Dislocation Emission Research Articles

Grain Boundary Segregation and Dislocation Emission from Grain Boundaries in High-entropy Alloys

Grain Boundary Segregation and Dislocation Emission from Grain Boundaries in High-entropy Alloys

A new method for predicting dislocation emission of grain boundary in bicrystal Mg

A new method for predicting dislocation emission of grain boundary in bicrystal Mg

The fatigue crack propagation in nanocrystalline materials in the hydrogen environment

In this article, the effect of the external corrosion environment on dislocation emission is introduced, and the microscopic mechanism of fatigue crack propagation (FCP) in a hydrogen environment is studied. Meanwhile, based on the discrete distributed dislocation method, the corrosion FCP model of nanocrystal materials is established. In the framework of our model, the FCP is mainly determined by the irreversibility of the crack tip dislocation. The research results indicate that the aggregation of hydrogen atoms at the crack tip (CT) promotes the dislocations emission and increases the FCP rate. Meanwhile, considering the application of materials in different hydrogen circumstances, the effects of changes in the distance between hydrogen atoms and the crack tip and the radius of hydrogen atoms on crack propagation rate are studied. The results show that the accumulation of hydrogen atoms at the crack tip promotes the fatigue crack growth rate in nanocrystalline materials. Compared with the air environment, the maximum increase in crack propagation rate is about 45% under the action of hydrogen environment. Meanwhile, increasing the grain size and dislocation emission angle helps to increase the crack propagation rate. The research in this paper provides a theoretical basis for evaluating the corrosion fatigue life of metal materials.

Dislocations generated by pre-strain dominate the subsequent plastic deformation

Plasticity of sub-micron scale metals is well known to be controlled by dislocation source. Here, we report an in-situ study of how the dislocation structures generated by pre-strain in submicron-sized copper affect the subsequent plastic deformation by conducting nanocompression testing in a transmission electron microscope. The results show that short isolated dislocations first undergo directional intermittent jumping motion along specific glide directions under the applied force. And the jumping frequency and jumping distance of dislocation motion can be determined by the density and strength of obstacles distributed in the crystal. Then, the dislocation wall formed by dislocation pileup and entanglement during pre-strain can effectively pin the mobile dislocation in the early stages of compression, but at high stress levels it will perform overall motion in the form of dislocation avalanches and lead to the strain burst emissions of the material. The in-situ dynamic evolution of dislocation avalanche reveals the emitted dislocations and associated glide planes introduced by pre-strain can preferentially trigger dislocation avalanches of the dislocation wall.

Crack tip dislocation activity in refractory high-entropy alloys

Dislocation activities play an important role in the deformation and failure of refractory high-entropy alloys (RHEAs). However, the impact of chemical short-range ordering (SRO) on dislocation activities at a crack tip in RHEAs remains unclear. Here, we investigate the effect of SRO on the dislocation nucleation, propagation, and reaction at a crack tip in a body-centered-cubic (BCC) MoTaTiWZr RHEA, using a combination of molecular dynamic simulations and Monte Carlo methods. Our results indicate that this RHEA is energetically favorable to form SRO, developing a pseudo-composite microstructure with low-energy clusters (LECs), medium-energy clusters (MECs), and high-energy clusters (HECs). The HECs at the crack tip are favorable sites for dislocation nucleation whereas the MECs surrounding the HECs function as a strong matrix to stabilize the weak HECs. At elevated temperatures, the HECs near the crack tip transform to severely distorted BCC and disordered structures, which can cause the breakup, absorption, and annihilation of emitted dislocations and nucleation of new dislocations. Our work reveals the interesting role of SRO in altering the dislocation activities at the crack tip of RHEAs and suggests alternative routes for designing superior RHEAs at both room and elevated temperatures.

Molecular dynamics modeling of hydrogen-induced plastic deformation and cracking of ɑ-iron

In this work, molecular dynamics modeling was conducted to study hydrogen (H)-induced plastic deformation and cracking of polycrystal α-Fe. Under cyclic loading, the number of vacancies and the stress intensity increase with H atom concentration and the number of loading cycles. However, the effect of cyclic loading on cracking is not as significant as the increment of H concentration. As the H concentration increases, the dislocation generation and emission are enhanced in the {110}<111> slip system, but are inhibited in other slip systems. There is a critical H atom concentration, below which the plastic deformation of α-Fe is facilitated by H atoms. When the critical H concentration is exceeded, the dislocation emission is inhibited by H atoms at grain boundaries, where the H atoms can pin dislocations, causing piling-up of the dislocations to generate a stress concentration.

