Molecular dynamics simulation of the structural evolution of misfit dislocation networks at γ/γ′ phase interfaces in Ni-based superalloys

The structural evolution of misfit dislocation networks at γ/γ′ phase interfaces in Ni-based single crystal superalloys under tensile loading and temperatures is simulated by molecular dynamics. From the simulation, we find that, with increasing load or temperature, the patterns of dislocation networks on the (100), (110) and (111) phase interfaces change from regular to irregular or disappear. Under the same load and temperature, the dislocation networks of the different phase interfaces show different degrees and patterns of damage. The density and stability of the dislocation networks decrease with increasing temperature. When the interfacial dislocation networks become more regular, the γ/γ′ interfaces become more stable. The simulated results are supported by related experimental findings. Moreover, based on MD simulations, the averaged stress–strain responses for different phase interfaces under loading are presented. The results indicate that the combined influences of temperature and load play an important role for the structure evolution of misfit dislocation networks at γ/γ′ phase interfaces of Ni-based superalloys.     

Decorrelated movements of Shockley partial dislocations in the γ-phase channels of nickel-based superalloys at intermediate temperature

In nickel-based superalloys with high volume fraction of γ′ precipitates, dislocations have to experience high curvatures in order to enter narrow channels by glide in the {111} planes of the fcc γ matrix. Observations of in situ dynamic sequences performed in a transmission electron microscope on several industrial superalloys have shown the occurrence of decorrelated movements of Shockley partial dislocations, originating from perfect dislocation dissociation. By evaluating the effective stress acting on each one of these partial dislocations, as well as their respective flexibility, the possible occurrence of such movements for some particular dislocation characters and channel widths is accounted for. These movements can play an important role in the creep behaviour of these materials in the low deformation rate regime.     

The ternary Ni-AI-Co embedded-atom-method potential for γ/γ＇ Ni-based single-crystal superalloys： Construction and application＊

An Ni-AI-Co system embedded-atom-method potential is constructed for the γ（Ni）/γ＇（Ni3A1） superalloy based on experiments and first-principles calculations. The stacking fault energies （SFEs） of the Ni（Co, A1） random solid solutions are calculated as a function of the concentrations of Co and A1. The calculated SFEs decrease with increasing concentrations of Co and A1, which is consistent with the experimental results. The embedding energy term in the present potential has an important influence on the SFEs of the random solid solutions. The cross-slip processes of a screw dislocation in homogenous Ni（Co） solid solutions are simulated using the present potential and the nudged elastic band method. The cross-slip activation energies increase with increasing Co concentration, which implies that the creep resistance of γ（Ni） may be improved by the addition of Co.     

Coherent and Incoherent Interactions between Discrete-Soliton Trains in Two-Dimensional Light-Induced Photonic Lattices

The coherent and incoherent interactions between discrete-soliton trains are numerically investigated in lightinduced two-dimensional photonic lattices. The solutions of discrete-soliton trains for diamond and square lattices are obtained by Petviashvili iteration method. It is found that for both the kinds of lattices, two in-phase （out- of-phase） discrete-soliton trains attract （repel） each other, and the intermediates are always accompanied with energy transfer. While the interaction forces between two incoherent discrete-soliton trains are always attractive.     

Short range order heterogeneity on plastic mechanisms in γ-phase nickel-based superalloys

The fine structure of slip lines in the γ-phase of Ni-based superalloy single crystals has been investigated by in situ atomic force microscopy observations of the specimen surface during deformation. For the first time, the dislocation structure is discussed in terms of short range order heterogeneity. A stress-driven unlocking mechanism is observed of dislocation pile-ups scanning the short range order in the bulk material. This is strongly enhanced by the propagation of other pile-ups a few hundreds of nanometers apart. A modelling based on elasticity is proposed and discussed.     

The interaction of dislocation loop and γ' precipitate in nickel based alloys

The nucleation and growth of interstitial loops during irradiation has a : ontrolling effect on the subsequent swelling behaviour of metals. In nickel based alloys containing ordered γ' precipitate (Ni₃Al, Ti), interactions occur between the nucleated loops and γ' particles. This effect has been studied in two nickel based alloys using a High Voltage Electron Microscope.

