In-situ XRD study on the effects of stress relaxation and phase transformation heat treatments on mechanical and microstructural behaviour of additively manufactured Ti-6Al-4V

Additively Manufactured (AM) titanium (Ti) components are routinely post-thermal heat treated (HT), to reduce internal stresses, as well as to obtain more desirable microstructural features, yielding improved mechanical performance. Currently, there is no consensus on the optimum HT method for AM Ti-6Al-4V, as the mechanism for the main phase transformation (alpha' (martensite) -> alpha + beta (equilibrium)) is still ambiguous. In this study, stress relaxation and phase transformation in the alloy are investigated in detail, via isothermal heat treatments and in situ high temperature X-ray Diffraction (XRD). The latter was carried out at heating rates of 5 and 200 degrees C/min. The relationship between crystallographic evolution during isothermal treatments and mechanical behaviour was determined. Isothermal holding at 400 degrees C resulted in an increase in ultimate tensile strength (UTS) and yield strength (YS) by 3.4% and 2.1%, respectively, due to the relief of tensile microstrain. It was found that isothermal treatment conducted between 550 and 700 degrees C promotes martensitic decomposition, resulting in the formation of a transitional - alpha(tr) phase, which has an asymmetrical hexagonal crystal lattice. The formation of this alpha(tr) phase was determined to be the main factor contributing to a major decrease in ductility.