Role of extrinsic and intrinsic toughening mechanisms in graphene nanosheets reinforced magnesium matrix layered composites

Constructing a layered structure is an effective approach to achieve a balance between strength and toughness. The toughening mechanisms of layered composites are generally considered to include reinforcement bridging, pullout, and crack deflection. However, the above toughening mechanisms are more applicable to ceramic matrix composites with predominantly cleavage fracture. For metal matrix composites with a larger fracture process zone, the energy consumption in crack propagation may be dominated by the plastic deformation at the crack tip. Therefore, elucidating the dislocation emission behavior and the interaction between graphene nanosheets (GNSs) and crack is the key to revealing the toughening mechanisms. In this work, the extrinsic and intrinsic toughening mechanisms for the layered GNS/Mg composites were investigated systematically. The three-dimensional reconstruction of the fracture morphology, combined with statistical analysis and molecular dynamic (MD) simulations, elucidates that macroscopic crack deflection in layered composites does not always result in a microscopically longer crack propagation path. The mitigation and redistribution of stress concentration and the promotion of dislocation emission at the crack tip induced by the GNS layers are proposed to be critical toughening mechanisms in the GNS/Mg layered composites.

Interaction between cracks and dislocations in body-centered cubic crystals under mixed loads

A dislocation emission model for body-centered cubic (BCC) crystals with micro-scale grain size is described. When the critical condition of dislocation emission is reached, dislocations will be emitted from the crack tips and move along the slip planes at the crack tip. If the stress concentration at the crack tip is large enough or the distance between the crack tip and the grain boundary is small, the dislocation will move along the slip plane and accumulate at the grain boundary. Three BCC metals have been used to calculate the dislocation emission in this article. The results show the lattice friction has a significant effect on dislocation emission and distribution. Dislocations that emitted from crack tip have a shielding effect on stress concentration near the crack tip. The shielding effect of the dislocations on plane (110) is larger than that of the dislocations on plane (100), resulting in a more easy propagation of the crack in plane (100).

Microstructure and activation volume of a Cu-6 wt %Zn brass processed by equal channel angular pressing

The generation of a high density of twins few tens nanometers in size can enhance both strength and ductility. The present work aims to produce highly twinned microstructure in Cu-6 wt % Zn alloy of a moderate stacking fault energy (SFE) by equal channel angular pressing (ECAP) using a die with angles ϕ= 110° and Ψ = 0. X ray diffraction (XRD) shows that a high density of defects is stored up to 2 passes then a recovery takes place. EBSD imaging reveals a high density of twins and a fibrous microstructure. Grains close to 100 nm in size are formed. Strain rate sensitivity (SRS) was investigated in compression and nanoindentation (NI). Compression curves show that deformation occurs by slipping with a contribution of twins to hardening. A high contribution of shear bands was revealed during ECAP and compression tests. The activation volume V* in compression and NI is in the range 70b3 - 100b3, consistent with the emission of dislocations from grain boundaries and twin boundaries The experimental values of V* were compared to those expected from defect densities.

The interaction of crack and dislocations in cyclic loading in the body-centered cubic (BCC) crystal

In engineering, the mode of fatigue crack propagation in crystals is very important for studying the first stage of fatigue crack propagation. In this paper, a model of fatigue crack propagation in crystals is established by using the discrete dislocation method. The propagation rate of fatigue cracks in different crystals and the dislocation movement and distribution on the slip plans (SP) in front of cracks during loading and unloading are obtained by calculation the model. The effects of different dislocation motion behaviors on fatigue crack growth rate during loading and unloading are analyzed, including dislocation emission, rebound and annihilation. Compared with the case without considering dislocation shielding effect, the influence of dislocation shielding effect on fatigue crack propagation is given. At the same time, the effects of different grain sizes and different grain boundary (GB) angles on fatigue crack propagation are studied.

