Effect Of Trace Elements On The Microstructures And Properties Of Cast K416B Nickel-based Superalloy

Nickel-based superalloys are widely used in aerospace and other fields due to their excellent high temperature strength,good oxidation resistance and hot corrosion resistance.K416B alloy is a typical cast nickel-based high-W superalloy.Due to the high content of W,a W-rich phase is easy to precipitate during the solidification process,which affects the stability of the alloy.In addition,K416B contains a large amount of low-melting eutectic（about 17%）,which is easy to initiate cracks during tensile and stress rupture properties tests,which seriously affects the mechanical properties of the alloy.C,B,Zr,Hf,and rare earth elements La,Ce are usually added to the alloy as grain boundary strengthening elements.Due to its low solubility in the alloy,the formation of carbides and other precipitation phases affects the formation of eutectic during solidification and the interdendritic convection is weakened.Rare earth elements such as La and Ce are segregated interdendritic,which increases the density of the interdendritic liquid and reduces the density difference caused by the segregation of a large amount of W in the dendrites.Therefore,we reduced the eutectic content in the alloy by manipulating trace elements.It has great significance for the element segregation of the alloy,improving the properties of the alloy and promoting the engineering application of K416B.The results show that with the increase of C content,the number of carbides increases.When the C content reaches 0.27%,the carbides developed Chinese-like morphology.At the same time,with the C content increases,the initial melting temperature of the alloy increases.At the same casting temperature,the undercooling degree of alloy solidification decreases,the nucleation rate decreases.With the increase of carbon content,the grain size of the alloy increases significantly.The increase of C content carbides formed to consumed part of the（γ+γ′）eutectic forming elements during solidification,causing the（γ+γ′）eutectic content decreases significantly.In addition,α-W and M₆C were found in 0 wt.%C alloys and 0.27 wt.%C alloys,respectively,but not found in 0.1 wt.%C alloys haved W-rich phase.With the increase of C element,the alloy stress rupture properties increase.During the stress rupture properties test,the crack initiation area was mainly at the grain boundary and（γ+γ’）eutectic,while the 0.27 wt.%C alloy larger grain size and small amount of grain boundary and the eutectic content were low,which was not conducive to crack initiation and growth,so the repture life of the alloy was increased.The 0.1 wt.%C alloy had the most excellent yield strength and tensile strength relative to the other two alloys.During the tensile test of K416B alloy,α-W and M₆C carbides were the main crack sources.As the content of La and Ce in the alloy increases,the（γ+γ’）eutectic content increases,and the carbides transform from blocky-like to Chinese-like.The reason for that was La and Ce content increase makes carbide forming elements easier to diffuse into the liquid phase and eutectic forming elements to segregate interdendritic during solidification.The increase of La and Ce content contribute to grain refinement in the alloy.Since the La and Ce content were increased,the eutectic content were increased and the grain size reduction,so that the alloy crack initiation easied during stress rupture properties test.Therefore,the repture life of the alloy decreases with added La and Ce to the alloy.The tensile properties of the alloys improved with the addition of La and Ce.Due to the Chinese-like carbides had pinning effect on the grain boundaries and hinder the sliding of the grain boundaries.The increase of Zr and Hf content makes the carbide morphology change from Chinese-like to irregular block.Because Zr and Hf were carbide forming elements,increasing the content make the interfacial energy of carbide formation increase.The increase of Zr and Hf content reduces the initial melting temperature of the alloy,so that the liquid phase has enough time to feed in the later stage of solidification,and the porosity of the alloy is reduced.The repture life of the alloy increases with the addition of Zr.Due to with the addition of Zr,the porosity of the alloy is reduced,and purify the grain boundary to increase the grain boundary strength.With the Hf content increases from 0 wt.%to 1.0 wt.%,the repture life of the alloy increases.Because addition of Hf can increase the mismatch degree ofγ/γ’,increase the activation energy of dislocation climbing overγ’,and increase the curvature of the grain boundary to hinder the sliding of the grain boundary.When the Hf content increased to 2.0wt.%,the reptuew life of the alloy decreased due to the excessive eutectic in the 2.0 wt.%Hf alloy,and the eutectic was the crack initiation region in the stress rupture properties test.