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Abstract

NiTi shape memory alloys (SMA) exhibit specific functional properties which are based on a stress and/or temperature induced martensitic phase transformation. There is increasing interest in small SMA components, such that there is a need to characterize their mechanical properties and phase transformation behaviour at small length scales. In the present study we use nanoindentation with several spherical indenter tips to study the formation of stress induced martensite in NiTi SMA. From our polycrystalline specimen, individual austenite grains with specific crystallographic orientations were selected for nanoindentation using electron back scatter diffraction (EBSD). We use a specific NiTi alloy, such that the effective thermodynamic equilibrium temperature between austenite and martensite corresponds to room temperature. This property allows a post mortem characterization with an atomic force microscope (AFM). The remnant indents were then heated in-situ in the SEM and microstructural and topographical changes during the back transformation from martensite to austenite were investigated. The crystallographic sink-in patterns disappear during heating which proves their martensitic origin. Our results provide clear experimental evidence that the crystallographic anisotropy of nanoindentation is governed by the crystallographic anisotropy of the stress induced formation of martensite. A comparison between the indents before and after the heat treatment allows an experimental assessment of the reversible strain. The results are rationalized on the basis of Kalidindiś definition of the strain. Concluding we derive from Kalidindiś work a new method to directly determine the maximum reversible strain value r from AFM data before and after shape recovery.