Designed bimodal size lamellar O microstructures in Ti_2AlNb based alloy: Microstructural evolution, tensile and creep properties

摘要

Microstructure evolution, tensile and creep properties of designing bimodal size lamellar O phases by thermo-mechanical processing including conventional forging, isothermal forging process and heat treatment for Ti-22Al-25Nb (at%) orthorhombic alloy were investigated. The microstructures were obtained by different solution- and age-treatment temperatures, and analyzed by the BSE technique. The creep behavior of the alloy was studied at 650 ℃/150 MPa for 100 h in air. The tensile strength of the alloy at room temperature and 650 ℃ was also investigated. The experimental results showed that the microstructure of the isothermally forged alloys at 1080 ℃ contained the non-uniform distribution of lamellar O phases and B2 matrix. The advantage of the bimodal lamellar size distributed alloy can be concluded that firstly, the coarse lamellar O formed during solution process makes the alloy owns good elongation and secondly, the fine lamellar O precipitate during the aging process strengthens the alloy. The volume fraction and mean thickness of the lamellar O could be well controlled by the heat treatment. The yield strength was sensitive to the thickness of lamellar O, increase in the aging temperature leads to a decrease in strength and an increase in ductility. The relationship between creep resistance of alloys and microstructural features, such as the volume fraction of each phases, the morphology of lamellar O phases was also discussed.