M. Todai, K. Wada, Osaka University, Suita, Osaka, Japan; M. S. Choi, T. Fukuda, T. Kakeshita, Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, Suita, Osaka, Japan
Instability of B2-type structure (β-phase) in Ti-based alloys have been investigated by using Ti-(50-x)Pd-xFe (14 ≤ x ≤20) shape memory alloys. A first order martensitic transformation occurs in the alloys with x = 14, 16 and 18. The transformation temperature decrease with increases iron content and disappears in the alloy with x = 20. The electrical resistivity of the alloy with x = 20, however, exhibits a negative temperature coefficient in electrical resistivity, which is also observed in alloys with x = 14, 16 and 18 prior to the martensitic transformation. Moreover the alloy with x = 20 exhibits anomalies at low temperatures such as low Debye temperature and low Young's modulus. Electron diffraction patterns of this alloy exhibit diffuse scatterings along <ζζ0> direction in addition to reflections of the B2-type structure. The diffuse scatterings consist of two parts: one is a rod-like streak appearing adjacent to the B2-reflections, and the other is a satellite appearing near the tip of the streak. The intensity and the position of the latter depend significantly on temperature while those of the former are almost independent of temperature. These anomalies in the alloy with x = 20 resemble those observed previously in Ti-Ni-Fe alloys whose R-phase transformation is suppressed. Possibly these anomalies are common nature of the instability of the B2-type structure in Ti-based alloys originating from their electronic structures.
Summary: The so-called precursor phenomena appearing in alloys exhibiting thermoelastic martensitic transformation have been studied by using iron doped TiPd SMA. The anomalies observed in this system was found to resemble those observed in iron doped TiNi SMA.
