Regularities in superplasticity of titanium alloys in relation to their initial structure and phase composition

Abstract

The effect of the initial structure and phase composition on superplastic properties of pseudo α (Ti-4Al-2V), α + β (Ti-6Al-4V) and near β (Ti-5Al-5Mo-5V-1Cr-1Fe) titanium alloys was studied in the temperature range of 773 – 1223 K. It was shown that in the coarse-grained Ti-4Al-2V alloy superplasticity is not realized in the temperature range studied. It is assumed that this is due to the low concentration of β stabilizing elements and difficult development of phase transformation that promote the conversion of the lamellar structure into a globular one. In the case of two other alloys with a coarse-grained structure a decrease in the yield stress and an increase in elongation to failure over 150% are observed at temperatures above 1073 K. All the alloys with a fine-grained structure exhibit a superplastic flow with elongations to failure over 300% regardless of their phase composition. The formation of an ultrafine-grained structure in the alloys results in a decrease in the temperature of the beginning of the superplastic flow realization up to 823 K independently of the phase composition as compared with coarse-grained and fine-grained alloys. The β phase volume fraction and the nature of its precipitation have a significant effect on the features of the development of a superplastic flow and the maximum values of the relative elongation to failure. This effect is apparently due to the stabilization of the ultrafine-grained state by the precipitations of the β-phase along grain boundaries (Ti-6Al-4V alloy) or the formation of a micro-duplex two-phase structure (near β alloy).