On restructuring the microstructure of Ti-6Al-7Nb alloy before surface engineering

Abstract

This paper focuses on the effect of heat treatment processes on the microstructure of medical-grade Ti-6Al-7Nb alloy, with an objective to acquire a globular microstructure suitable for subsequent surface treatment such as Laser Shock Peening (LSP). In this study, the titanium alloy samples initially received solution treatment at various temperatures between 800 °C to 1100 °C, followed by subsequent cooling carried out at different rates, ranging from furnace cool to water quench. A number of samples were then subject to ageing at 800 °C for either 2 h or 4 h. The microstructure revealed that alpha (α) martensite was formed on cooling after solution treatment from 980 °C which is a typical of bimodal microstructure. What is more, air cooling and furnace cooling for 2 h and 4 h were also conducted after solution treatment at 900 °C and 950 °C. A globular microstructure was formed with furnace cooling from both temperatures. Furthermore, adjacent grain crystallographic misorientation was characterized by Electron Backscatter Diffraction (EBSD). The results show that solution treatment at 950 °C for 4 h combined with subsequent furnace cooling is the best heat treatment for obtaining a globular microstructure with lowest misorientation. Additionally, after LSP, a gradient change in misorientation was formed as shown in the Kernel Average Map (KAM). This work not only offer a straightforward way to develop a globular microstructure, but also reveal the corresponding microstructure in Ti-6Al-7Nb alloy at various temperatures for future metallurgical research.