Mechanical anomaly observed in Ni-Mn-Ga alloys by nanoindentation

Abstract Reduced elastic modulus (E r ) and hardness (H) for NiMnGa alloys were investigated using nanoindentation over a large compositional range generated via diffusion couple annealing. Diffusion couples were assembled from selected starting alloys and annealed at 900 °C for 120 h. Compositions generated through diffusion couples, as examined by electron probe micro-analyzer, covered large part of the β phase region. Microstructural analyses by scanning electron microscopy revealed that, for each diffusion couple, an interface separating austenite and martensite was created due to variation of the martensitic transformation temperature at different compositions. Nanoindentation measurements were carried out across the interdiffusion zone for each diffusion couple, and a correspondence between E r or H and composition is established. The measured E r and H had larger scatter for the martensitic phase than for the austenitic phase. A decrease of E r and H was observed with Mn or Ni substituting for Ga, and Ni substituting for Mn in the austenitic phase. However, an opposite trend was found in the martensitic phase: an increase of E r and H was observed with Mn or Ni substituting for Ga, and Ni substituting for Mn. A minimum value of E r and H was always observed near the interface between austenite and martensite. The softening of the elastic constants due to pre-martensitic transformation contributed to the sharp decreases in E r and H near the interface region. The results demonstrate that the influence of lattice softening is significant over a large compositional range on the mechanical properties of NiMnGa alloys and potentially in other materials that have a martensitic transformation.