Research On Heat Treatment Of A Directionally Solidified Ni-based Superalloy

During the directional solidification process of superalloy melt,solute redistribution at the solid/liquid interface of various elements will result in the inhomogeneity of composition and microstructure of castings,manifested in the typical "cross" dendrite morphology,significant differences in size and morphology of γ’ phase between dendritic core and interdendrite,and the precipitation of y/y’eutectic,which will deteriorate the properties of superalloy.Therefore,in order to meet performance requirements,heat treatment is needed to improve uniformity of composition and microstructure.DZ125L superalloy is the first generation of directionally solidified nickel-based superalloy with excellent physical and mechanical properties,and it was developed based on the composition of Rene 125 superalloy,combined with low segregation technology.However,after standard heat treatment,the dendrite morphology is still obvious,and there are still γ/γ ’eutectic residue.Therefore,to further expand the commercial value of DZ125L superalloy,this study aims at select an optimized heat treatment process for the composition optimized DZ125L superalloy,and compare microstructure,element segregation,mechanical properties and structure stability of the standard and optimized heat treated superalloy.In this study,DZ125L superalloy was prepared by vacuum induction melting and directional solidification technology.The characteristic temperatures were determined by differential scanning calorimetry and metallographic water quenching.The microstructure and distribution of major elements were characterized by optical microscope,scanning electron microscope,energy dispersive spectroscopy and electron probe X-ray microanalyser.The mechanical property was researched by room temperature tensile and high temperature stress rupture.The fracture morphology and dislocation configuration were observed by scanning electron microscope and transmission electron microscope.The microstructure stability was researched by long term aging.The main research results are as follows:The as-cast DZ125L superalloy is composed of γ matrix,γ’ phase,γ/γ’ eutectic and carbide,and the γ’ phase at dendrite core and interdendrite differ greatly in size,where the average size of γ’ phase at dendrite core is about 300 nm,the average size of γ’ phase at interdendrite is about 660 nm.The area fraction of γ/γ’ eutectic is about 1.1%.The solution temperature of γ’ phase is 1209℃,and the incipient melting temperature is in the range of 1270℃～1280℃.After standard heat treatment,the dendrite morphology is still obvious,and the area fraction of residual γ/γ’ eutectic is 0.65%.The size difference of γ’ phase between dendrite core and interdendrite is still large,where the average size of γ’ phase at dendrite core is about 342 nm,the average size of γ’ phase at interdendrite is about 936 nm.The area fraction of γ’ phase and γ/γ’ eutectic decrease with the increase of heating time at 1240℃,1250℃ and 1260℃,while the area fraction of γ’ phase and γ/γ’ eutectic decrease firstly,then increase,and decrease lastly at 1230℃.The change in the area fraction of γ’ phase and γ/γ’ eutectic during solution heat treatment at 1230℃ was caused by the rapid dissolution of γ’ phase in the dendritic core,which led to the diffusion of Ta element from dendrite core to interdendritic region through γ matrix.During aging heat treatment,with the increase of high-temperature aging temperature,the cube degree and the size of γ’ phase increase,the area fraction of γ’ phase increases and then decreases.When the high-temperature aging temperature is 1080℃,with the increase of heating time,the cube degree and the size of γ’ phase increase,the area fraction of γ’ phase increases slowly.When the high-temperature aging temperature is 1100℃,the cube degree of γ’ phase increases and then decreases,the size of γ’ phase increases,and the area fraction of γ’ phase remains basically unchanged.The optimized heat treatment process is 1260℃×2 h,AC+1080℃×8 h,AC+900℃×16 h,AC.After optimized heat treatment,the cubic degree of γ’ phase is high,the average size is about 354 nm and the area fraction is 74.23%.Comparing the mechanical properties of DZ125L superalloy under different heat-treated states,it can be seen that the standard heat-treated superalloy has better room temperature tensile ductility,while the optimized heat-treated superalloy has higher room temperature tensile strength and longer creep rupture life at 980℃/235 MPa.Among them,percentage elongation,percentage reduction of area of room temperature tensile property and percentage elongation of stress rupture property at 980℃/235 MPa of standard heat-treated superalloy are 3.5%,3.25%and 4.8%higher than optimized heat-treated superalloy,respectively.Yield strength and tensile strength of optimized heat-treated superalloy are 212 MPa and 52.5 MPa higher than standard heat-treated superalloy.Creep rupture life at 980℃/235 MPa of optimized heat-treated superalloy is nearly 1 times longer than standard heat-treated superalloy.Comparing the microstructure stability of DZ125L superalloy under different heat-treated states,it can be seen that the optimized heat-treated superalloy has better microstructure stability than standard heat-treated superalloy.Among them,in the optimized heat-treated superalloy,after aging at 850℃/1000 h and 950℃/1000 h,no TCP phase precipitates,no γ’ phase raft,and the M23C6 carbides mostly precipitate among grain boundaries.In the standard heat-treated superalloy,after aging at 850℃/1000 h and 950℃/1000 h,no TCP phase precipitates,but the γ’phases at interdendritic region raft at 850℃/200 h and 950℃/200 h,and the M23C6 carbides are formed around MC carbides or precipitate from the matrix.