In-situ measurement of α formation kinetics in a metastable β Ti-5553 alloy using laser ultrasonics

Abstract

In this study, we explored the potential of laser ultrasonics for metallurgy (LUMet) as an advanced in-situ technique to measure the α formation kinetics during high temperature isothermal holdings of a Ti-5553 (Ti-5Al-5Mo-5V-3Cr, wt%) alloy. The measured changes in ultrasound velocity were seen to increase linearly with the α phase volume fraction in the investigated temperature range. Using the law of mixture an analytical model was developed to rationalize this experimental observation based on the temperature dependent macroscopic elastic constants and densities of the α and β phases. The transformation kinetics were modeled in the frame of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory and the JMAK exponents coincide with the sequence of precipitation observed. Further, a partial time-temperature-transformation (TTT) diagram, constructed from the LUMet data, was found to be consistent with results from in-situ electrical resistivity measurements reported for the same alloy. Considering the advantages of the real-time, non-contact and non-destructive nature of LUMet, this sensor technique provides an excellent monitoring capacity for tracking microstructure evolution in Ti-alloys.