Theoretical Study On Structure,mechanical Properties And Phase Transition Of Ni₃X(X=Ti?Nb)

Nickel-based superalloys are a type of superalloys with nickel as the matrix.Because of its high strength,high toughness and excellent high temperature performance(such as good fatigue,oxidation and creep resistance at high temperature),it is widely used in the manufacture of aerospace engine,gas turbine and other key components.With the rapid development of aerospace,electric power and national defense industry,the properties of nickel-based superalloys need to be further enhanced.In this paper,the structural,mechanical properties and phase transition of Ni3X(X=Ti,Nb)intermetallic compounds were studied by first-principles method.Using plane wave pesudopotential method based on density functional theory,we explored the relative stability of different Ni3Ti1-xNbx structures and determined the critical point of structural transformation with x.At the same time,the trend of mechanical properties of Ni3Ti1-xNbx with composition and structure was revealed.The diffusion kinetics and phase transition mechanism from ?" phase to ? phase for Ni3Nb were analyzed.The main conclusions are as follows:1.According to the fitted curve of the formation energy,Ni3Ti1-xNbx tends to transform from D024 to D019 when the concentration of Nb atoms increases up to 29.5%while the structure will transform from D019 to D0a when the concentration increases up to 71.2%.That is to say,when x is 0.295 or 0.712,Ni3Ti1-xNbx will undergo structural transformation.2.The calculated results of elastic constants have shown that with the increase of Nb content,C11 and C13 gradually increase,while C44,C55 and C66 gradually decrease.Ni3Ti1-xNbx system is more prone to shear deformation.It indicates that the possibility of shear deformation of the system increases further with the increase of the Nb content.3.It is found that the intrinsic stacking-fault(ISF)energy of ?"-Ni3Nb is the lowest.It means that ISF plays a decisive role in the phase transition,which is consistent with the experimental findings of abundant ISF in ?" in Inconel 718.In the?" phase,some atoms move along the basal plane with a vector of 1/6[11-2],forming the stacking sequence with the characteristic of ?-like phase.It suggests that ?nucleation may take place by the growth of these faults into the matrix.