Formation mechanism of titanium solid solution and its influence on equiaxed behavior of α phase of Ti–5Al–5Mo–5Cr–2Zr-xNb alloys

Abstract

To clarify the formation mechanism of titanium solid solution (Tiss) and its effect on the microstructure, Ti–5Al–5Mo–5Cr–2Zr-xNb alloys are prepared by vacuum induction melting and multi-directional forging (MDF). The results show that the alloy phase is composed of grain boundary α (αGB), primary α (αp), secondary α (αs) and residual β phases. The crystal structure of Tiss is consistent with that of the β phase. Nb atoms rapidly spread along the dislocations and aggregated to the α phase boundary during MDF, and Nb atoms dissolved in the lattice of the β phase to form Tiss. There are (0002)α∥(3¯21¯)Tiss and (011¯0)α∥(121¯)Tiss orientation between titanium solid solution and equiaxed αp phase. Therefore, the equiaxed behavior of the αp phase is characterized by the formation of new phase boundaries caused by dislocations captured by Tiss. The tensile strength and toughness increased when the Nb content increased from 3 to 9%, which is attributed to the increase in the α phase content and the solid solution strengthening of Nb. When the Nb content increased to 12 wt.%, the tensile strength and toughness decreased due to the debonding of the lamellar αp phase. The equiaxed αp phase improves the plasticity of the titanium alloy, so the strain decreases with decreasing content.