Effect Of Thermal Exposure On Microstructure And Mechanical Properties Of Semi-solid Forged Ti-Cu Alloy

Titanium alloys are widely used in many fields due to their excellent properties,especially in the aerospace field.Titanium alloy parts will be exposed to high temperature conditions for a long time during aircraft service.After the alloy undergoes thermal exposure,the microstructure and stress distribution state will change significantly,resulting in a rapid deterioration of the alloy properties and affecting the service life of the structural parts.Aiming at the problems of Ti-Cu alloys in high-temperature environments,the effects of their microstructure and mechanical properties under different thermal exposure conditions are studied to provide theoretical guidance for improving the thermal stability of Ti-Cu alloys in high-temperature environments.In this paper,by studying the influence of thermal exposure on the microstructure stability of three Ti-Cu alloys,combined with the law of microstructure evolution,the coarsening behavior of Ti₂Cu phase during thermal exposure was quantitatively studied,and the microstructure evolution model at different coarsening stages was established;The effect of microstructure evolution induced by thermal exposure on the mechanical properties of Ti-Cu alloys was systematically studied,and the strengthening mechanism of Ti₂Cu with different morphologies relative to the mechanical properties of Ti-Cu alloys was clarified.The study found that with the increase of thermal exposure time,the three alloys showed the phenomenon of structure coarsening.However,due to the better thermal stability of the whole lamellar structure of Ti-7Cu alloy,the coarsening rate of the structure is relatively slow.During the thermal exposure process,the granular and lamellar structure in the Ti-Cu alloy became coarser.Which is consists of rapid coarsening stage and stable coarsening stage.In the rapid coarsening stage,the volume fraction of Ti₂Cu phase increases by 6.44%,while the volume fraction of Ti₂Cu phase in the stable coarsening stage is only an increase of6.13%.These two stages are respectively controlled by the terminal migration mechanism and the Ostwald maturation mechanism.The granular Ti₂Cu phase destroys the stability of the lamella structure and causes coarsening behavior.After the thermal exposure time increases from 0 h to 200 h,Ti₂Cu in Ti-14Cu alloy of the phase spacing is reduced by 48.7%.As the distance between the Ti₂Cu phases is reduced,the slip length is reduced,resulting in a decrease in the elongation of the alloy.After the coarsening of the Ti₂Cu phase,the dislocation density near the?/Ti₂Cu interface decreases,and the tensile strength of the alloy decreases.During the thermal exposure process,the temperature has a more significant effect on the stability of the Ti-Cu alloy structure.As the temperature rises promotes the diffusion of solute atoms,the layered Ti₂Cu phase accelerates the coarsening rate with the increase of thermal exposure temperature,and thermal exposure at 400?.The thickness of the Ti₂Cu phase in the Ti-2.5Cu alloy middle layer was as high as 1.59?m after only 100 h.The Cu atoms in Ti-14Cu alloy have a large solid solubility in the?-phase matrix.Cu atoms are activated and diffused in the process of increasing the thermal exposure temperature,resulting in a higher coarsening rate of Ti₂Cu phase than Ti-2.5Cu alloy,and a large amount of coarsening of Ti₂Cu phase.As a result,the dislocation density in the alloy decreases,and the residual thermal stress has a dominant influence on the performance.The strength of the alloy decreases.The coarsening of the Ti₂Cu phase and the increase of the lamellar Ti₂Cu phase reduce the dislocation slip length and density,Resulting in a decrease in the plasticity of the alloy.