Compliant Lattice Modulations Enable Anomalous Elasticity in Ni-Mn-Ga Martensite

TL;DR

This study uncovers the unique elastic properties of modulated Ni-Mn-Ga martensite, revealing a shear instability linked to lattice modulations that explains its high twin boundary mobility and potential for advanced applications.

Contribution

It demonstrates a shear elastic instability in modulated Ni-Mn-Ga martensite and proposes a lattice-scale mechanism explaining its exceptional twin boundary mobility.

Findings

Confirmed strong shear elastic instability in 10 M Ni-Mn-Ga

Linked lattice instability to lattice modulations

Proposed a dynamic faulting mechanism for modulation sequence

Abstract

High mobility of twin boundaries in modulated martensites of Ni-Mn-Ga-based ferromagnetic shape memory alloys holds a promise for unique magnetomechanical applications. This feature has not been fully understood so far, and in particular it has yet not been unveiled what makes the lattice mechanics of modulated Ni-Mn-Ga specifically different from other martensitic alloys. Here, results of dedicated laser-ultrasonic measurements on hierarchically twinned five-layer modulated (10 M) crystals fill this gap. Using a combination of transient grating spectroscopy and laser-baser resonant ultrasound spectroscopy, it is confirmed that there is a shear elastic instability in the lattice, being significantly stronger than in any other martensitic material and also than what the first-principles calculations for Ni-Mn-Ga predict. The experimental results reveal that the instability is directly related to the lattice modulations. A lattice-scale mechanism of dynamic faulting of the modulation sequence that explains this behavior is proposed; this mechanism can explain the extraordinary mobility of twin boundaries in 10 M.