Microstructure Evolution During Aging Heat Treatment of Metastable Ti-15V-3Cr-3Al-3Sn β Titanium Alloy

Abstract

The cold formability of metastable Ti-15V-3Al-3Sn-3Cr (Ti-15333) beta (β) titanium alloy has led to its widespread use in the aerospace industry. This alloy gets its hardness from the precipitation of the α phase in the β matrix. The focus of this investigation is to determine how high-temperature aging affects its microstructure evolution. Various thermal aging tests have been performed in a horizontal tube furnace in an inert gas environment for up to 6 h from 450 °C to 700 °C with a step gap of 50 °C. After each heat treatment, the resulting microstructure and associated phase change and composition have been analyzed and reported by making use of a scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) techniques. Results show that the supersaturated β phase obtained after solution treatment at 800 °C gives way to the more stable α phase during aging and that the α phase fraction rises as the temperature increases with accelerated precipitation at 500 °C and 550 °C. Raising the aging temperature and prolonging the duration results in the formation of α phase precipitates at grain boundaries and within the grains themselves. This transition is accompanied by a shift in morphology from globular to lath-shaped. EDS mapping, together with SEM observation, points to the α phase becoming coarser and less dense with a further rise in aging temperature to 600 °C and beyond. XRD analysis of the aged samples detects the presence of only hcp α and bcc β with β phase split taking place at 500 °C. Aged samples displayed a gradual increase in Vickers hardness, reaching a maximum before subsequently decreasing. This observation aligns with the concept that mechanical properties evolve in tandem with microstructural changes. The most favorable properties were achieved following a 6 h aging process at temperatures of 500 °C and 550 °C.