| In this study,a novel high strength metastableβtitanium alloy,Ti-3.9Al-6.8Mo-1.2V-2.8Cr-1Zr-2Nb,was taken as the research object.The gradient heat treatment of the alloy was carried out to obtain a large number of microstructure characteristics on a single sample and optimize the mechanical properties by adjusting the microstructure.By means of scanning electron microscope（SEM）,optical microscope（OM）and transmission electron microscope（TEM）,the effects of solution temperature and aging temperature on the size,morphology and volume fraction of primary and secondary phases were studied.The aging hardening behavior of the alloy was studied by microhardness test,and combined with tensile test,the influence of solution and aging temperature changes on the mechanical properties of the alloy was systematically studied,the solution and aging temperature was quickly optimized,and the strength and ductility of the alloy were improved.The main conclusions are as follows:（1）The morphology and volume fraction of primaryαphase are very sensitive to the change of solution temperature.The volume fraction of primaryαphase decreases from 30%of 746℃to 3%of 819℃.After solution treatment at 844℃and above,there are no primary phases in the alloy,and all metastableβphases are obtained.The morphology of primaryαphase is also sensitive to temperature.At the lower temperature（746-770℃）,there are two morphologies of primaryαphase:sphericalαp with a diameter of 1-5μm,and rodαr with a diameter of submicron.At the solution range of 795℃to 819℃,only gloublar primaryαphase is obtained.（2）After solid solution at 746-909℃and aging at 550℃for 8 h,αsphase precipitates in the alloy.Under the same aging condition,the volume fraction ofαs phase increases with the increase of solution temperature in the range of 746-819℃,but the size and phase spacing decrease.This results in an increase in yield strength and a decrease in ductility.In addition to the coarseαs phase,2-4μm wide PFZ can be observed at some grain boundaries at solution temperature of 844℃.In the temperature range of 865-909℃,the increase of solution temperature has no obvious effect on the morphology and volume fraction of secondary phase.（3）Under the same solution treatment condition at 746℃,the aging temperature has a great influence on the phase transition behavior and the morphology of the secondary phase of the alloy.After aging at 453-503℃for 8 h,the precipitates in the alloy areωiso phase andαs phase.The precipitation ofωiso phase reduces the ductility of the alloy.At 526℃or higher,onlyαs phase is precipitated.With the increase of aging temperature,αs phase coarsenes along width direction and becomes shorter along length direction.And the volume fraction goes down.The coarsening of secondary phase and the reduction of volume fraction make the strength decrease,but the ductility increase obviously.（4）After solution at 746℃for 2 h and aging at 608℃for 8 h,the microstructure of the alloy are as follows:globular primary phase with average diameter of 3.17μm,and The volume fraction of is 25.1%;rod-likeαr phase with average width and length of 0.21 and 0.88μm,and the volume fraction is 4.9%,αs phase with average width and spacing of 61 nm and 101 nm,and the volume fraction of is 27.2%.The tensile results show that the alloy has a superior combination of strength and ductility,with yield strength and elongation of 1140 MPa and 15.6%,respectively.The homogenous strain gradient between phases with different sizes and properties during deformation improves the strength and ductility. |
PDF Full Text Request