Effect Of Heat Treatment On Microstructure And Mechanical Properties Of Pure Molybdenum

Metal molybdenum is widely used in aviation,aerospace,power,nuclear reactor,and other fields due to its high melting point,high strength,low linear expansion coefficient,and good high-temperature performance.Pure molybdenum sheet has poor ductility and toughness due to its processing characteristics that decrease with the increase of the plastic-brittle transition temperature.Pure molybdenum sheet during rolling deformation and heat treatment process has characteristics such as high starting recrystallization temperature,lack of nucleation particles,and easy growth of non-uniform crystal grains,which seriously restricts the deep processing and application range of molybdenum materials.Therefore,it is of great significance to study the effects of rolling deformation and heat treatment on the structure and properties of pure molybdenum sheet,reduce its internal stress,regulate the structure of the molybdenum sheet through recrystallization,and improve the comprehensive performance of molybdenum sheet in service.In this paper,four kinds of pure molybdenum plates with deformation amounts of70%,80%,90%,and 95% were taken as research objects.The heat treatment process with the heat treatment temperature of 800-1300°C and heat preservation for 1-2 hours was adopted.The microstructure and properties of pure molybdenum plates were characterized by metallographic microscope,scanning electron microscope,differential scanning calorimetry,and microhardness test.The effects of deformation degree and heat treatment parameters on the microstructure evolution and mechanical properties of deformed pure molybdenum plates were studied.The results show: In the process of heat treatment of rolled molybdenum sheet,the grain of rolled molybdenum sheet changes from flattened rolling to equiaxed as the heat treatment temperature increased.The recrystallization grain size of the large variable pure molybdenum sheet decreased obviously and the structure was more uniform.The recrystallized grain size of the pure molybdenum sheet with 95%deformation decreased by 43.2% compared with that with 90% deformation.In the process of heat treatment,the measure of raising the heat treatment temperature was more helpful to the recrystallization process than the measure of extending the holding time.The texture composition of pure molybdenum sheet was different at different temperature.In the lower temperature heat treatment process,the texture components in the pure molybdenum sheet with high deformation were Goss texture and Copper texture.In the high-temperature heat treatment process,the texture components were Goss texture,γ-fiber,and cubic texture.The four molybdenum pure molybdenum sheets have Goss component textures after they have been completely recrystallized.As the heat treatment temperature increases,their orientation gradually deflected toward the γ-fiber component texture,leading to a decrease in strength;the transition path of the weak texture Copper texture.It was transitioned from {112} <110>to {110} <110> and finally converted to {001} <100>.Increasing the heat treatment temperature can more effectively increase the recrystallization volume fraction in the pure molybdenum sheet.The 95% large plastic deformation pure molybdenum sheet has a recrystallization volume fraction of 97.96%after heat treatment at 1100°C and was completely recrystallized.The JMAK equation is used to describe heat treatment at 900°C under large plastic deformation of 95%pure molybdenum plate Avirami index was 3.6.The amount of deformation mainly affected the initial temperature of recrystallization grain growth during the recrystallization of pure molybdenum sheet.The strarting temperature of recrystallization grain growth of pure molybdenum sheet with 95% large plastic deformation was 45°C lower than that of sheet with 70%deformation.When the heat treatment temperature was in the annealing temperature range of950-1150°C,the effect on the hardness of the pure molybdenum sheet was more significant than that of the extended holding time.After complete recrystallization,the microhardness of pure molybdenum plates was maintained at 205.25± 10 HV.The softening of pure molybdenum sheet during heat treatment is caused by the synergistic effect of texture evolution and nucleation and growth of recrystallized grains.