Plastic instabilities in epitaxial NiMnGa Heusler films

Magnetic shape memory alloys are known to undergo stress- and temperature-driven phase changes. Here we study the specific case of a NiMnGa Heusler alloy that has an austenitic phase at room temperature. Upon cooling or the application of mechanical pressure, the austenite can transform into martensite, allowing for large reversible strain cycling and making such alloys to promising actuating materials. In order to shed more light on the mechanical switching behavior and possible dissipative processes, we probe the nano-scale plasticity of 0.5 and 2 µm thick epitaxial NiMnGa films with nanoindentation. A distinct pop-in signature is seen as the first departure from Hertzian elastic contact mechanics at small film thicknesses. This pop-in behavior persists across four orders of loading rates and over a broad temperature regime from 40°C to -30°C, which encompasses the transformation temperature to martensite. The statistics of the incipient plastic events are well described by a Weibull distribution. Atomic force microscopy reveals surface signatures around indents that indicate residual martensite, which is further confirmed with transmission electron microscopy imaging of the structure underneath indents. Instead of the expected modulated martensite (14M, 10M) that forms during a temperature-driven phase change, regions underneath indents contain non-modulated (NM) martensite. NM martensite exhibits a higher spontaneous strain and often forms at lower temperatures and higher strains. Therefore, it is concluded that the pop-in signature during nanoindentation originates from an athermal martensitic transformation, where the confinement effects result in huge and complex deformation inducing a partly irreversible transition to NM martensite.