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

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 (α_GB), primary α (α_p), secondary α (α_s) 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 α_p phase. Therefore, the equiaxed behavior of the α_p 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 α_p phase. The equiaxed α_p phase improves the plasticity of the titanium alloy, so the strain decreases with decreasing content.