The evolution of microstructure and mechanical properties of Ti-5Al-5Mo-5V-2Cr-1Fe during ageing

The phase transformations and compositional changes occurring during thermo-mechanical processing and subsequent high temperature ageing of Ti-5Al-5Mo-5V-2Cr-1Fe (wt.%) were investigated using scanning transmission electron microscopy (STEM) and atom probe tomography (APT). High resolution STEM revealed nano-sized alpha (<10 nm) and athermal omega (similar to 1-3 nm) formed during accelerated cooling from 800 degrees C and slow heating to an ageing temperature of 650 degrees C. Nuclei of alpha were found to form heterogeneously in the beta matrix as well as at the omega phase. APT revealed pronounced Mo compositional fluctuations in the beta matrix. No direct connection was established between Mo-rich or Mo-lean regions and alpha or omega nuclei. APT also failed to detect the omega phase, which supports theories that it forms by a shuffle mechanism, without any compositional difference from the beta phase. Very small alpha particles, after initial ageing, showed only a minute change in composition with respect to the beta matrix, indicative of a displacive-diffusional transformation. With further ageing, growth of the alpha lamellae was accompanied by compositional changes according to the diffusion rates of beta-stabilising elements. Pile-up of the slowest diffusing solutes (Mo, V) at the alpha/beta interface were pronounced in the initial stages of ageing. The best combination of mechanical properties (1200 MPa ultimate tensile strength with 15% total elongation) was recorded after 3.6 ks of ageing. (C) 2015 Elsevier B.V. All rights reserved.