In-situ synchrotron radiation study of the aging response of Ti-6Al-4V alloy with different starting microstructures

The aging behavior of a Ti-6Al-4V alloy with different starting microstructures was evaluated by means of synchrotron X-ray diffraction, scanning-transmission electron microscopy and micro-hardness measurements. Initial microstructures were produced by thermal and thermomechanical treatments and comprised different morphologies of alpha phase (martensitic, lamellar, bimodal and globular), as well as the presence or absence of the beta phase. Results show that one or more of the following phenomena can take place during aging and contribute to the hardening of the alloy: beta decomposition into fine secondary alpha laths; transformation of the metastable martensitic alpha' into the equilibrium alpha phase; and precipitation of the intermetallic Ti3Al. The composition and distribution of the beta phase was shown to affect the precipitation of secondary alpha during aging, while the composition of the alpha phase plays a key role on the formation of Ti3Al. In situ X-ray diffraction studies of the early stages of aging show the kinetics of the alpha' -> alpha conversion by the reduction in FWHM of XRD reflections, indicate the contribution to hardening by the increase of the c/a ratio and the consequent limitation of active slip systems and depict the chemical homogenization and decomposition of the beta phase.