Session: 01-01: Shape Memory Alloy Characterization I

Paper Number: 98015

98015 - Empirical Relationships for Calculating the Fracture Toughness of Ni2mnga Magnetic Shape Memory Alloys Accounting for Their Elastic Anisotropy and Magneto-Mechanical Loading 

Ni₂MnGa Magnetic Shape Memory Alloys (MSMAs) are a unique class of smart materials that exhibit a high reorientation strain of up to 10%, and a fast deformation rate of up to 10 kHz, under a magnetic field and/or mechanical stress. These characteristics make Ni₂MnGa MSMAs an attractive material for developing actuators, power harvesting, and sensing devices. The macroscopic response of Ni₂MnGa MSMAs, which is the basis of applications, is driven by its distinctive microstructure comprising of tetragonal martensite variants with magnetic easy axes approximately aligned with the short side of the tetragonal unit cell. The martensite variant with magnetic easy axis aligned with the external magnetic field or mechanical load, grows at the expense of the other variant, resulting in twin boundary motion, strain, and change in the material’s magnetization.

When the Ni₂MnGa alloy is deployed in applications, like the ones stated above, it undergoes continuous reorientation from the magnetic field preferred variant state to the stress preferred variant state and vice versa, and during this process it is observed that cracks begin to develop in it, ultimately leading to fracture. The susceptibility of Ni₂MnGa MSMAs to crack nucleation and growth is a major factor restricting the expansion of the field of applications of these alloys.

The evolving microstructure in Ni₂MnGa and the magneto-mechanical loading that influences it, make studying their fracture mechanics a complex problem. Thus, it is critical to understand the fracture mechanics of these alloys in relation to the MSMA microstructure, and how the magneto-mechanical loading impacts the life of the material, i.e., how fast or slow a nucleated crack would grow under a certain combination of magnetic and mechanical loading.

Our prior experimental results indicated crack length dependency on the external magneto-mechanical loading conditions but didn’t quantify the effect of the material’s elastic anisotropy. Since fracture toughness captures the variation in elastic modulus and crack growth, this paper reports on an updated empirical relationship for fracture toughness calculations that accurately accounts for the material’s elastic anisotropy and its magneto-mechanical loading conditions. This is a first attempt at developing a robust relationship between fracture toughness and the evolving MSMA microstructure.

Presenting Author: Glen D'Silva Northern Arizona University