The Correlation Between Re-addition And W-addition And Its Synergistic Effect On The Ideal Shear Strength Of γ’-Ni₃Al Phases In Ni-based Single Crystal Superalloys

Ni-based single crystal(SC”superalloys are strategic for the blade application in advanced aero engines owing to their excellent high-temperature mechanical properties,which benefits from the typical microstructure with high volume fraction ofγ′-Ni₃Al phases coherently embedded in a FCCγ-Ni matrix.Theseγ′phases with high volume fraction up to 70%are a dominated phase.The alloying effects forγ′-Ni₃Al phases by multiple additions are currently still under debate.In this work,the interactions between Re-W atoms,Re-Re atoms as well as Re atom and the intrinsic point defects inγ′-Ni₃Al phases are investigated by the first-principles calculation.Further,their synergistic effects on the ideal shear strength ofγ′-Ni₃Al phases have been analyzed.The following provides the main contents of this thesis.Firstly,using the first-principles calculation,the interactions between Re-W atoms and their synergistic effects on the ofγ′-Ni₃Al phases are investigated.A correlation energy function?（）was used to evaluate the interaction between point defects caused by Re-addition and W-addition,i.e.,Re and W defects.For the site preference of Re and W atoms inγ′-Ni₃Al phase,we have calculated the formation enthalpy of point defects.The results indicate that the interaction between Re and W defects does not affect the site preference of Re and W at Al sites,and W in preference to Re substitutes for Al atoms inγ′-Ni₃Al phases.As a result of strong repulsion,Re and W atoms tend to stay away from each other inγ′-Ni₃Al phases.For the preferred sites of Re and W,the strengthening effect in the Re model is greater than in the W model,but the strengthening effect of double Re addition is weak compared with multiple addition of Re and W.Large?（）between Re and W defects is demonstrated to be disadvantageous for the strengthening ofγ′-Ni₃Al phases,and the stronger the repulsion is,the lower is.The analysis of electronic and geometric structures reveal Re/W-addition induced strengthening mainly originates from a stronger Re/W-Ni bonding than Al-Ni bonding,but a significant ascent of inγ′-Ni₃Al-Re-W systems can be attribute to a corporative effect of electronic interaction and local elastic strain energy,and the local elastic strain energy is dominant.Further,the interactions between Re-Re atoms and their synergistic effects on the of γ′-Ni₃Al phases are explored by the first-principles calculation.A correlative energy function?（）has been adopted to evaluate the interaction between point defects caused by Re-additions,i.e.,Re and Re.The results show either attraction or repulsion doesn’t change the site preference of Re at Al sites,and Re atoms tend to stay away from each other in the double Re-addition system.The in Re+Re complexes is not only a function of distances between Re and Re defects,but also depends on the direction[θ,φ]from Re to Re defects.The double Re-addition at preferential Al-Al sites may further elevate the in single Re-addition systems,but an extra substitution of Re for Ni atoms,especially the clustering of Re atoms,is harmful to the strengthening ofγ′-Ni₃Al phase.A careful analysis of?（）reveals the improvement of ofγ′-Ni₃Al phases induced by Re-addition can be attributed to a week repulsive interaction between Re and Re defects,and a strong attractive interaction caused by Re-clustering should be responsible for the detrimental effect of excess Re-additions to some extent.Finally,the synergistic effects of Re and intrinsic point defects on the mechanical properties ofγ′-Ni₃Al phases are investigated.It is found that Al antisite defect(i.e.,Al”is occurred in Al-richγ′-Ni₃Al phase,and the Ni antisite defect(i.e.,Ni”is emerged in Ni-richγ′-Ni₃Al crystal.However,both in Al-rich and Ni-rich system,Re atom is substituted for Al atom(i.e.,Re”.Further,the elastic properties ofγ′-Ni₃Al crystals have been calculated.The results indicate the strength ofγ′-Ni₃Al phases are improved by the Re,Al and Ni point defects.In addition,the Re+Ni<100>and Re+Al?<110>complexes are easily emerged inγ′-Ni₃Al phases,and their strengthening effects are high compared with corresponding isolated point defects.In other words,a synergistic effect exists between Re-addition and antisite point defects,which can further improve the strength ofγ′-Ni₃Al phases.The analysis of electronic structure reveals the beneficial effect of Al defect mainly originates from the strong Al-Ni bondings around point defect,and the strong Ni-Ni bondings should be responsible for the improved mechanical properties of Ni model.Furthermore,the interaction between Re and Al defects can lead to the Re-Ni bondings to be stronger,which is favorable to strengthening theγ′-Ni₃Al phases.