Research On Effect Of Thermal Cycle On Microstructure And Mechanical Properties Of Semi-solid Forging Ti-Cu Alloy

Titanium alloy is an indispensable material for key parts of aerospace vehicles.As titanium alloy parts experience repeated impacts at different temperatures during service,the alloy microstructure will change.The effect of thermal cycling on the evolution of alloy structure is different from that of long-term isothermal heat treatment.The deterioration of the microstructure caused by repeated thermal shocks will cause the alloy service cycle to be shortened rapidly and seriously affect the application of the alloy.Therefore,studying the effects of thermal cycling on the microstructure evolution and mechanical properties of Ti-Cu alloys can provide a theoretical basis for the changes in the microstructure and properties of titanium alloys under thermal cycling conditions.In this work,by studying the influence of the number of thermal cycles at 400?on the microstructure and properties of the hypoeutectoid alloy Ti-2.5Cu,the eutectoid alloy Ti-7Cu,and the hypereutectoid alloy Ti-14Cu,and the effect of thermal cycle temperature on the evolution of the alloy structure under the conditions of low thermal cycles and low thermal cycles.Combined with the law of microstructure evolution,the thermal stability of microstructures of Ti₂Cu phases with different morphologies under thermal cycling conditions is analyzed.Finally,the effect of microstructure evolution on the changes of mechanical properties of the alloy under different thermal cycle conditions was systematically studied,and the strengthening mechanism of Ti₂Cu relative to Ti-Cu alloy properties was discussed.It's found that the Ti₂Cu phase in the Ti-7Cu alloy is fully lamellar distributed on the?matrix,and it exhibits better thermal stability of the structure during thermal cycling.After 50times of thermal cycles at 400?,the interfacial energy of?/Ti₂Cu phase increases under the action of accumulated heat,which causes the blocky Ti₂Cu phase in Ti-2.5Cu alloy to merge and grow,but it deteriorates the mechanical properties of the alloy.Ti-14Cu alloy grain boundaries are distributed with large-size blocky Ti₂Cu phases formed by peritectic solidification,and the grain boundaries,as a kind of surface defect,have a greater impact on the stability of the surrounding lamellar Ti₂Cu phase structure.As the number of thermal cycles increases,the lamellar structure on both sides of the blocky Ti₂Cu phase gradually coarsens and transforms into a blocky Ti₂Cu phase,and a metastable Ti Cu phase is formed when the thermal cycle reaches 50 times.After 200 thermal cycles,the lamellar Ti₂Cu phase was coarsened,which seriously affected the plasticity of the alloy and caused the alloy elongation to drop by as much as 81.3%.During thermal cycling,temperature has a more significant effect on the structure of Ti-Cu alloy.Due to the low activation energy of Cu atom diffusion,when the thermal cycling temperature reaches 500?,only 20 thermal cycles can promote the grain boundary size of Ti-2.5Cu alloy from 0.56?m increased to 0.74?m.At the same time,it is found that the“lamellar+blocky”structure is extremely sensitive to the thermal cycle temperature.The volume free energy and interface energy of the alloy increase during the temperature increase of the thermal cycle,which promotes the interface migration rate during the coarsening of the lamellar Ti₂Cu phase to accelerate significantly After 20 thermal cycles at 600°C,the lamellar Ti₂Cu phases on both sides of the grain boundary coarsened a lot.Under repeated heating,the difference in the thermal expansion coefficients of the two phases in the Ti-Cu alloy increases,resulting in thermal mismatch,causing interface mismatch to promote atom diffusion along the interface defects,and accelerating the coarsening of the lamellar Ti₂Cu phase during the process of increasing the thermal cycle temperature,Resulting in an increase in the average size of the Ti₂Cu phase and a decrease in the dislocation density,which greatly affects the mechanical properties of the alloy and promotes the transformation of the alloy from plastic fracture to brittle fracture.This work has studied the effect of microstructure evolution and Ti₂Cu phase morphology changes on mechanical properties of Ti-Cu alloy during thermal cycling,which can provide theoretical basis for Ti-Cu alloy composition design,microstructure control,etc.It is of great significance for the long-term stability of Ti-Cu alloy service.