Time dependent nanomechanical properties of NiMnGa Heusler alloy

Abstract

In this work nano-indentation has been adopted as a technique to measure the Time dependent deformation (TDD) and other related mechanical properties of NiMnGa Heusler alloys. The measurement of TDD utilizes two different techniques namely: CRL (Constant Rate of Loading) and CSR (Constant Strain Rate) modes. Tests to measure the behavioral dependence on the holding time were also done. The loading rates for CRL technique were varied from 0.1 mN/s to 1 mN/s. Strain rates for CSR mode were varied from 0.05 to 2. The tests were done at room temperature on Ni50Mn28Ga22 alloy which was prepared by TIG arc melting. Hardness and strain rate, both are inversely dependent on depth of indentation and time. The amount of plastic work was calculated and its dependence on loading rate, holding time and strain rate has been studied. The parabolic dependence of the fraction of plastic work to the time of holding was obtained by least squares method. The amount of recovery also declined as the loading rate and strain rates were increased. Microstructural studies were also performed to observe the changes that are inflicted underneath the surface during indentation. The microstructure was initially a twinned martensitic structure having two different variants which developed into a single martensitic variant under the influence of load. Nano twins were also observed in the microstructure.