For the case of Nimonic 80A alloy containing 18% volume fraction : gamma;' precipitate, dislocation loop-particle interactions obeyed the developed isotropic elasticity theory.²'³'¹² Consequently, rather low dislocation densities were developed and the swelling resistance was high during electron irradiation. In Nimonic 115A alloy, loop nucleation and growth was dependent on the availability of interfacial dislocation surrounding the γ' particles.

With regard to the swelling behaviour of γ' hardened alloys, it : s concluded that several mechanisms contribute to make these materials resistant.

Coherency strains at the γ' particles reduce the density of : limbing dislocations.

The γ' precipitate affects the climb efficiency of the : ucleated dislocations by:

pinning the dislocation line, thus introducing a line tension force : hich opposes dislocation climb and reduces swelling;

reducing the available volume of material in which dislocation loops : an nucleate and grow.     

Influence of rhenium and ruthenium on the local mechanical properties of the γ and γ′ phases in nickel-base superalloys

The effect of rhenium and ruthenium on the hardness of the γ′ precipitates and the γ matrix in nickel-base superalloys was investigated using a nanoindenting atomic force microscope. The partitioning behaviour of the alloying elements and the lattice misfit between the γ and γ′ phase were determined in fully homogenised samples to explain the alloying effects. Rhenium strongly strengthens γ as it predominantly partitions to γ and has a strong solid solution-hardening effect. Ruthenium strengthens both γ and γ′ due to a more homogeneous partitioning behaviour. Ruthenium was found to cause less partitioning of rhenium to γ. This results in a stronger increase of the γ′ hardness. The change in the nanoindentation-derived hardness of both phases could be mainly attributed to the solid solution strengthening of Re and Ru.     

High temperature materials for aerospace applications: Ni-based superalloys and γ-TiAl alloys

Two outstanding aerospace-oriented high-temperature materials, the single-crystal nickel-based superalloys for high-pressure turbine blades and the γ-TiAl-based alloys for low pressure turbine blades, are being presented here. In both cases, the optimisation of their mechanical properties is based on a high knowledge of metallurgy, mixing together different aspects such as processes, alloy design, deformation mechanisms, impact of oxidative environment or interaction between protective layers and protected alloy. Historical evolutions are recalled and put into perspective with more recent research activities.     

Authority or Autonomy? Exploring Interactions between Central and Peer Punishments in Risk-Resistant Scenarios

Game theory provides a powerful means to study human cooperation and better understand cooperation-facilitating mechanisms in general. In classical game-theoretic models, an increase in group cooperation constantly increases people’s gains, implying that individual gains are a continuously varying function of the cooperation rate. However, this is inconsistent with the increasing number of risk-resistant scenarios in reality. A risk-resistant scenario means once a group does not successfully resist the risk, all individuals lose their resources, such as a community coping with COVID-19 and a village resisting a flood. In other words, individuals’ gains are segmented about the collaboration rate. This paper builds a risk-resistant model to explore whether punishment still promotes collaboration when people resist risk. The results show that central and peer punishments can both encourage collaboration but with different characteristics under different risk-resistant scenarios. Specifically, central punishment constrains the collaboration motivated by peer punishment regardless of risk, while peer punishment limits the collaboration induced by central punishment only when the risk is high. Our findings provide insights into the balance between peer punishment from public autonomy and central punishment from central governance, and the proposed model paves the way for the development of richer risk-resistant models.     

Shearing of γ′ precipitates in Ni-base superalloys: a phase field study incorporating the effective γ-surface

An extended phase field model of dislocations in Ni-base superalloys is presented. It incorporates the recently developed effective γ-surfaces for both matrix and precipitate phases, obtained from atomistic simulations. These novel γ-surfaces feature extrinsic stacking faults as additional local minima. Thus, they offer an increased number of available dislocation dissociation pathways within the phase field system. The new model has been used to simulate a variety of mechanisms for γ′ precipitate shearing proposed in literature. A critical assessment is made based on the modelling observations.     

π0–η–η' Mixing in the Theory with Four-Quark Interactions

JETP Letters - Mass formulas, mixing angles, and decay constants of the pseudoscalar π0, η, and η' mesons have been obtained in the theory with four-quark interactions with an...     

Relation between the mass coefficients for rotational and γ-vibrational modes in well-deformed nuclei

The ratio of the mass coefficients for the γ-vibrational and rotational motion for the well-deformed axially symmetric nuclei is calculated basing on the Cranking model and the Random Phase Approximation. The results obtained show that the calculated ratio is equal to 2–3, i.e., significantly larger than unit. This is in a qualitative agreement with the experimental data.     

