Diffusivity And Strengthening Of Ni-based Superalloys: Atomistic Simulations

Due to their excellent material properties such as good creep, fracture resistance,anti-oxidation and corrosion resistances, Ni-based superalloys are widely used inmodern industry production fields such as aerospace, petroleum and navigation so on,and become a key material to produce the blade of aero-engines and industrial gasturbines. As well known, it contains more than ten kinds of alloying elements in theNi-based superalloys, as well as the complex phase structures. Because of thecomplexity of microstructure, experiments are difficult to measure the distribution ofsolutes and the characteristics of intrinsic structure of superalloys. Especially, directconfirmations of the solution strengthening mechanism and atomic diffusivity arestill challenges due to the difficulties in carrying out radiotracer experiment. On theother hand, atomistic simulations have recently evolved to effective tools to studythe interaction between solute atoms and matrixes, which can play a role in exposingthe strengthening mechanism derived from solid solution, precipitated phase andphase interface. Meanwhile, it also can describe the atomic arrangement of the phaseinterface and the diffusivity of all kinds of alloying element.In this work, the predictions of self-and impurity diffusion coefficients arediscussed from first-principles in Ni-based superalloys. The solute-vacancy bindingenthalpies and entropies are determined. The positive solute-vacancy bindingenergies indicated solute-vacancy pair is not a favorable binding. Considering thefive-jump frequency model, the results show that impurities atoms may exert aninfluence on the movement of host Ni atoms. It indicates that addition Co atomactivates the movement of surrounding host Ni atoms, and yet addition Ru and Reatoms deactivate the movement. Based on the calculation results, we observe that thebiggest migration barrier of impurity can be obtained by introducing Re atom. Thebonding interaction between impurity atom and nearest-neighbor Ni atoms ispresented by more obvious dumbbell-shaped regions in the case of Re impurity,which gives rise to a maximum migration energy. Meanwhile, calculated resultsshow that correlation factor for solute Co and Ru are not a constant but changingwith temperature. Re atom are more difficult than Co element to return their originalposition back as temperature is rising. The clear trend in diffusivity is: Co> Ni> Ru>Re.The prediction and analysis of diffusivity for Ni and Al in stoichiometric Ni3Al have been implemented in the current work with first-principles calculations. Theformation energies of four intrinsic point defects and various migrati on energies instoichiometric L12Ni3Al are determined. It is also noteworthy that the calculateddiffusion coefficient for Ni is larger than that of Al over the entire temperature range.As for the off-stoichiometric Ni3Al structure, considering the dependence ofcomposition in Ni diffusivity, we conclude that Ni diffusion coefficients decreasemonotonically with increasing Ni composition. Meanwhile, the calculations showthat the Al antisite defect concentration play a decisive role in the Al diffusivity, andthe Al diffusion coefficients rise with the increasing of Al content.Molecular dynamics coupled with modified analytic embedded atom model(MAEAM) potentials have been used to study the solid solution strengtheningmechanism in Ni-based superalloys. By means of simulating the edge dislocationmoving in the Ni matrixes, alloying elements impede the motion of dislocation andpresent solution strengthening effects. At the same concentration, the introduced Ruatoms obtain a stranger hardening than Co solute on dislocation moving; however, asfor Re solute, even though the introduced Re concentration is low in γ phase, thesolid solution strengthening is obvious. Hence, it can reach to the conclusion that theRe element has most effective strengthening, and followed by Ru element, the last isfor Co element. The interactions between alloying elements and Ni atoms situat ed atglide plane of dislocation are the key factor for the solid solution strengthening,which result in the stacking fault energy increasing and consuming more energy inthe dislocation moving.The nano-indentation processes of monocrystal Ni, Al and Ni3Al is studied.Analysis of the structure shows that these indentations fall into two processes:elastic deformation and plastic deformation. However, the indentation of Ni3Alemerge another deformation mechanism, such as pseudo-elastic deformation, whichis attributed to forming the nanotwins. The low ratio of intrinsic stacking faultenergy and unstable stacking fault energy in the Ni3Al are liable to generate partialdislocation, which can pin up below the indenter and form the nanotwins.Considering the pinning-up effects, the presence of nanotwins postpones theappearance of plastic deformation and hence harden the superalloys. As for theindenter size effect, the results show that the size indenter is smaller; the flow stresscorresponding to plastic deformation is higher.The Re distribution and its influence on the Ni/Ni3Al interface are studied withMonte Carlo (MC) simulations. It’s found that the interface is not an abrupt face at the atomic scale, but an order-disorder one with8-10atomic layers width.Meanwhile, the interface width increases with the temperature rise. As Re solutes arerandomly introduced, it’s found that Re atoms will preferentially occupy the Al sitesand integrate with its nearest neighbor Ni atoms. It also reveals that there is noenrichment of Re in the interface. The interfacial width has been measured bycomposition variation in the interface as a function of Re solute concentrations,while the width decreases with increasing solute concentrations. As the Re contentcontinues to increase, the γ’ precipitate phase reaches to the solubility limit of Resolute. The solubility value increases with increasing temperature.