Ultrasonic Characterization of Microstructural Changes in Ti-10V-4.5Fe-1.5Al β-Titanium Alloy

Abstract

Ultrasonic measurements have been carried out in Ti-10V-4.5Fe-1.5Al b-titanium alloy specimens subjected to b annealing at 1173 K (900°C) for 1 hour followed by heat treatment in the temperature range of 823 K to 1173 K (550°C to 900°C) at an interval of 50 K (50°C) for 1 hour, followed by water quenching. Ultrasonic parameters such as ultrasonic longitudinal wave velocity, ultrasonic shear wave velocity, shear anisotropy parameter, ultrasonic attenuation, and normalized nonlinear ultrasonic parameter have been correlated with various microstructural changes to understand the interaction of the propagating ultrasonic wave with microstructural features in the alloy. Simulation studies using JMatPro Ò software and X-ray diffraction measurements have been carried out to estimate the a-phase volume fraction in the specimens heat treated below the b-transus temperature (BTT). It is found that the a-phase (HCP) volume fraction increases from 0 to 52 pct, with decrease in the temperature from 1073 K to 823 K (800°C to 550°C). Ultrasonic longitudinal and shear wave velocities are found to increase with decrease in the heat treatment temperature below the BTT, and they exhibited linear relationships with the a-phase volume fraction. Thickness-independent ultrasonic parameters, Poisson's ratio, and the shear anisotropy parameter exhibited the opposite behavior, i.e., decrease with increase in the a-phase consequent to decrease in the heat treatment temperature from 1073 K to 823 K (800°C to 550°C). Ultrasonic attenuation is found to decrease from 0.7 dB/mm for the b-annealed specimen to 0.23 dB/mm in the specimen heat treated at 823 K (550°C) due to the combined effect of the decrease in the b-phase (BCC) with higher damping characteristics and the reduction in scattering due to randomization of b grains with the precipitation of a-phase. Normalized nonlinear ultrasonic parameter is found to increase with increase in the a-phase volume fraction due to increased interfacial strain. For the first time, quantitative correlations established between various ultrasonic parameters and the volume fraction of a-phase in a b-titanium alloy are reported in the present paper. The established correlations are useful for estimation of volume fraction of a-phase in heat-treated btitanium alloy, by nondestructive ultrasonic measurements.