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

摘要

Because of the nature of the plasma spray processes, physical and mechanical properties of the vacuum plasma sprayed structures of Ti-6Al-4V alloys are completely different compared to conventionally manufactured alloy. In order to reach to desirable mechanical and physical properties, vacancy and internal defects must be reduced, splats boundaries must be eliminated, and the optimal phase compositions should be obtained through the post deposition heat treatment. In order to have appropriate heat treatment processes, it will be needed to study the kinetic behavior of the as-sprayed microstructure at elevated temperatures. In the current study, the kinetic of solid transformations in Ti-6Al-4V alloys produced by vacuum plasma spraying process was studied based on Johnson-Mehl-Avrami (JMA) theory. In the kinetic behavior of this alloy, the dependency and lack of equality of the transformation rate constant with temperature caused an irregularity at 900°C. This irregularity showed deference between transformation mechanism above and below 900°C. At lower temperature (<900°C) curves constant gradient showed lack of change in the transformation mechanism including homogeneous nucleation and grown of α-phase. At higher temperature (>900°C) the gradient change indicated change in the transformation mechanism (first mechanism was formation of α-phase grain boundary and second mechanism was α-plate nucleation and grown from the grain boundaries). The value of the transformation rate constant in the kinetic study of the as-sprayed Ti-6Al-4V alloy was much higher than that produced from casting method. By using the results obtained from kinetics of β → α + β phase transformation at different constant temperatures, TTT diagram for the as-sprayed Ti-6Al-4V alloy was developed.