Non-martensitic Needle-like Structures on Ni-Ti Alloys - Occurrence and Origin

In the present work the occurrence of needle-like structures that are observed on martensitic and austenitic NiTi after metallographic preparation and etching is described. The investigations show that the needle-like structures form in less than 10s during the etching process if solutions containing HF are applied. The needle-like structures look very similar to martensitic structures. Hence it is tempting to interpret them as martensite. Distinguishing the structures from martensite is not straightforward since standard analysis e.g. by EDX does not yield any differences in chemical composition of the needle-like structures and NiTi. Application of surface sensitive methods like grazing incidence X-ray diffraction clearly shows that the needle-like structures are different from martensitic structures of NiTi. In the present study the options of altering the etching process with the objective of avoiding the formation of the needle-like structures are presented and the origin of the needle-like structures is discussed