Fracture toughness evaluation of Ni2MnGa magnetic shape memory alloys by Vickers micro indentation

Abstract

This paper reports on an experimental study focused on the evaluation of fracture toughness of Ni2MnGa magnetic shape memory alloys, and the effect of the microstructure evolution during magneto-mechanical loading conditions on this material property. The shape memory effect in Ni2MnGa MSMAs is driven by magnetic field induced or stress induced motion of twin boundaries. The Ni2MnGa microstructure consists of tetragonal martensite variants with magnetic easy axis aligned with the short axis (of the unit cell), which may align in either the direction of the applied magnetic field or mechanical stress causing a reorientation of the microstructure. The reorientation strain and the change in the material’s magnetization during variant reorientation, drive the development of MSMA based applications. However, the brittle nature of the material causes cracks to develop in the MSMA element, potentially hampering the function of the material in a specific application. Having an understanding of the fracture toughness of the material is hence considered key for the design of future robust MSMA based devices. To evaluate the alloy’s fracture toughness and its dependence on the material’s microstructure, the Vickers micro indentation technique was deployed on Ni2MnGa samples under no load, as well as under several magnetic and/or mechanical loading conditions. Measurements of the indentations suggested a material hardness dependency on the microstructure, which is determined by the external loading condition. Experimental results also revealed that variant reorientation, caused by the magneto-mechanical loading, from the stress preferred state (V1) to the field preferred state (V2) decreases the fracture toughness, and identified V2 as the variant state more susceptible to fracture.