Microstructure And Mechanical Properties Of Ti-6Mo-5V-3Al-2Fe-2Zr Alloy Under Different Heat Treatment States

The metastable beta titanium alloy is one of the ideal lightweight metal materials for aerospace due to its high specific strength.With the continuous development of aerospace industry,more and more high performance requirements were put forward for the metastable βtitanium alloy.In this study,metastable β titanium alloy Ti-6Mo-5V-3Al-2Fe-2Zr(wt.%)was taken as the object.Microstructure,tensile properties at room temperature and low cycle fatigue behavior of the alloy after heat treatment with different process paths were studied.In order to provide reference for the formulation of heat treatment process and fatigue performance of the alloy in industrial application.The main research results are as follows :The alloy was treated by solution and single-stage aging,solution and double-stage aging and solution and air cooling and furnace cooling.After heat treatment of different process paths,secondary α phase precipitated in the alloy.Moreover,the morphology,size and number of secondary α phases were different.For the solution and single-stage aging alloy,the short rodlike intracrystalline α phases with uniform size and dispersed distribution were precipitated in the grain,and the continuous grain boundary α phases were precipitated at the grain boundary.The intracrystalline α phases and grain boundary α phases were precipitated in the alloy under solution and different double-stage aging conditions.The size of some secondary α phases was relatively large,and a large number of smaller secondary α phases were included between the coarse secondary α phases.Temperature of pre-aging in double-stage aging was the main factor affecting the average phase spacing of secondary α phase.After solution and air cooling and furnace cooling treatment,acicular intracrystalline α phases were precipitated in the alloy.Except formimg the grain boundary α phase in the alloy,and the WGB α phase,which was nucleated along the grain boundary α phase and grows in parallel to the grain was also formed.Alloys under different heat treatment states exhibited different tensile properties at room temperature.The strength-plastic matching of solution and air cooling and furnace cooling alloy was relatively good.The tensile strength,yield strength and elongation of the alloy were1421 MPa,1324MPa and 7.7%.Second was solution and single-stage aging alloy.The strengthplastic matching of solution and double-stage aging alloy was relatively poor.Combined with the microstructure and the tensile test results at room temperature of alloys under different heat treatment states,the influence law and mechanism of secondary α phase on the strength of alloys were quantitatively analyzed.The results showed that the average spacing of secondary α phase was the main factor affecting the strength of the alloy.Strength of alloy decreases with the increase of secondary α phase spacing.Under alternating load,the cyclic stress response behaviors of solution and single-stage aging and solution and air cooling and furnace cooling alloys were similar.The alloys exhibited cyclic stability at low strain amplitudes of 0.6%-0.8%.However,at higher strain amplitudes of0.9% and 1.0%,the alloys exhibited different cyclic softening.The cyclic softening phenomenon of the alloy under high strain amplitude was the result of the coupling effect of friction stress change and back stress change.Under all the applied strain amplitudes,the alloy under the solution and single-stage aging showed longer low-cycle fatigue life.