Nucleation and Growth Transformations in Vacuum Plasma Sprayed Ti-6Al-4V Alloy

Abstract

Abstract This study investigates isothermal transformation kinetics in Ti-6Al-4V alloys structures produced by vacuum plasma spray forming. As-sprayed samples were homogenized in the β phase followed by fast cooling to the two-phase temperature region, then quenching to suppress further transformation. The microstructure of heat-treated specimens was examined by optical microscopy and equilibrium phases were measured using image analysis. The kinetics of the β → α+β phase transformation are revealed by plotting the amount of α-phase obtained over a 10 to 60 s interval at isothermal temperatures of 800, 850, 900, and 950 °C. Corresponding phase transformation rates are also calculated based on Johnson-Mehl-Avrami (JMA) theory. At temperatures below 900 °C, the main phase transformation mechanism is homogeneous nucleation and growth of α-phase. At higher temperatures, phase transformations are driven by two mechanisms: the formation of α-phase in grain boundaries and α-plate nucleation and growth.