First Principles Study On The Effect Of (Cr,Re) Elements On The Structure And Electronic Properties Of ? Phase

Nickel-based single crystal superalloys are widely used in the manufacture of aero-engine turbine blades due to their excellent high-temperature properties.As the performance requirements of superalloys increase,a large number of refractory elements are used to improve their temperature-bearing capacity,but excessive addition of refractory elements will promote the precipitation of topological close-packed(TCP)phases,which will consume the alloy in the?matrix.Element,weaken the solid solution strengthening effect of the?matrix.Therefore,the existence of the TCP phase severely limits the use of superalloys.As an important element in the first high-temperature alloy Ni₈₀Cr₂₀series,Cr is the forming element of the?phase in the TCP phase.Re is the representative element in the second-generation nickel-based single crystal superalloy,which increases the temperature-bearing capacity of the nickel-based single crystal superalloy by about30°C,and is also the element forming the?phase in the TCP phase.However,the mechanism of Cr and Re on the formation of?phase is still unclear.In this paper,using the first-principles calculation studied the structural stability,electronic properties and elastic properties of the?-Co₇M₆(M=W,Mo,Nb)phase,which appears most frequently in nickel-based superalloys,The preferential occupancy tendency of Cr element in?-Co₇W₆and the preferential occupancy tendency of Re element in?-Co₇Mo₆phase.The results are as follows:(1)The three intermetallic compounds Co₇M₆(M=W,Mo,Nb)all meet theconditions of energy stability and mechanical stability.The reason for their stability is that there is a strong d-d orbital hybridization between the atoms of each system,and there is a covalent bond in the system.The binding energy,density of states and charge density show that Co₇W₆is the most stable among the three intermetallic compounds,followed by Co₇Mo₆and Co₇Nb₆.Theelastic properties show that they are the most difficult to deform in[001]direction and the most easy to deform in[111]direction.Among the three intermetallic compounds,Co₇W₆has the highest Young's modulus,the most difficult to deform and the highest hardness,followed by Co₇Mo₆and Co₇Nb₆.(2)After Cr atom is doped with Co₇W₆,when Cr atom occupies W positon,the system stability will be reduced,and when Cr atom occupies Co position,the system stability will be enhanced.The state density and charge density show that when Cr atom occupies W position,the d-d orbital hybridization intensity between Cr atom and surrounding atoms weakens,and the binding ability decreases.When Cr atom occupies Co position,the opposite is true,indicating that Cr atom tends to occupy Co position in Co₇W₆.(3)After Re doping Co₇Mo₆,the stability of the system will be enhanced whether Re atoms occupy Mo or Co sites.However,the stability of Re atomoccupying Mo site is weaker than that of Re atom occupying Co site.The state density and charge density indicate that the d-d orbital hybridization betweenthe Re atom and the surrounding atom is enhanced after the Re atom occupiesthe Mo site,and the bonding strength is enhanced,but its strength is also.