Microstructure and mechanical properties of heat-treated Ti-5Al-2Sn-2Zr-4Mo-4Cr

The effects of heat treatment parameters on the microstructure, and mechanical properties and fractured morphology of Ti-5Al-2Sn-2Zr-4Mo-4Cr with the equiaxed, bi-modal and Widmanstätten microstructures were investigated. The heating temperatures for obtaining the equiaxed, bi-modal and Widmanstätten microstructures were 830, 890 and 920 °C, respectively, followed by furnace cooling at a holding time of 30 min. The volume fraction of primary α phase decreased with increasing the heating temperature, which was 45.8% at 830 °C, and decreased to 15.5% at 890 °C, and then the primary α phase disappeared at 920 °C during furnace cooling. The variation of volume fraction of primary α phase in air cooling is similar to that in furnace cooling. The increase in heating temperature and furnace cooling benefited the precipitation and growth of the secondary α phase. The equiaxed microstructure exhibited excellent mechanical properties, in which the ultimate strength, yield strength, elongation and reduction in area were 1035 MPa, 1011 MPa, 20.8% and 58.7%, respectively. The yield strength and elongation for the bi-modal microstructure were slightly lower than those of the equiaxed microstructure. The Widmanstätten microstructure exhibited poor ductility and low yield strength, while the ultimate strength reached 1078 MPa. The dimple fractured mechanism for the equiaxed and bi-modal microstructures proved excellent ductility. The coexistence of dimple and intercrystalline fractured mechanisms for the Widmanstätten microstructure resulted in the poor ductility.