Effect Of Solidification Method And Tantalum On Microstructure Evolution Of Directionally Solidified Ni-based Superalloy DZ411

With the increase of inlet temperature of gas turbine,hot corrosion resistant superalloys with higher temperature capacity and more stable microstructure are required.In this work,the temperature field and grain structure of DZ411 alloy during directional solidification by high-rate solidification(HRS)and liquid metal cooling(LMC)method were simulated by ProCAST software.Three alloy ingots with different Ta contents(2.72,3.10,and 4.00 wt.%)were obtained by arc melting.The directional solidification of three alloys with different Ta contents is carried out by LMC method,and the three alloys are heat-treated under the same conditions.Then,the fully heat-treated alloys are held at 950?for different times.The effects of Ta on directionally solidified microstructure,heat treatment microstructure and isothermal long-term aging microstructure stability of the alloy were investigated.Provide theoretical support for the development of hot corrosion resistant directionally solidified Ni-based superalloys.ProCAST simulation results show that the LMC method increases the temperature gradient of the alloy,resulting in the decrease of the width of the mushy zone.Meanwhile,the solidification interface of the alloy is smoother and the grain orientation deviation is smaller.In addition,the LMC method has higher temperature gradient and cooling rate.The experimental results of different solidification methods show that the size of dendritic structure and microstructure(carbides,?'phase)obtained by LMC is smaller than that of alloy obtained by HRS,and the area fraction of different phase is larger than that of alloy obtained by HRS.Both the JMatProsimulation and differential scanning calorimetry(DSC)experimental results show that the solidification freezing range of the alloy increases with the increase of Ta content.Therefore,the primary and secondary dendrite arm spacing of the alloy increase during the directional solidification process.In addition,more carbides and eutectic are formed in the interdendritic region,which is attributed to the addition of Ta,thus aggravating the segregation of Ta in the liquid phase.Besides,the addition of Ta increases the saturation of?matrix,resulting in the increase of nucleation rate of?'phase in?matrix,the distance between?'phases decreases and the element diffusion fields overlaps,thus,the size of?'phase decreases.Furthermore,the increase of Ta content increases the mismatch between the?matrix and the?'phase,the cubicity of the?'phase increases.After solution treatment,the elements in the alloy are distributed uniformly.The dissolution of eutectic leads to the increase of matrix saturation,thus the dissolution ability decreases gradually.In the 4.00 Ta alloy,the matrix dissolves the most eutectic and the matrix saturation is the largest,so some eutectic cannot be dissolved.The increase of Ta content leads to the increase of MC carbides in the alloy,thus the M₂₃C₆carbides formed by MC decomposition increase during the solution treatment.During the secondary aging process,Ta is the formation element of??phase.With the increase of Ta content,the number of secondary??phase precipitated on the?matrix channel increase.The increase of Ta content reduces the interfacial energy between the?and??phases,resulting in the decrease of coarsening coefficient of??phase,thus,the coarsening trend of??phase decreases according to Lifshitz-Slyozof-Wagner(LSW)model.Meanwhile,the results of atomic mapping show that the increase of Ta content causes more Ta to replace Al in?'phase,and the diffusion rate of Ta in??phase is smaller than that of Al.Therefore,the increase of Ta content reduces the coarsening trend of??phase and delays the coalescence of??phase during isothermal long-term aging.