Ab Initio Study On The Mechanical And Thermodynamic Properties Of ?? Phase Ni-Co Based Superalloys

As an important high-temperature resistant structural material,superalloys are mainly used in aerospace turbine blades.The alloy has excellent strength at high-temperature with good resistance to oxidation and corrosion under extreme environments due to the??precipitation?L1₂?and chemical properties of the components.The research and development on the Ni-based superalloys have deepened the understanding of the superalloys properties and thereby prompting the improvement of the alloy properties.However,low solidus temperature is the limiting factor in the development of Ni-based superalloys.Presently,the new Co-based and Ni-Co based superalloys with high solidus temperature have resulted in precipitation hardening.The mechanical and thermodynamic properties of these alloys are the hot spots in the field of superalloys.This paper calculates the elastic constant,bulk modulus,shear modulus,polycrystalline elastic constant,formation enthalpy,mix enthalpy and superlattice intrinsic stacking fault energy?SISF?of?NiCo?₃AlTm?Tm=Cr,Mo,Ti?superalloys based on density functional theory.The influence of composition and magnetic moment on the mechanics and thermodynamics has been investigated.Presence of Co in the Ni site of the alloy will increase the lattice constant and bulk modulus,whereas Cr,Mo,and Ti in the Al site will have different behavior due to magnetic properties.The formation enthalpy of Ni-Co based superalloys is negative,indicating an exothermic reaction.Positive mixing enthalpy value may indicate phase separation in the quaternary alloy.The Gibbs energy will be negative after considering the configuration entropy,indicating that the quaternary alloy may be stable within a limited temperature range.In this manuscript,the Curie temperature of Ni₃Al calculated by the average field method is 38.5K,which is in good agreement with the experimental value.The results of show that the Curie temperature of??-Ni-Co-Al-Cr?Mo?superalloys is very low,and this alloy series is paramagnetic.The coefficient of thermal expansion of the alloy in the paramagnetic state is smaller.In the ferromagnetic state,the increase of Co content will reduce the shear modulus,whereas,with increasing Co content,the paramagnetic ratio of Ni to Co will show an extreme value.The Ni-Co-based superalloys exhibit ductility and anisotropy as evident from the polycrystalline results.The Young's modulus of the ferromagnetic state has obvious anisotropy,and the degree of anisotropy of the alloy will be reduced in the paramagnetic and non-magnetic states.In the ferromagnetic state,the alloy has good ductility which can be improved by Co whereas,the elements Cr,Ti and Mo have opposite effect.Stacking fault energy the Ni-Co based superalloys decrease with increasing Co content,but increases with the content of Cr,Mo.Paramagnetic state and non-magnetic state values,which are relatively close,show significant differences with the ferromagnetic state ratio.There is huge influence of various components on the SISF energy.Under different magnetic states,the law governing the change of the single crystal elastic constant of the alloy is varied.In the ferromagnetic state,the calculation of the alloy will underestimate the shear modulus.With the element Cr,the C?value of the paramagnetic Ni-Co based superalloys is maintained,whereas,C₄₄ is expanded which will be lower than that of the non-magnetic whole.Analysis of the equilibrium volume,bulk modulus,Poisson's ratio and elastic constant of the alloy under different magnetic states shows that under low concentration doping,the difference between the paramagnetic and nonmagnetic states is slight,whereas,ferromagnetic and paramagnetic states are significantly different;Paramagnetic and non-magnetic are the same,but the gap between ferromagnetic and paramagnetic is still visible,with many of these numerical differences being⁵0%.The change of SISF energy of the Ni-Co based alloy is uniform with the elastic constant.There will be obvious differences in different magnetic states,and the difference can be up to 20mJm^-2.The above data shows that the magnetic states in alloy calculation cannot be overlooked.