Indentation-induced two-way shape-memory effect in NiTi

The indentation-induced shape-memory behavior in both an aged and a solutionized austenitic nickel-titanium (NiTi) alloy was investigated by Vickers indentation. The depth recovery ratio of the indents as well as the formation of protrusions on the surface after planarization was analyzed after the sample was heated above and cooled below the phase transformation temperature using a white light interferometer. For comparison a solutionized, fully austenitic sample was subjected to the same indentation parameters and temperature changes. The results show that indentation induces a one-way and two-way shape-memory effect in the aged material containing small fractions of martensite and precipitates. In contrast, the solutionized fully austenitic sample shows less pronounced one-way compared to the aged material and no two-way shape recovery. This phenomenon is discussed in terms of martensite stabilization by precipitates and dislocations.