Direct microstructural evidence for the stress induced formation of martensite during nanonindentation of NiTi

When a pseudoelastic NiTi alloy is loaded and subsequently unloaded, stress induced martensite forms and disappears. It is challenging to directly observe and characterize the local martensitic features in such alloys. In the present study we use a specific NiTi alloy where stress induced martensite is thermally stable during unloading to prove the formation of martensite during nanoindentation. TEM investigations provide direct microstructural evidence for the martensitic phase transformation during nanoindentation. Subsequent in-situ heating in TEM shows how the stress induced martensite transforms back to austenite.