Modulating grain boundary-mediated plasticity of high-entropy alloys via chemo-mechanical coupling

Intrinsic chemical disorder and the subsequent interfacial roughening have posed formidable challenges in elucidating the grain boundary (GB)-mediated plasticity in high-entropy alloys (HEAs). Here using a self-propelling atomistic algorithm to probe the complex energy landscape of the CoCrFeMnNi HEAs, in conjunction with location-specific perturbations across GBs exposed to different environments, we investigate atomic-reconfiguration ensembles near GBs and their sensitivities to various chemo-mechanical conditions. Two distinct modes, collective and random, are discovered, and their partitions are dictated by multiple factors, including the activation energy window, external mechanical loading, and local compositions. Remarkably, Fe disproportionately promotes the collective events and facilitates the slip activities near GBs, while Cr atoms suppress the emission of partial dislocations from GBs. These findings imply promising solutions – via synergistic combination of microalloying, heat treatment, and mechanical loading – to selectively trigger desired plasticity modes at needed deformation stage, and hence to achieve an enhanced tunability of HEAs’ mechanical behaviors.

Effect of symmetrical and asymmetrical tilt grain boundaries with as the tilt axis on the shock response of bi-crystalline Ni

Aim of this article is to study the effect of tilt grain boundaries on the shock response of single and bi-crystalline nickel. Nickel is commonly used in many structural applications, but research on dynamic response is very limited or in an immature stage. In this article, molecular dynamics-based simulations were performed in conjunction with embedded atom method potential to quantify the response of shock in bi-crystalline Ni containing symmetrical or asymmetrical tilt grain boundaries. Simulations were performed under the influence of adiabatic conditions. Compressive ultra-shock pulse of width 2 ps was simulated in the simulation box; stress, particle velocity, dislocation emission and phase transformation were quantified for symmetrical and asymmetrical tilt grain boundaries in bi-crystalline Ni. It was concluded from the simulations that higher mis-orientation angle between the crystals of Ni help in retarding the shock front velocity and stress. The alternation in the shock front behavior was associated with the change in deformation governing mechanism. The emission of dislocations and formation of Lomer Cottrell lock governs deformation in symmetrical tilt grain boundary configurations under shock pulse loading. It was concluded from the set of simulations that plastic deformation blunts the shock front in bi-crystalline Ni, whereas the sharp shock front was observed in single crystal Ni. Radial distribution function in post shock atomistic configurations was performed to capture the stability of the crystal in post shock scenario. Asymmetrical tilt grain boundaries are not as efficient in restricting the shock front as symmetrical tilt grain boundary configurations.

Effect of helium bubbles on the interface shear strength of copper/niobium multilayer

The influence of the helium bubble on the interfacial shear strength of copper/niobium multilayers and the corresponding deformation mechanisms are studied through molecular dynamics. Strong anisotropy of the shear strength is demonstrated, and the ratio of the highest strength to the lowest is approximately 4.3. The interface bubble leads to the dislocation emission under the shear along the high-strength directions and results in the severe decrease of the interface bearing capability, while the loss of the interface bearing is negligible for the case along the weak/medium-strength directions. The contradictory effects of the bubble on the interface shear strength under different shear directions are explained by the competition between the bubble-related dislocation emission and propagation and the regular uniform interface slip. For the shear along the high-strength direction, the interface could bear relatively high shear stress which is large enough to activate the dislocation from the bubble. Consequently, the corresponding deformations are dominated by the mechanism of bubble-related dislocation emission and its propagation. For the shear along the weak/medium-strength direction, the interface itself is sheared under relatively low shear stress before the effective nucleation of dislocation. Therefore, the corresponding deformation mechanism is controlled by the regular interface shear.

