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

Sanjay Singh,P Devi,S Chadov,T Hickel,D Pandey,A S Gibbs,M G Zavareh,S W D'Souza,C Felser,B Dutta

doi:10.1038/s41467-017-00883-z

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.

Highlights

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
We present here experimental evidence for macroscopic symmetry breaking in the PM phase of Ni2MnGa as a result of 10% Pt substitution leading to a robust Bain distortion
We have presented evidence for robust Bain distortion in the PM phase of Ni2MnGa as a result of 10% Pt substitution using high resolution synchrotron X-ray powder diffraction (SXRPD)

Summary

Introduction

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 appearance of a precursor state, widely known as the premartensite (PM) phase, is an interesting feature of compounds/alloys exhibiting martensitic phase transitions. This has been extensively studied in conventional shape memory alloys (SMAs)[1,2,3,4,5,6,7,8,9]. The PM state appears in between the parent austenite and martensite phases with preserved parent phase symmetry (cubic) in the sense that the austenite peaks in the diffraction patterns do not exhibit any signature of symmetry breaking or in other words any evidence for Bain distortion. While the martensite phase of Ni2MnGa exhibits macroscopic symmetry breaking[26, 44], the PM phase does not exhibit any evidence of symmetry breaking transition

Methods

Results

Conclusion