Effect Of Cu And Co On Nb Segregation And Laves Phase Of Inconel718 Alloy

Inconel 718 alloy is an age-strengthened Ni-Fe-Cr based superalloy,which is widely used in aerospace,petrochemical industry,nuclear energy and other fields due to its good high temperature properties such as oxidation resistance,corrosion resistance and thermal stability.However,in Inconel 718 alloy,Nb segregation occurs in the process of casting,welding and laser cladding.Serious Nb segregation will promote the formation of a large number of Laves phase in the alloy.Laves phase belongs to hard brittle phase,which will greatly reduce the performance of the alloy and the difficulty of subsequent homogenization heat treatment.Based on this,the influences of Cu and Co on Nb segregation and Laves phase stability of Inconel 718 alloy were analyzed by first principle calculation and experimental studies.The influence mechanisms of Cu and Co on Nb segregation and Laves phase of Inconel718 alloy were revealed from the micro level.XRD and SEM were used to study the cast structures and morphologies of Laves phase.The melting point,hardness and initial melting temperature of Laves phase were measured before and after doping.The results are as follows:1.The model of Ni-Fe-Cr-Nb solid solution system has been successfully constructed.The first principle calculation shows that both Cu and Co elements can be successfully doped into Inconel 718 alloy Matrix to form stable crystal structure.However,due to the differences of atomic radius and interaction between atoms of doping elements,lattice distortion occurs in the doping system.After Cu doping,the absolute value of the formation heat and the binding energy of the system decrease,indicating that the stability of the system is reduced after Cu doping,while the absolute value of the binding energy of the system increases after Co doping,indicating that Co improves the stability of the Ni-Fe-Cr-Nb system.The results show that the valence electrons between the doped elements and other atoms are hybridized after the doping of Cu and Co.The state density at Fermi level increases due to the doping of Cu,while the state density at Fermi level decreases due to the doping of Co.The charge density diagrams show the change of the interaction between atoms.The doping of Cu and Co leads to the decrease of the bonding strength between Nb and its surrounding atoms,which reveals the essence of Cu and Co promoting Nb segregation.2.The first principle calculation shows that the doping of Cu and Co atoms will cause the lattice distortions of Laves phase.The stabilities of the system decreased after doping.Cu 3d,Fe 3d and a small amount of Nb 4d,Cu 4s orbital valence electrons are overlapped and hybridized at Fermi level,and the low bonding strength between Cu atom and adjacent atoms is the basic reason for Cu atom to reduce the stability of Laves phase.after Co doping,Fe 3d,Nb 4d and Co 3d orbital valence electrons hybridize,resulting in the increase of density of state at Fermi level and the decrease of system stability.The calculation of elastic properties shows that all the doping systems have mechanical stabilities.After Cu doping,the hardness of the system increases,the ductility of the position system of the doped Fe atom decreases,and the ductility of the position system of the doped Nb atom increases.after Co atom doping,the hardness of the system increases,the hardness of the position system of the doped Nb atom decreases;before and after doping,the system has ductility,but the ductility after doping shows a decreasing trend.3.The experimental results show that the doping of Cu and Co can cause the segregation of Nb in Inconel 718 alloy Matrix and promote the formation of Laves phase.Cu doping can reduce the initial melting temperature of Laves phase and alloy melting point,while Co doping has no obvious effect on the initial melting temperature of Laves phase,and the melting point of the alloy is slightly increased.