Preparation And Composition Design For Novel Co-based Wrought Superalloy

In order to improve the efficiency of aero-engines,the turbine inlet temperature of the aero-engine continues to increase,and the operation temperature of its key hot-end component turbine disks also continue to increase.At present,the widely used turbine disk alloy is nickel-based wrought superalloy strengthed by ?'-Ni3(Al,Ti)phase.A large number of refractory elements are added to improve its high temperature performance,which makes the hot workability of the alloy sharply decline.The service temperature is limited to 750? below,can not meet the needs of the next generation of turbine disk materials.The novel ?'-Co3(Al,W)strengthened Co-Al-W-based superalloy has a smaller tendency to solidify and segregate,a larger hot working window,and potential high temperature strength is regarded as the next generation of turbine wrought superalloy.At present,this alloy has the problems of simple components,large specific gravity,insufficient stability of ?',and unreasonable alloying elements,which leads to the precipitation of harmful phases,which limits the application of this alloy.In this work,the CALPHAD method is used to design and optimize the composition of the alloy by predicting the influence of the alloy composition and heat treatment system on the alloy phase composition.The phase transition law is used to guide the alloy preparation and hot compress performance research.The following results were obtained:(1)The composition range of low-W,high-Ta and Ti alloys has been preliminarily determined Co-9Al-(3?5)W-(15?30)Ni-(2?3)Ta-(3?5)Ti(at.%),Using the Pandat calculation software and cobalt-based superalloy thermodynamic database,the influence of W,Ni,Ta and Ti on the equilibrium phase precipitation behavior was calculated.The results show that:W,Ni,Ta and Ti can improve the stability of ?',W and Ta will increase the precipitation tendency of harmful phase ?-DO19,Ti and Ni can suppress the precipitation of harmful phase ?-D019 and B2 phase,respectively,and optimize the alloy composition to Co-9Al-3W-30Ni-2Ta-3.5Ti(at%).(2)The calculation results of the phase transformation law of the alloy indicate that the equilibrium precipitation phases of the alloy are ?,?',?-D019 and Co7Ta2,and the initial precipitation temperatures are 1355?,1169?,700? and 288?,respectively.Further formulate the alloy heat treatment system:1250?/12h homogenization,1150?/6h solution treatment,900?/4h+750?/16h two-step aging treatment.(3)A vacuum-induced melting method was used to prepare a cast alloy sample with qualified components without macrosegregation.After the above heat treatment,the elemental segregation and dendrite structure of the cast alloy were eliminated.The DSC analysis result is consistent with the calculated phase transition law,which verifies the accuracy of thermodynamic calculations.SEM and XRD results show that the equilibrium phase in the alloy is a typical ?+?' two-phase structure.(4)The hot compression deformation behavior at elevated over the temperature range from 1100-1200? with the strain rates from 0.01?1s-1 were investigated by Gleeble3500 type thermal compression simulator.The results show that the average values of strain rate sensitivity exponent,m,were 0.158,0.169 and 0.164 and the average values of deformation activation energy Q were 538.79 kJ/mol,1147.64 kJ/mol and 1110.26 kJ/mol,respectively during hot press deformation in the ?+?' two-phase region(1100?,1150?),? single-phase region(1180?,1200?)and the full temperature range(1100-1200?),By summarizing the experimental data,the true stress ?(MPa)and strain rate ?(1/s)and deformation temperature T(K)between mathematical models.By summarizing the experimental data,the mathematical model amoung the true stress ?(MPa),the strain rate?(1/s)and the deformation temperature T(K)of the Co-9Al-3W-30Ni-2Ta-3.5Ti alloy during hot compression deformation are obtained.