Direct observations of dynamic and reverse transformation of Ti-6Al-4V alloy and pure titanium

Abstract

This paper focused on the characterization and mechanism of the dynamic transformation from the alpha to beta phase during the hot deformation of Ti-6Al-4V alloy and pure titanium. The investigation employed in-situ neutron diffraction and atomistic simulations for a comprehensive understanding of the process. Dynamic transformations were observed during deformation of the Ti-6Al-4V alloy and pure titanium below the beta-transus temperatures (temperatures at which (alpha + beta) / alpha → beta). In the dynamic transformation of Ti-6Al-4V, the lattice parameters of the beta phase continuously increased during deformation in the tensile and transverse directions, a trend inconsistent with the behavior of flow stress. The anomalous variations in lattice parameters were attributed to element diffusion (i.e. a decrease in vanadium content in beta phase). During isothermal holding after unloading, the in-situ neutron diffraction results for Ti-6Al-4V and pure titanium indicated a sluggish reverse transformation from the beta to alpha phase. The mechanism of dynamic transformation was explored through in-situ neutron diffraction and atomistic simulations, which revealed twofold effects of deformation on dynamic transformation. Firstly, deformation led to a significant rise in the Gibbs energy of the alpha phase relative to the beta phase, expanding the beta phase region and diminishing the alpha phase region. Secondly, deformation lowered the energy barriers associated with dynamic transformation, facilitating the activation of dynamic transformation more readily than in the equilibrium state before deformation.