Incommensurate–commensurate transition and nanoscale domain-like structure in iron doped Ti–Ni shape memory alloys

Abstract

Precursor phenomena of displacive transformation have been studied by optical and transmission electron microscope observation and X-ray diffraction of Ti–(50 − x)Ni–xFe (x = 2, 4, 6, 8 in at.%) alloys. We found that a Ti–44Ni–6Fe alloy exhibits a second-order-like incommensurate–commensurate transition without latent heat and discontinuity in lattice parameters. In other words, diffuse scatterings appear in an electron diffraction pattern at an incommensurate position on cooling; they move gradually towards 1/3⟨ 110⟩ as the temperature decreases and lock into the commensurate position at 180 K. The commensurate phase is not expanded along one of the ⟨ 111⟩ directions, unlike the R-phase formed by a first-order transformation in Ti–48Ni–2Fe and Ti–46Ni–4Fe alloys. In addition, the commensurate phase shows a nanoscale domain-like structure, which is inherited from the incommensurate state of the parent phase. Thus, the anomalies in physical properties observed in the incommensurate state are most likely the precursor phenomena of the commensurate phase in the Ti–44Ni–6Fe alloy. In the case of a Ti–42Ni–8Fe alloy, the incommensurate state remains even at 19 K.