Study On Composition Design,γ/γ’ Evolution And Hot Deformation Behavior Of CoNi-based Superalloys

As the increase of aeroengine thrust-weight ratio,the demand for the temperature bearing capacity of superalloys for turbine disc is also increasing.However,with the increase of alloying extent,the elaborate control of microstructure and hot working of wrought superalloy for turbine disk become more and more difficult.Therefore,exploring new generation turbine disc materials that can be used at higher temperatures has become an urgent problem.Recently,the CoNi-based superalloys strengthened by precipitates with L12 structure are expected to be candidate materials for turbine disks used at higher temperatures due to their large composition design space and the combined advantage of casting and forging.As a result,it has become a research hotspot at home and abroad.In this work,senary CoNi-based superalloys with high γ’ solvus temperature,low alloy density and high-temperature oxidation resistance were designed by thermodynamic calculation of phase diagram.Meanwhile,the hightemperature stability of γ/γ’ microstructure,the coarsening behavior of γ’ precipitates,the effect of γ/γ’ structure on high temperature property and the hot deformation behavior of CoNi-based superalloy were studied.The thermodynamic calculation results show that Ni component significantly widens the γ+γ’ two-phase region,while Cr component narrows the γ+γ’ two-phase region and promotes the precipitation tendency of detrimental μ phase.Substitution W with the same content of Ti(Ta)increases the content of γ’ precipitates and γ’solvus temperature,as well as decreases the content and the precipitation temperature of DO19Co3W.During the aging treatment at 900℃ for 3000 h,Co-30Ni-9Al-6W-6Cr-2Ti(1.5Ta)superalloys can sustain the γ/γ’ microstructure stability and no detrimental secondary phases are observed.The coarsening kinetics of γ’ precipitates in CoNi-based superalloy was characterized by the classical LSW model during aging treatment at 800～950℃ for a short time.Besides,the coarsening rate of γ’phase increases and the coarsening activation energy decreases attributed to the addition of Ni,Ti component.A large number of fine secondary γ’ phases are precipitated from CoNi-based superalloys at a lower second-step aging temperature,and thus introduce the microstructure with bimodal size distribution of γ’ phase,resulting in the increase of γ’volume fraction and high-temperature strength.The phenomenon of temperature dependent yield strength was found in CoNi-based superalloy,and the yield strength abnormal behavior appeared in the temperature range of 600～700℃,mainly attributed to the cross-slip of a/2<110>screw dislocation pairs.Interestingly,the stacking faults shearing γ’ precipitates occur at peak temperature(700℃)in alloy with bimodal size distribution of γ’ phase,whereas the stacking faults appear above the peak temperature of 900℃ in the alloy with unimodal size distribution of γ’ phase.Combined the processing map of CoNi-based superalloy at deformation degree of 50%with the microstructure evolutions in "safety zones",the optimum processing parameters are determined to be deformation temperature range of 1100～1150℃ and strain rate range of 0.01～0.03 s-1.The discontinuous dynamic recrystallization(DDRX)and continuous dynamic recrystallization(CDRX)process occur simultaneously during hot deformation,and DDRX process is the dominating softening mechanism.During hot deformation,the recrystallization nucleation sites include the original grain boundaries,carbides and deformation microbands,where the DDRX mainly occurs at the original grain boundaries;the nucleation near carbides occurs by the particle stimulated nucleation mechanism;the microband assisted DRX occurs through CDRX or DDRX process.The effect of DDRX and microband assisted DRX was enhanced with increasing deformation temperature or decreasing strain rate.The effect of CDRX was firstly enhanced and then weakened with the deformation degree increasing from 30%to 70%.