Robust Bain distortion in the premartensite phase of platinum substituted Ni2MnGa magnetic shape memory alloy

TL;DR

This study provides clear evidence of robust Bain distortion in the premartensite phase of Pt-substituted Ni2MnGa, challenging the view of premartensite as a symmetry-preserved precursor to martensite.

Contribution

It reveals that the premartensite phase undergoes a symmetry-breaking Bain distortion via an isostructural transition, offering new insights into the stability and nature of premartensite in MSMAs.

Findings

Robust Bain distortion observed in premartensite phase.

Premartensite phase results from an isostructural transition.

Premartensite does not preserve cubic symmetry as previously thought.

Abstract

The premartensite phase of shape memory and magnetic shape memory alloys (MSMAs) is believed to be a precursor state of the martensite phase with preserved austenite phase symmetry. The thermodynamic stability of the premartensite phase and its relation to the martensitic phase is still an unresolved issue, even though it is critical to the understanding of the functional properties of MSMAs. We present here unambiguous evidence for macroscopic symmetry breaking leading to robust Bain distortion in the premartensite phase of 10% Pt substituted Ni2MnGa. We show that the robust Bain distorted premartensite (T2) phase results from another premartensite (T1) phase with preserved cubic-like symmetry through an isostructural phase transition. The T2 phase finally transforms to the martensite phase with additional Bain distortion on further cooling. Our results demonstrate that the premartensite phase should not be considered as a precursor state with the preserved symmetry of the cubic austenite phase.