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Highlights • An anisotropic elastic–plastic model was developed for predicting rafting behavior. • Variation in mismatch degree has a strong effect on rafting in the elastic stage. • Variation in mismatch degree has very slight impact on rafting in the plastic stage. • Three elastic-constant differences have different influences on rafting behavior. Abstract An anisotropic elastic–plastic model was developed to investigate the rafting behavior of Ni-based single crystal superalloys. This model was developed by introducing the plastic constitutive equation of face-centered cubic (FCC) single crystal in the framework of Eshelby's equivalent inclusion theory. The Hill's equivalent stress was calculated when applying a tensile or compressive loading along the [001] direction. The calculated results successfully predict the rafting direction for alloys with a positive or negative mismatch, consistent with pervious experimental and theoretical studies. Moreover, based on this model, the mismatch degree and the elastic-constant differences of the matrix and precipitate phases and their effects on the speed of rafting are carefully discussed. Regardless of a positive or negative mismatch alloy, the larger absolute value of mismatch degree can more effectively accelerate the process of rafting in Ni-based single crystal superalloys. However, when the alloys enter the plastic deformation stage, the variation in mismatch degree slightly affects the speed of rafting. For the elastic-constant differences (C 11 r ′ − C 11 r , C 12 r ′ − C 12 r ,and C 44 r ′ − C 44 r), the smaller the value of C 11 r ′ − C 11 r , or the greater the value of C 12 r ′ − C 12 r , the more effective the acceleration of γ′ rafting; whereas the value of C 44 r ′ − C 44 r has no effect on the rafting of alloys. The research results provide a new theory and method for studying the rafting behavior and its influencing factors for Ni-based single crystal superalloys. Graphical abstract Fig. Material model of Ni-based single crystal superalloys and representative volume element (RVE) for micromechanical analysis. (a) Original material microstructure: Cubical γ′ precipitates are uniformly embedded in the γ matrix, and the volume fraction of γ′ phase is 70%. (b) Simplified material model. (c) RVE for micromechanical analysis. (d) Variations in Hill's equivalent stress with the external tensile stress. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• TMF behavior at atomistic scale is studied by MD simulation for the first time. • The cyclic deformation mechanisms of superalloys under TMF loadings are revealed. • Stress–strain and stress response curves are obtained under TMF and IF loadings. • The failure reason and fatigue life of superalloys are analyzed under TMF loadings. In this paper, the thermomechanical fatigue (TMF) properties of Ni-based single crystal superalloys are studied by molecular dynamics simulations. Two different cyclic deformation mechanisms of superalloys are found under TMF loadings. The sample has a higher cyclic stress range, plastic strain energy density and a shorter fatigue life under Out-of-phase TMF loading than those under In-phase TMF loading. Moreover, the low-temperature tension half-cycle is more favorable for dislocations and stacking faults to cut into the γʹ precipitate phase, causing an earlier failure of superalloys under Out-of-phase TMF loading, which attracts more attention in actual operation. [ABSTRACT FROM AUTHOR]

The evolution of misfit dislocation network at γ / γ ′ phase interface and tensile mechanical properties of Ni-based single crystal superalloys at various temperatures and strain rates are studied by using molecular dynamics (MD) simulations. From the simulations, it is found that with the increase of loading, the dislocation network effectively inhibits dislocations emitted in the γ matrix cutting into the γ ′ phase and absorbs the matrix dislocations to strengthen itself which increases the stability of structure. Under the influence of the temperature, the initial mosaic structure of dislocation network gradually becomes irregular, and the initial misfit stress and the elastic modulus slowly decline as temperature increasing. On the other hand, with the increase of the strain rate, it almost has no effect on the elastic modulus and the way of evolution of dislocation network, but contributes to the increases of the yield stress and tensile strength. Moreover, tension–compression asymmetry of Ni-based single crystal superalloys is also presented based on MD simulations. [ABSTRACT FROM AUTHOR]

Creep phenomenon is a common mechanical behavior in nanocrystalline nickel (Ni) at high temperature that may affect component function and lead to material failure. In this paper, we systematically study the effects of temperature, stress, and grain size (GS) on creep behavior at high temperature in nanocrystalline Ni by molecular dynamics simulation. Three nanocrystalline Ni models with different GS and a single-crystal Ni model are built for creep behavior simulation. Stress exponent, GS exponent and microstructure characteristics during creep simulation are used to describe steady-state creep mechanism. An obvious creep phenomenon is observed in nanocrystalline Ni, but not in single-crystal Ni; primary creep and steady-state creep phenomenon both occur in nanocrystalline Ni during creep simulation. With increase of temperature and stress level, decrease of GS, creep mechanisms are discovered to change from (1) lattice diffusion and grain boundary (GB) sliding to (2) GB diffusion and GB sliding and then to (3) dislocation nucleation. The similar creep mechanism transition tendency is also found in smaller GS sample at lower temperature. The variation of stress exponent and GS exponent obtained from the simulation results is consistent with creep mechanism theory. [ABSTRACT FROM AUTHOR]

Nanoscale fatigue crack growth was investigated by introducing a cohesive zone model based on molecular dynamics simulations. The evolutions of the microstructure and stress in fatigue crack growth over two different cyclic loading regimes were investigated using pre-existing centre crack specimens. Under increasing strain amplitude cyclic loading, dislocations emitted and persistent slip bands formed around the fatigue crack tip, which retarded crack propagation and changed the stress distributions; under constant amplitude cyclic loading, crack opening and closing was accompanied by void formation in the crack plane. Different fatigue loading regimes resulted in different crack propagation mechanisms and stress distributions; the peak stress was accompanied by microstructure evolution ahead of the crack tip, which induced the variation in fatigue crack growth rates and crack opening displacements. [ABSTRACT FROM AUTHOR]

