Microstructure Evolution And Mechanical Behavior Of IN740H Alloy Under Different Deformation Conditions

IN740H alloy is a nickel chromium cobalt based superalloy developed on the basis of IN740alloy by increasing the content of Al,reducing the content of Ti and controlling the content of Si,with excellent high temperature durable creep performance,long-term aging microstructure thermal stability,excellent processing and welding performance and low cost,the heat exchange tube and high temperature steam pipeline for A-USC reheater at 700℃have been widely used.Hot extrusion is a key process in the preparation of IN740H alloy pipes,due to its many alloying elements and high content,it has the characteristics of large deformation resistance and narrow hot working temperature range during processing,hot working is very difficult,therefore,the thermal modification behavior of the alloy is then explored by thermal simulation,and the evolution of the tissue of the thermal processing parameters is explored,which has important engineering significance for guiding heat extrusion production.This paper uses the Gleeble-3500 thermal simulation machine to perform an lsothermal hot compression test of IN740H alloy in solid solution state under various deformation temperatures and strain rates,flow stress curve is obtained by collecting and processing the test data,the influence of different thermal processing parameters on the flow stress is analyzed,and a high-temperature constitutive model is constructed through the Arrhenius equation to predict the flow stress.Drawing thermal processing map based on DMM dynamic model theory,microstructure and deformation mechanism of alloys under different deformation conditions is studied by means of OM,SEM and EBSD,through high temperature tensile tests at different temperatures,the high temperature fracture mechanism of the alloy is explored,the microstructure evolution and mechanical behavior of IN740H alloy under different deformation conditions are systematically analyzed.The flow stress curve indicates that at a constant strain rate,the peak stress of IN740H alloy decreases as the deformation temperature increased,and at constant deformation temperature,the peak stress of alloy decreases with a decrease in the strain rate.The constitutive equation of the alloy is established by Arrhenius formula to simulate the high temperature deformation process of the alloy.DMM dynamic model theory is used to construct a hot working map,and the safe region of hot deformation and instability region of the alloy are getted,comparative analysis with thermal deformation microstructure and derive the best hot processing parameters.The rheological instability diagram and power dissipation diagram show that the alloy exhibits optimal thermal processing characteristics at the temperature range of 1120-1200℃and the strain rate of 0.01-0.32 s^-1.Microstructure analysis shows that the recrystallized grains of the alloy in the hot deformation structure are obviously refined compared with the original grains,this is due to the increase of the free energy between the new grains and the original grains with an increase in deformation,an increase in temperature or a decrease in strain rate,dynamic re-crystalline grain growth is increased,the nucleation rate and growth rate of dynamic recrystallization are increased,the recrystallized grains grow,and the recrystallization behavior is more likely to occur.EBSD study of the hot-deformed microstructure reveals that discontinuous dynamic recrystallization（DDRX）is the dominant nucleation mechanism during hot compression of IN740H alloy,with many miscatenations in the grain boundary,the dislocation is blocked,this blocking will force the boundary to slide the crystal grains having a higher fault density,reduce the dislocation density in the region of the sliding grain boundary,and then the recrystallized nucleus of DDRX is generated.The fracture mechanism of IN740H alloy at high temperature is mainly ductile fracture,the tensile strength of the alloy generally shows a downward trend,the reduction of area and elongation after fracture show a downward trend first and then an upward trend,this is because the fracture mechanism of the alloy changes from brittle fracture to ductile fracture at 750℃.