Effect of Fe content on the coupling between lattice distortion and atomic shuffle of the R phase in Ti50Ni50-xFex alloys

The R phase in NiTi-based shape memory alloys is known to have two crystallographic changes from its B2 parent phase: a lattice distortion and an atomic shuffle. The lattice distortion induces a crystal shape change and is thus more responsive to mechanical influences. In contrast, the atomic shuffle has no mechanical effect and is more likely affected by alloy chemical composition. In an earlier study we investigated the decoupling of the two under mechanical constraints in nanocrystalline matrices. In this study we investigated the effect of alloy composition, more specifically Fe content, on the coupling between the two during B2 ↔ R transformation by means of thermal and electrical analyses, in-situ X-ray diffraction measurements and density functional theory (DFT) calculations. It was found that Fe substitution for Ni in equiatomic NiTi reduces the magnitudes of both lattice distortion and atomic shuffle, with the latter appearing to be more hindered. At 6 at.% Fe substitution, atomic shuffle is completely inhibited whereas the lattice distortion is reduced only by approximately 50%. This is consistent with DFT calculations, which reveal that the magnitude of atomic shuffle follows the order of Ni > Ti >Fe and that increasing Fe content decreases the magnitude of shuffle of all three elements. This is compounded by the additional mechanical constraint of the nanocrystalline grain size which leads to the complete suppression of the B2 ↔ R transformation in the Ni₄₄Ti₅₀Fe₆ alloy.