First-Principles Study Of Interactions Between Alloying Elements And Defects In Ni-Based Single-Crystal Superalloys

Nickel-based single-crystal superalloys are widely used in the manufacture of key components for gas turbines and aero-engines due to their excellent creep resistance and fracture toughness.During its service,the creep process of the alloy is accompanied by many microstructure evolution,such as the diffusion and segregation of alloy atoms,the emission and annihilation of dislocations,the formation and recovery of the stacking faults and the rafting of Ni₃Al phase.In order to achieve better performance,a large number of refractory alloying elements are added to nickel-based superalloys to regulate these microstructures.Nowadays,One of the most important obstacles to the design of high-performance nickel-based single-crystal superalloys is that the influences of alloying elements on these microstructures is not yet well understood.On the one hand,the addition of a large number of refractory alloying elements increases the diversity of the alloy’s compositions space.The traditional experimental alloy design based on trial and error is obviously difficult to explore the vast multicomponent phase space and also incapable of giving deep insights on the role of alloying elements at the electronic and atomic level,which increases the difficulty and cost of alloy development.On the other hand,the complexity of the synergistic interactions between alloyling elements makes it impossible to discern the mechanism of different alloying elements on these microstructures,making the regulation more difficult.Therefore,it is crucial to investigating the effects of different alloying elements on these microstructures in nickel-based single-crystal superalloys.This not only help to understand the mechanism of strengthening of different alloying elements on individual microstructures,but will also help to find alternative alloying elements that are superior or have similar strengthening effects.In this thesis,a comprehensively study of the interaction of different alloying elements with the point defects,the generalized stacking faults,and Ni/Ni₃Al phase interface in nickel-based single-crystal superalloy was performed,as well as the distribution characteristics and the effect of alloying elements on thermodynamic stability of nickel-based single-crystal superalloy.The achieved results and conclusions are as follows:（1）The substitution formation energy of the solutes and its effects on vacancy formation energy were studied.For Ni,the elements in groups ⅢB and ⅣB are easily dissolved in it and form stable solid solutions.For Ni₃Al,most of the alloying elements are not easily dissolved in it,but there is an obvious occupancy preference for Al or Ni site.Most of the alloying elements in groups ⅢB-ⅦB show a strong Al site preference,while only Pd,Pt and Au showing a strong Ni site preference.Furthermore,in Ni and Ni₃Al,the larger the radius of the alloying atom,the smaller the nearest vacancy formation energy of the solute.Y and Ce significantly promote the formation of vacancies,whereas Cr,W and Re inhibit the formation of vacancies,which is conducive for hindering the diffusion of alloying elements and the movement of dislocations.（2）The effects of the alloying elements on the generalized stacking faults energy were investigated.For Ni,most of the alloying elements decrease the unstable stacking faults energy(γ_usf)of the[01（?）]（l 11）and[11（?）]（111）slip systems and also decrease the stable stacking faults energy(γ_sf)of the[11（?）]（111）slip system.The larger the atomic radius of the alloying elements,the more pronounced the decrease in γ_usf andγ_sf of Ni.For Ni₃Al,except Mn and Fe,the elements in groups VB-VIIB and the first column of group VIII increase the value of γ_usf of different slip systems of the Ni₃Al phase,which makes the slip deformation more difficult.However,the elements in groups IIIB-VIIB also increase the value of γ_sf,which reduce the stability of the stacking faults configuration.In order to quantify the effects of the alloying elements on the slip deformation,the R₁(γ_usf^X/γ_usf⁰)and R₂((γ_usf^X-γ_sf^X)(γ_usf⁰-γ_sf⁰)),indices are proposed.It is obtained that Re is an excellent strengthening alloying element that significantly increases the barrier of the recovery process of the stacking faults for the Ni phase,and also enhances the barrier of the formation process of the stacking faults for the Ni₃Al phase.W and Mo exhibit similar effects as Re.We predicted that Os,Ru,and Ir are also good strengthening alloying elements,which show the strengthening effects on both the formation and recovery process of the stacking faults for Ni and Ni₃Al.（3）The interaction between alloying elements and γ/γ’ boundary were explored.It was found that the elastic strain energy of the（001）γ/γ’ boundary is the lowest,which indicates that the（001）γ/γ’ boundary is soft and easy to form a stable interface.When the solute were added into Ni,except Cr and Co,most of the alloying elements increase the lattice constant of the y phase and decrease the lattice misfit.And the lattice misfit continues to decrease with the atomic concentration increasing.We found the interface formation energy,elastic strain energy and chemical energy increase with the absolute value of the lattice misfit going up.When the solute content below 4.69 at.%,the addition of elements Mn,Cr,Re,Mo,W,Ti,Nb and Ta decreases the interface formation energy and enhance the stability of the（001）γ/γ’ boundary.（4）The effect of Re on the microstructure evolution of the Ni-Al-Re ternary alloys was explored.The results demonstrate that as the temperature decreases,the Ni₃Al phase gradually precipitates and grows,and the concentration of Re in the Ni phase gradually increases.The addition of Re promotes the ordering of Al-Al and raises the order-disorder transition temperature of the Ni₃Al phase.At low temperatures,a small amount of Re is distributed in the Ni₃ A1 phase and occupy the Al position,while most of the Re is distributed in Ni and some of Re segregate at the Ni side of the Ni/Ni₃Al phase boundary.When the Re concentration exceeds 2.0 at.%,these enriched Re tend to favor the formation of a large amount of Ni8Re-and Ni4Re-like local structures.From the perspective of solid-solution strengthening,these local cluster structures consumes the Re atoms in solid solutions and would have a negative influence on the creep resistance of the superalloys.