arrest of R-B19′ Transformation in Iron-Doped Ti–Ni Shape Memory Alloy

抄録

We have studied the R-B19′ martensitic transformation behavior of Ti–(50−x)Ni–xFe (at%) alloys. The R-B19′ transformation start temperature (M_s) decreases from 234 K to 102 K as x increases from 2.0 to 3.7 at%. This transformation is suppressed in the alloy with x = 4.0 (4.0Fe alloy), although the equilibrium temperature is estimated to be 104 K. The estimated free energy difference at M_s (driving force) for the R-B19′ transformation increases as M_s decreases from 45 J/mol (M_s = 234 K) to 48 J/mol (M_s = 102 K). If we extrapolate this relation linearly, M_s of the 4.0Fe alloy is expected to be ∼50 K. Nevertheless, the R-B19′ transformation is completely suppressed in the 4.0Fe alloy. Presumably, insufficient thermal activation energy is the main reason for the suppression of the R-B19′ transformation in the 4.0Fe alloy.

Fig. 7 Temperature dependence of free energy difference between the R-phase and the B19′ phase of Ti–(50−x)Ni–xFe alloys. The M_s(B19′) temperature of each alloy is indicated by solid circles.