Goos-Hänchen shifts at the interfaces between left- and right-handed media

The Goos-H?nchen shift caused by total internal reflection at the interface between two media is analyzed. For two media of the same handedness the Goos-H?nchen phase shift opposes the phase variation associated with propagation through the incident medium. The Goos-H?nchen lateral shift is in the same direction as the horizontal component of the incident energy flux. Conversely, for two media of opposite handedness the Goos-H?nchen phase shift reinforces the phase variation associated with propagation through the incident medium. The lateral shift is in the opposite direction of the horizontal component of the incident energy flux.     

Doping effects on the stacking fault energies of the γ’ phase in Ni-based superalloys

The doping effects on the stacking fault energies(SFEs),including the superlattice intrinsic stacking fault and superlattice extrinsic stacking fault,were studied by first principles calculation of the/phase in the Ni-based superalloys.The formation energy results show that the main alloying elements in Ni-based superalloys,such as Re,Cr,Mo,Ta,and W,prefer to occupy the Al-site in Ni3 AI,Co shows a weak tendency to occupy the Ni-site,and Ru shows a weak tendency to occupy the Al-site.The SFE results show that Co and Ru could decrease the SFEs when added to fault planes,while other main elements increase SFEs.The double-packed superlattice intrinsic stacking fault energies are lower than superlattice extrinsic stacking fault energies when elements(except Co) occupy an Al-site.Furthermore,the SFEs show a symmetrical distribution with the location of the elements in the ternary model.A detailed electronic structure analysis of the Ru effects shows that SFEs correlated with not only the symmetry reduction of the charge accumulation but also the changes in structural energy.     

The wettability and interfacial characterization between γ-TiAl alloy and ceramic reinforcements

The wettability and interfacial characterization of γ-TiAl alloy on TiC_0.78, TiN_0.87, and VNx substrates were studied using the Sessile Drop method at 1758 K. The equilibrium apparent contact angle of liquid γ-TiAl alloy are 8° on TiC_0.78 substrate, 22° on TiN_0.87, but deficient for VNx substrate because of the gradual appearance of solidification phenomena. The spreading mechanism of γ-TiAl/TiC_0.78 was ascribed to the product control model, which was determined by the new Ti₂AlC formed at the interface. The decreased apparent contact angle of γ-TiAl on TiN_0.87 substrate with the slow rate resulted from the combined effect of Ti adsorption at the interface and the decreased surface tension and viscosity of the liquid. The γ-TiAl/VNx system was a dissolution system. The effect of dissolution on apparent contact angle is reflected in the initial stage and progressed synchronously with the movement of the triple-phase line.     

The ternary Ni–Al–Co embedded-atom-method potential for γ/γ Ni-based single-crystal superalloys: Construction and application

An Ni–Al–Co system embedded-atom-method potential is constructed for the γ(Ni)/γ(Ni3Al) superalloy based on experiments and first-principles calculations. The stacking fault energies(SFEs) of the Ni(Co, Al) random solid solutions are calculated as a function of the concentrations of Co and Al. The calculated SFEs decrease with increasing concentrations of Co and Al, which is consistent with the experimental results. The embedding energy term in the present potential has an important influence on the SFEs of the random solid solutions. The cross-slip processes of a screw dislocation in homogenous Ni(Co) solid solutions are simulated using the present potential and the nudged elastic band method. The cross-slip activation energies increase with increasing Co concentration, which implies that the creep resistance of γ(Ni)may be improved by the addition of Co.     

Hydrogen vibrations in γ-TiH and γ-ZrH under high pressure

Abstract

The effect of high hydrostatic pressure on the vibrational spectra of monohydrides γ-TiH and γ-ZrH was studied by inelastic neutron scattering, the maximum experimental pressure being, respectively, P=15 and 17·5 kbar. There was observed a uniform linear pressure-induced energy shift of each spectrum compared to that at ambient pressure with proportionality factors of 1·02 for y-TiH and 1·03 for γ-ZrH. The observed linear shift of the one-phonon hydrogen modes and the bound multiphonons (biphonons and triphonons) is indicative of a pressure-induced increase of the anharmonicity parameters in the model Hamiltonian representing the strength of two-phonon and three-phonon interactions.