Thermal Decomposition of Massive Phase to Fine Lamellar α/β in Ti-6Al-4V Additively Manufactured Alloy by Directed Energy Deposition

The occurrence of a massive phase (alpha(m)) transformation and a thermal decomposition of an alpha(m) to fine lamellar alpha/beta with an alpha lamellar width of approximately 1 mu m have been identified in additively manufactured Ti-6Al-4V alloy by directed energy deposition. The beta to alpha(m) phase transformation during additive manufacturing generated a high dislocation density, which was calculated by the kernel average misorientation. This caused a significant change in the Gibbs free energy of alpha, resulting in the nucleation of the beta phase becoming preferable in the area with high dislocation density at temperature below the beta-transus temperature. Subsequent annealing at 850 degrees C altered the massive morphology to lamellar due to the formation of a beta phase between alpha lamellar boundaries.