The effect of thermo-mechanical processing and ageing time on microstructure and mechanical properties of powder metallurgy near β titanium alloys
Powder metallurgy near β Ti-5Al-5Mo-5V-1Cr-1Fe and Ti-5Al-5Mo-5V-2Cr-1Fe (in wt.%) alloys were investigated to establish the microstructure-mechanical properties relationships during thermo-mechanical processing and after isothermal ageing. Thermo-mechanical processing included high temperature deformations (900 and 800 °C) and soaking (800 °C) after solution treatment, whereas ageing was performed at 650 °C. The flow behaviour of both alloys at 900 °C displayed a short yield drop followed by an increase with strain due to generation of dislocations at grain boundaries. However, a continuous softening was observed after reaching peak stress at 800 °C; microstructural features at corresponding temperatures caused such variation. Following the heat treatment, the alloys were characterised using electron microscopy, X-ray diffraction and tensile test. In addition to β matrix phase, α with different morphologies were observed in the aged microstructures. While the highest ultimate tensile strength for Ti-55511 and Ti-55521 are found to be 1298 and 1267 MPa for samples aged 8 h, the maximum ductility for Ti-55511 and Ti-55521 (14 and 15%) are achieved for 1 h aged samples, which still maintained a high strength of 1200 and 1145 MPa, respectively. The good combination of tensile mechanical properties could be attributed to the design of processing schedule with slow heating rate to ageing temperature. The obtained variation in tensile properties of the alloys was associated with the different volume fractions of α phase, which is related to Cr content effect.