| At the present stage,aerospace structural components put forward more stringent indicators on the mechanical properties of titanium alloy materials,so the research on high strength-toughness titanium alloy is favored by people..The research object of this paper is a newly developed metastable ? titanium alloy.At present,the research on it is insufficient,including the study of its phase transformation process and the heat treatment of large bars,the relationship between microstructure and properties.Therefore,further research is particularly important.In this paper,the grain growth kinetics,microstructure evolution,aging hardening characteristics and phase transformation kinetics of the alloy after solution and aging in the ? phase region were systematically studied.The mechanical properties of the alloy after solution and aging in the single-phase region and two-phase region were compared,and the strength and toughness of the alloy were matched.The heat treatment process was optimized.After the research in this paper,the following main conclusions are obtained:When the alloy is dissolved below 890?,the stability of ? grain size is good,and it is not sensitive to the change of temperature and time.But after heat treatment above 890?,the grains coarsen sharply.So for the alloy,the suitable solution temperature range is 920?.The uniformity of solid solution grain size decreases first and then increases with the extension of time,and the higher the temperature,the shorter the time to reach the minimum value of uniformity ?,the worse the uniformity of grain size.The growth index of the alloy increases with the increase of temperature,ranging from 0.22 to 0.39,and the growth index increases sharply above 890?.The grain growth activation energy of the alloy increases with the extension of holding time,and its value is between 46.2 kJ/mol and 100.6 kJ/mol.Considering the suitable aging temperature of the alloy,the alloy has high aging response speed at 450-600?,so the suitable aging temperature is above 450?.When aged at 400?,the ? phase precipitated in the crystal is more uniformly dispersed than that at the grain boundary.When aging at 500?,the morphology change process of ?phase is 'point into line,line into sheet'.After a long time aging,the size of ? phase tends to be uniform.When aging at 600?,the precipitation rate and density of a phase at the grain boundary are greater than those in the crystal,and the growth direction of a phase is perpendicular to the grain boundary and extends to the crystal.Alloy aging temperature window is wide,aging strengthening effect is good,with heat treatment strengthening operability,is an ideal structural material.The JMA equation can accurately describe the isothermal aging phase transformation kinetics of alloy at nose temperature(520?).When the solution and aging treatment was carried out in the ? phase region,the original ? grain size had little effect on the strength of the aged alloy.When the ? grain size was in the range of 150-210 ?m,the fracture toughness of the alloy was less affected by the grain size.When the ? grain size was in the range of 240-326 ?m,the fracture toughness decreased with the increase of the grain size.When the solution was aged in the two-phase region,with the increase of solution temperature,the volume fraction of primary a phase gradually decreased,and the morphology of a phase changed from rod-like to spherical,while the KIC value of the alloy also increased.Based on the mechanical properties test results of the single-phase region and the two-phase region,it is found that the solid solution and aging treatment in the single-phase region can achieve good matching of strength and toughness of the alloy,while the solid solution and aging treatment in the two-phase region can achieve good matching of strength and plasticity.The best heat treatment system for strength and toughness matching of the alloy at each strength level is given. |
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