First-principles Study Of Effects Of The Transition Metal Elements On The Mechanical Properties In Ni-based Superalloys

Ni-based single-crystal superalloys are widely used for blade applications in advanced airplane engines and power-generation turbines,thanks to their excellent elevated-temperature mechanical properties such as mechanical strength,creep strength,ductility and oxidation resistance.Transition metal?TM?elements have important effects on the superalloys,and more than ten TM elements are added to Ni-based superalloys to achieve extraordinary high-temperature performance.Using first-principles methods,we investigated effects of TM elements on the mechanical properties of?phase,??phase and model superalloys in Ni-based superalloys.Furthmore,strengthening mechanism of these TM elements on the mechanical properties are analyzed.By doing uniaxial tensile tests and examining the crystal mechanical stability during the tensile tests,we studied the uniaxial tensile behaviors of?and??phase along three characteristic directions.These two single phases will become mechanical unstable well before reaching their stress peak during the[001]and[111]tension.Furthermore,we investigated the effects of alloying elements on the ideal tensile strength of?and??phase.We found that the effect of alloying elements on tensile strength shows obvious directionality.The electronic mechanism underlying the strengthening effect of alloying elements is determined by analyzing the electronic structures during the tensile tests in detail.Using Wagner-Schottky model,the site occupancy behavior of 3d?Sc–Zn?,4d?Y–Cd?and 5d?Hf–Au?TM alloying elements in??-Ni₃Al were predicted.Additionally,the strengthening ability of TM elements for increasing the ideal compressive strength under different site occupancy is systematically explored.Our results show that both site preference and strengthening ability of TM alloying elements are related to their d-electron number.Except the elements at the ends of the series?i.e.,Cu,Ag,Au,Zn and Cd?,the preference for Al-site of other TM elements increases as the d-occupation number increases along a series.The elements with a half-filled d-band?such as Cr,Mo,Tc and Re?manifest the strongest potency to improve the ideal compressive strength of??phase.The analyses of charge redistribution show that the elements with a half-filled d-band have strongest interaction with Ni atom in??phase.Finally,combining the quasi-harmonic approximation and rule of mixture method,we predicted the response of elastic properties to temperature for?/??two phase.The predicted elastic constants,Young's modulus,bulk modulus and shear modulus exhibits linear decreasing dependence to temperature,which is consistent with the experimental rerults of the CMSX-4 superalloys.Furthermore,by doing first-principles calculations on the?/??multilayer,we explored the influence of Mo,W and Re on the elastic properties of rafted model superalloys.We also calculated the charge redistribution of the ternary model superalloys to reveal the strengthening mechanism of Mo,W and Re on the elastic properties of superalloys.