Lattice distortion and atomic shuffle coupling of the R phase under grain size confinement in a Ni₄₈Ti₅₀Fe₂ alloy

The R phase in NiTi-based shape memory alloys is well known to exhibit a lattice distortion and an internal atomic shuffle relative to its parent B2 phase, yet over the past decades little is known of the correlation between these two structural changes. This study presents a first attempt to clarify their correlation in a nanocrystalline NiTiFe alloy. The lattice distortion produces a crystal shape change for the shape memory and pseudoelastic properties of these alloys and is subjected to the influences of mechanical constraint of a nanocrystalline matrix. In contrast, the internal atomic shuffle does not produce a crystal shape change, thus is not directly affected. This may cause decoupling of the two crystallographic changes. This study investigated the correlation between the two in the R phase by means of in-situ synchrotron high-energy XRD analysis and DFT calculations. The findings of this study provide new insights into the B2 → R transformation process in constrained environments and offers two significant implications. They help to understand many earlier observations of peculiar behaviour of the transformation, such as the pre-martensitic or precursor phenomena, the incommensurate and commensurate phases, and the R-like structures. They also help to guide the design of novel NiTi-based alloys of superior properties, such as NiTi strain glass alloys which are characteristic of severely suppressed lattice distortion transformation with significant atomic shuffle.