First-Principles Calculation Of Re-Effect In Dislocation Slip Mediated Creep Of Ni-Based Single Crystal Superalloys

Ni-based single crystal（SC）superalloys,which mainly consist ofγ′precipitates with L1₂ structure embedded coherently in fccγmatrixes,are widely used for the key materials in aerospace engine combustor owing to their outstanding mechanical properties and creep resistances at high temperature.Their high-temperature strength and creep resistance primarily arise from three aspects,i.e.,the solid solution strengthening ofγmatrixes,the anomalous yield and precipitation strengthening ofγ′phases as well as theγ/γ′interfacial strengthening.The strengthening of superalloy in practical application is to add refractory elements into the alloy material as pinning,which can effectively impede the nucleation and movement of dislocations,therefore,the plastic deformation occurs difficultly,as a result,the mechanical properties of the superalloys can be improved.In this work,we study the synergistic effects of the interaction among Re,W,Ta,Ti and Mo on the nucleation and movement of dislocations inγ-Ni as well asγ′-Ni₃Al.The following provides the main contents of this thesis.Firstly,using the first-principles calculation,the influence of Re-clustering on resistances to dislocation slip mediated plastic deformations in theγphase of Ni-based SC superalloys is investigated.An isolated substitution of Re for Ni atoms（i.e.,）,which can reduce intrinsic stacking fault energy,raise unstable stacking fault energy and enhance ideal shear strength,is demonstrated to be beneficial to reinforce the resistance to dislocation slip mediated plastic deformations.A careful analysis for the fault energy and inγ-Ni（Re）phases shows the distance between defects is dominant compared with their position orientation.is revealed to be very sensitive to the clustering degree of Re atoms,while-surfaces mainly depend on the size of Re-clusters.As the interaction between defects being characterized by a correlation energy function_ⅠⅡand_ⅠⅡⅢ,it is found that a strong repulsion caused by Re-clustering is unfavorable for the strengthening of Ni-based SC superalloys.Further,an impact of Re-effect on dislocation slip mediated creeps in theγ′-Ni₃Al phase is investigated.The analysis of energy factors associated with the formation enthalpy of point defects reveals Re prefers to occupy Al sublattice sites inγ′-Ni₃Al phase.In the case of double Re-additions,the Re-Re pair prefers to occupy Al-Al sites,and Re atoms at preferred Al sites are prone to stay away from each other.By calculating the generalized stacking fault energies for, and systems in Ni₃Al,it is found that the Re occupying Al sites can improve the unstable stacking fault energy and promote the cross-slip of screw dislocations,but an extra substitution of Re for Ni atoms is harmful to the strengthening of γ′-Ni₃Al phase.As excessive Re atoms are added intoγ′-Ni₃Al phases to replace a part of Ni atoms,a distinct orientation-dependent fault energy in models is observed.As a result,the fault energy in+complexes not only depends on the distance between and defects,but also is related to the distribution and arrangement of and defects.By conducting a careful analysis of correlative energy reveals the strength of Ni₃Al phases induced by Re-addition can be attributed to a weak interaction between Re_A+Re_B complex defects.Finally,using the first-principles calculation,an impact of replacements of Re by W on dislocation slip mediated creeps ofγ′-Ni₃Al phases is investigated.The formation enthalpy of point defects reveals single solute atom（i.e.,Re,W,Ta,Ti and Mo）prefers to occupy Al sublattice sites,and solute-solute complex defects（i.e.,Re-W,Re-Mo,Re-Ta,Re-Ti,W-Mo,W-Ta,W-Ti,Mo-Ta,Mo-Ti and Ta-Ti）occupy Al-Al sites in γ′-Ni₃Al phase.An isolated solute defects can raise unstable stacking fault energy and in each slip system,and promote the cross-slip of screw dislocations,in which single Re and W can effectively impede the nucleation and movement of dislocations of Ni₃Al more than the Ta,Ti and Mo.A comparison of solute-solute complex defect,it is found that the W has a possibility in replacing part of Re in Ni-based SC superalloys without making a compromise on the mechanical properties.A comprehensive assessment reveals that a weak interaction between solute-solute complex defects should be responsible for the improvement of strengths ofγ′-Ni₃Al to some extent.The electronic structure analysis reveals the Re/W/Ta/Ti/Mo induced strengthening mainly originates from a strong Re-/W-/Ta-/Ti-/Mo-Ni bonding in theγ′-Ni₃Al phase.