SIZE EFFECTS DURING INDENTATION OF HIGH TEMPERATURE NiTiHf SHAPE MEMORY ALLOYS

Abstract

The application of NiTiHf alloys had been receiving considerable interest due to its high transformation temperature, high strength with excellent shape memory effect and superelasticity. In the ongoing work NiTiHf (with Hf content of 12%.at and 15%.at) alloys were used to study the size effects during indentation by using a Berkovich indenter of tip radius 100 nm. The alloys were processed at different conditions and their hardness was measured at different depth of penetration to establish a relationship between them. The shape memory alloys exhibits stress assisted phase transformations and show different hardness values at various depths and almost show size effects based on the type of indenter used and condition for nanoindentation. The NiTiHf alloy shows different phases based on composition and XRD analysis helps in predicting the type of phases along with presence of precipitates. The martensitic and austenite peak temperature is determined by DSC analysis to know the transformation temperature. To relate the size effects during indentation the nanoindentation of the samples is carried out and hardness value dependency is found out. Different from most ductile metals, NiTiHf reacts to the mechanical load of indentation based on strain gradient operated by density of geometrically necessary dislocation underneath the indentation tip. It shows that the hardness decreases rapidly with the increase of the indentation depth and eventually approaches a constant. The clear absence of size effect is also established by conducting nanoindentation at a higher level of penetration depth with spherical as well as berkovich indenter.