Microstructural evolution during post-heat treatment and its effect on the mechanical properties of directed energy deposited near β titanium alloy

In this work, the microstructural evolution of a directed energy deposited near β titanium alloy, Ti5Al2Sn2Zr4Mo4Cr, during post-heat treatment was investigated, and its effect on the mechanical properties was evaluated. The results showed that the equilibrium volume fraction of the α phase was reached in the first 30 min over a temperature range of 720–840 °C. Fine basketweave α laths (α_BW) were homogenously distributed within the prior β grains at 840 °C. Both fine α_BW and α colonies developed from the grain boundary α layers (α_GBW) were obtained in the sample solutions treated at 720, 760, and 800 °C. Water-quenching and air-cooling were employed to retain the β phase at room temperature. The α phase volume fraction increased significantly and reached the equilibrium volume fraction during the furnace-cooling process. In addition, the manner in which the samples were subjected to isothermal temperatures can strongly affect the evolution of the α phase. The homogenized specimen containing a single β phase exhibited the lowest strength and the highest plasticity. The strength increased and the ductility decreased after subtransus solution treatment. The difference in the mechanical properties can be attributed to the differences in the α and β phases caused by the various heat treatments. The modified rule of mixtures appropriately described the relationship between the yield strength and microstructure characteristics.