Pre-existing center crack and void growth in single crystal nickel under Mode I loading are investigated by introducing a cohesive zone model (CZM) based on molecular dynamics (MD) simulation. The microstructural evolution and stress distribution during crack and void growth are analyzed as are the associated mechanical properties. The results indicate that the crack and void have different fracture mechanisms. Crack-tip blunting occurs due to the [1 1 0] super-dislocations emission during crack growth, while for void growth the primary micro-mechanism is the formation of stacking faults, which result in the different growth rates, opening displacements, and stress states. Based on the calculation of the CZM, the crack has a greater growth speed and opening displacement, but a lower tensile stress and fracture strain than the void under the same loading conditions, and the high stress is accompanied by microstructural evolution during crack and void growth. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations have been performed to investigate the spontaneous phase transformation of copper nanowires. It is found that the spontaneous phase transformation exhibits distinct dependence on the cross-sectional geometry of the nanowires and can lead to the reconstruction of atoms into different atomistic configurations, e.g., pure hexagonal-close-packed crystals, fivefold deformation twins, and core/shell structures. For single-crystal copper nanowires, the critical cross-sectional size, above which no spontaneous phase transformation can occur, is determined. The physical mechanisms underlying the complicated transformation behavior are analyzed from the viewpoints of energy and stresses. [ABSTRACT FROM AUTHOR]

[Display omitted] • Established three embedded cubic models with orientations of (1 0 0), (1 1 0), (1 1 1). • Tensile deformation mechanisms of superalloys with different orientations are revealed. • Interfacial microstructure evolutions are observed during tensile loading. • The main slip system 1/6 〈1 1 2〉{1 1 1} is found during tensile loading. In this paper, molecular dynamics (MD) simulations are performed to investigate the tensile deformation mechanisms and mechanical properties of superalloys with different orientations. The research results show that the superalloys exhibit anisotropy of mechanical properties and are consistent with previous experimental research results. The anisotropy of mechanical properties is related to the differences of stability at the interfacial dislocation network and the 1/6〈1 1 2〉{1 1 1} slip system startup of superalloys with different orientations. When the yield point is reached, dislocations and stacking faults on both sides of the slip plane will contact each other and merge into a stacking fault band to penetrate into the γ' precipitates. Meanwhile, the dislocation density and the proportion of face-centered cubic (FCC) structure with different orientation models have a sudden change at the yield point. As the temperature increases, the yield strength and elastic modulus decrease in the (1 1 0) and (1 1 1) orientation models, whereas the (1 0 0) orientation model has an anomalous yield behavior. This is mainly related to the destruction of dislocation network, the change of deformation mechanism and the thermally activated cross-slip mechanism when the temperature increases. [ABSTRACT FROM AUTHOR]

Abstract: The microstructure evolution and stress distribution characteristics of a pre-cracked single crystal nickel at different temperatures are studied by molecular dynamics (MD) simulation. The simulation results indicate that the crack propagation process and stress distribution characteristics are closely related to the change of temperature inducing the microstructure evolution ahead of a growing crack in single crystal nickel. At 0K, the crack propagates rapidly without inducing microstructure evolution, the stress concentration is always at the crack tip of a growing crack throughout the crack propagation process. The crack propagation becomes slow and accompanies with microstructure evolution at elevated temperature, firstly crack tip blunting occurs at a certain distance ahead of crack tip due to the dislocation emission, then void nucleation and growth as well as the generation of slip bands. The microstructure evolution ahead of crack tip induces the change of stress distribution, the high stress value occurs at the site of the microstructure evolution (dislocation emission, void nucleation and the generation of slip bands), and a rather large plastic deformation range ahead of crack tip is necessary for the generation of slip bands. The failure process of single crystal nickel show the different crack propagation dynamics and plastic behavior, which are linked to the void nucleation and generation of slip bands ahead of the crack tip at elevated temperature. [Copyright &y& Elsevier]

• The cyclic deformation mechanisms are revealed at different temperatures. • Interfacial microstructure evolutions are obtained during cyclic loading. • Dislocation density tends to remain stable in the cyclic saturation stage. • Stress amplitude and hardening rate increase with increasing strain rate. The fatigue performance and deformation mechanism of Ni-based single crystal superalloys under cyclic tension–compression loading are studied by molecular dynamics simulations. The effects of temperature and strain rate on the cyclic deformation of superalloys are discussed. The results show that there are three different cyclic deformation mechanisms at different temperature ranges. In the low temperature range, dislocations and stacking faults shearing γ' phase is the main deformation mechanism. In the high temperature range, Orowan bypassing and climbing mechanism dominates the deformation. While in the medium temperature range, the deformation mechanism gradually transforms from the dislocation and stacking faults shearing the γ' phase to the Orowan bypassing and climbing. The simulation results also reveal that the superalloys exhibit a cyclic saturation stage after the initial cyclic hardening. The cyclic saturation stage is a dynamic equilibrium of dislocation proliferation and annihilation, indicating that the superalloys have excellent fatigue mechanical properties. Moreover, the dislocation density and the proportion of stacking faults increase with increasing strain rate, resulting in faster cyclic stability and higher stress amplitude for the superalloys. These results provide important information for understanding the fatigue mechanisms of superalloys from an atomistic perspective. [ABSTRACT FROM AUTHOR]