Effects of Multistage Aging Treatment on the Microstructure and Mechanical Properties of α plus β-Type Ti-6Al-7Nb Alloy

The influence of multistage heat treatments on the microstructure and mechanical properties of Ti-6Al-7Nb alloy used for functional structures was systematically studied. Cast Ti-6Al-7Nb alloy samples were subjected to hot forging, annealing, and multiple stages of heat treatments. The relationship between volume fraction of a-phase and the mechanical properties was investigated. Herein, multiple heat treatment steps were selected based on the dilatometric analysis, at the end of the beta transformation and the start of the beta transformation, followed by aging at low temperature, i.e., 500 C-degrees. A higher volume fraction of alpha- aphase leads to higher strength but lower ductility. The yield, ultimate tensile strengths, and ductility of the annealed alloy are 822 MPa, 1019 MPa, and 10%, respectively. The corresponding values for the alloy treated at 900 C-degrees followed by WQ are 777 MPa, 932 MPa, and 16%, while the aged alloy has 879 MPa, 967 MPa, and 12%, respectively. A trade-off between strength and ductility was attained. Besides, the microstructural evolution of the studied conditions was also covered in depth.