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

Abstract

Abstract. 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.

Building Orientation and Heat Treatments Effect on the Pseudoelastic Properties of NiTi Produced by LPBF

“…Microstructural analyses were carried out by a Nikon Eclipse LV150NL optical microscope (OM) and a Zeiss Sigma 500 field emission scanning electron microscope (FE-SEM) equipped with an EBSD detector. To perform OM and FE-SEM analyses, samples were etched with a solution composed by 5 mL of hydrofluoric acid (HF), 20 mL of nitric acid (HNO 3 ), and 25 mL of water, according to what reported in [50]. Water was used instead of acetic acid (CH 3 COOH) to avoid pitting and overetching.…”

Processing of Small Scale Nitinol Billets by Induction Heated Nonconventional Isothermal Extrusion (IHNCIE)

“…16. If higher magnification images are required, a combination of the optical imaging and SEM is normally used [38].…”

“…It is important that the correct acid solution is used during etching; otherwise, it is possible to form a nonrepresentative or reformed surface on the material [38]. Etching was carried out with an acid solution described in full detail in the experimental section.…”