Influence of phase transformation kinetics on the formation of α in a β-quenched Ti-5Al-5Mo-5V-3Cr-1Zr alloy

The effect of phase transformation kinetics on the formation of α in the β-quenched Ti-5Al-5Mo-5V-3Cr-1Zr metastable β titanium alloy is investigated as a function of heating rate by means of in situ high energy synchrotron X-ray diffraction complemented by metallographic analysis. Quantitative phase analysis based on the Rietveld method provides the continuous evolution of phase volume fractions and lattice parameters, revealing variations in the phase transformation sequence with increasing heating rate. The initial microstructure consists in a matrix of equiaxed metastable β grains with ωath particles located along dislocation lines as well as within domains associated with spinodal decomposition of β. During the first stage of slow heating diffusion-driven formation of the metastable phases ω and αiso″ takes place with further decomposition of β. This effect contributes to the formation of ω and is observed as a symmetric pattern of parallel domains that reflect compositional modulations in the matrix. Evidences of the role of dislocations during phase transformation are presented. Furthermore, the results show that the stable α phase forms via two different paths: (a) slow heating rates provide homogeneous distribution of fine α plates formed through the evolution of ω, while (b) fast heating rates suppress this mechanism and promote the formation of α only from β grain boundaries.