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

23. Oktober 2017

The premartensite phase of shape memory and magnetic shape memory alloys 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 magnetic shape memory alloys.

In a recent study, scientists from the Max Planck Institute for Chemical Physics of Solids in Dresden demonstrated unambiguous evidence for macroscopic symmetry breaking leading to robust Bain distortion in the premartensite phase of 10% Pt-substituted Ni2MnGa using high resolution synchrotron x-ray diffraction study. They show that the robust Bain-distorted premartensite phase results from another premartensite phase with preserved cubic-like symmetry through an isostructural phase transition. The Bain-distorted premartensite phase finally transforms to the martensite phase with additional Bain distortion on further cooling. These results demonstrate that the premartensite phase should not be considered as a precursor state with the preserved symmetry of the cubic austenite phase.  The gradual evolution of the Bain distortion can facilitate the emergence of an invariant habit plane.  Therefore, such alloys may exhibit better reversibility due to lower hysteresis, which will enhance their applicability as magnetic actuators and in refrigeration technology. 

CF ; SS / CPfS

Temperature dependent phase diagram of Ni1.9Pt0.1MnGa magnetic shape memory alloy.  Unit cells for different crystallographic phases (austenite, premartensite and martensite) are also shown. The T1 phase has 3M modulated structure with preserved cubic symmetry while T2  phase has 3M modulated structure with orthorhombic symmetry (Bain distorted phase). 

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