Laser Ultrasonic Measurements of Phase Transformation Kinetics in Lean Ti–Mo Alloys

Abstract

Molybdenum (Mo) is one of the most common $\beta$-stabilizing elements used in commercial titanium alloys. In comparison with other traditional $\beta$-stabilizers, Mo possesses the lowest diffusion rate in the $\beta$ phase and a rather low diffusivity in the $\alpha$ phase, therefore playing an important role in the $\alpha$/$\beta$ phase transformation rates. While most of the earlier studies focused on complex multi-component Ti-systems as well as highly alloyed binary Ti–Mo alloys, here we investigate quasi-binary Ti–Mo model alloys with systematically varied Mo content up to 6 wt pct, which is the relevant level for many commercial Ti-alloys. A Gleeble thermo-mechanical simulator coupled with a Laser Ultrasonics for Metallurgy (LUMet) sensor was used to measure the phase transformation rates in these alloys during continuous heating and cooling treatments at varying rates. The measurably different densities and elastic constants of the parent and product phases result in variation in the ultrasound longitudinal velocity, which is correlated to the transformed volume fractions. The rate of the thermal treatments was seen to have a larger effect on transformation rates during cooling, as compared to that on heating. Further, increasing the Mo content retards the phase transformations. The results obtained during continuous heating were compared with thermodynamic calculations, while those obtained during cooling were modeled using the additivity concept applied to the Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory. The LUMet data presented here together with theoretical background showed that this technique can be successfully used as an additional tool to characterize microstructural evolution during industrially relevant non-isothermal treatments of Ti-alloys.