Laser ultrasonic velocity measurement for phase transformation investigation in titanium alloy

We report on a contactless laser ultrasonic (LUS) method to monitor the phase transformation in a titanium alloy (Ti-6%Al-4%V) sheet by using a simple and high resolution LUS interferometer, based on two wave mixing in a fast BSO photorefractive crystal. The temperature dependent phase transformation from the α to the β phase in the Ti samples was observed around 1000°C. During heating the Ti sheets, the velocity of the laser generated longitudinal and shear ultrasonic wave, 5820 and 2950 m/s respectively, decreases fast up to 980°C which shows the start of the phase change region. At 980°C to 1020°C the bulk wave velocity is approximately constant (4820 and 2600 m/s) indicating the phase transformation process from the α to the β phase. Due to microstructural changes in the β phase at higher temperatures, the velocity of the bulk waves decreases slowly in comparison to that of the α phase.