Abstract
Ni-Ti shape memory alloy also called Nitinol is majorly used in applications of medical products such as orthodontic wire, stent, etc. In the present study, the microstructure analysis of the Nitinol ingot and samples from different mechanical process supplied by Metal Industries Research & Development Centre was conduct. Based on the results of analysis and the paper review, the investigation on the connection between the microstructure and mechanical properties e.g., the formability of Nitinol and the origin of cracks were done. And the correlation between the NiTi2 segregation and Ni2Ti4Ox oxides was probed then to realize the mechanism of formation. The microstructures analysis of Nitinol ingots from vacuum induction melting (VIM) and vacuum arc melting (VAM) shows that NiTi2 segregation formed along the grain boundary. And the internal stress in VIM ingots will cause crack along the distribution of NiTi2 segregation. The NiTi2 segregation in VAM ingots would also induce crack during the thermomechanical process and may cause the rupture in final. In the products, the NiTi2 segregation distributed uniformly and the small crack may exist in it. Therefore the existence of this brittle phase will decrease the yield rate of Nitinol and may cause the mechanical properties go down due to the small cracks in the product. Then the solution heat treatment at 1273K for 24, 36 and 48 h was conducted on VIM samples and shows that the amount of NiTi2 segregation can be decreased effectively. However, there still existed about 1% NiTi2 even though sustained for a longer time. Moreover, the NiTi2 segregation left behind may solute oxygen and become the Ni2Ti4Ox. Therefore the Material Studio 7.0 software was used to investigate the correlation between the NiTi2 and Ni2Ti4Ox by calculating the formation enthalpy of both phases based on first principle simulation. The result shows that the formation enthalpy decreases as increasing the amount of oxygen mixed in NiTi2 phase. It means that the oxygen mixed in NiTi2 phase will cause the phase transform into Ni2Ti4Ox which is more stable than former.