Effect of Cu and Mg co-segregation on the strength of the Al grain boundaries: A molecular dynamics simulation

Molecular dynamics simulations employing machine learning potential functions of Al-Cu-Mg ternary alloys, were performed to elucidate the segregation behavior of Cu and Mg solutes and study their effects on the mechanical properties of Al grain boundaries. The calculation results show that Cu segregation enhances the strength of the Al Σ5 (210)[001] grain boundary, while the Mg instead weakens the Al Σ5 (210)[001] grain boundary. Cu and Mg solutes lead to the Al Σ41 (540)[001] grain boundary embrittlement. Dislocation plays a crucial role in the analysis of plastic deformation. We found that the reinforcing effect of the Cu solutes lies in the dislocation pile-up in plastic deformation, and the stair-rod dislocations inhibited the movement of the Shockley dislocations. The embrittlement cause of Mg solutes is that Mg solutes inhibit dislocation emission and shorten the dislocation nucleation cycle. This work illustrates the relationship between solute strengthening mechanism and dislocation evolution at the atomic scale.

Grain size and scratching depth dependent tribological characteristics of CrCoNi medium-entropy alloy coatings: A molecular dynamics simulation study

In this work, we have investigated the impact of grain size and scratching depth on the tribological behavior of nanocrystalline CrCoNi medium-entropy alloy (MEA) coatings during scratching by molecular dynamics (MD) simulations. Regardless of the grain size, a monotonically increase in both friction coefficient and wear rate with increasing the scratching depth was observed in the nanocrystalline CrCoNi MEA coating. When its microstructure becomes coarser, the friction coefficient becomes lower. As a function of grain size, the evolutions of friction coefficient and wear rate can be divided into three regions, namely high friction coefficient (or wear rate) region in a grain size range of below 6 nm, relatively stable friction coefficient (or wear rate) region in a grain size range of 6–12 nm, and a relatively low friction coefficient (or wear rate) region in a grain size range of larger than 12 nm. According to the MD simulation results, the higher wear rate for a finer CrCoNi MEA coating can be explained by its lower hardness. In contrast, the reduction of friction force was mainly attributed to the scratching induced grain boundary (GB) sliding and dislocation emissions from GBs. Moreover, the scratching induced phase transformation from crystalline to amorphization and plastic deformation mode transitions also affect its tribological properties.

Molecular dynamics simulations of interface structure and deformation mechanisms in metal/ceramic composites under tension

The role of interfaces on plastic deformation of metal/ceramic composites is multifaceted, which is due to the fact that interfaces can emit, hinder, annihilate or store dislocations. In this work, molecular dynamics simulation was employed to investigate the interfacial structure and deformation mechanism of the coherent and semi-coherent Al/MgO(110) configurations under uniaxial tensile loading. In particular, rectangular mismatch dislocation networks were observed at the interface of a semi-coherent Al/MgO(110) configuration after relaxation, which were due to the difference in lattice constants between the constituent phases. Besides, the strain concentration at the dislocation nodes was found, being conducive to the emission of misfit dislocations into the Al matrix. Moreover, unlike the coherent configuration, the elastic zone in the stress-strain curve of the semi-coherent Al/MgO(110) configuration was relatively narrow and had two yield points. The first yield point was attributed to the nucleation of misfit dislocations from the interface dislocation nodes, whereas the second one was ascribed to the nucleation of lattice dislocations in the Al matrix. Finally, the failure stress of the semi-coherent configuration was less than that of the coherent system.

Development of ultrahigh-strength low-alloy Mg-0.7Al-0.3Ca-0.4Mn (wt.%) alloy with excellent ductility via controlling extrusion temperature

Mg-0.7Al-0.3Ca-0.4Mn (AXM070304, wt.%) alloy was subjected to extrusion with a ram speed of 1 mm/s and an extrusion ratio of 25:1. The extrusion temperature varies from 170 °C to 400 °C. The influence of extrusion temperature on microstructure and mechanical properties of the dilute AXM070304 alloy was systematically explored. With raising the extrusion temperature, the average size of the dynamic recrystallized (DRXed) grains was increased, while the intensity of the basal fiber texture was decreased. When the extrusion temperature was escalated from 170 °C to 400 °C, the tensile yield strength (YS) of the low-alloy AXM070304 extrusion alloy was decreased from 437 to 246 MPa, while the elongation to failure (EL) was increased from 2.9 to 16.9%. The alloy extruded at 200 °C obtains an YS of 412 MPa, an ultimate tensile strength (UTS) of 418 MPa and an EL of 14.8%, exhibiting excellent ductility and ultrahigh strength. The ultrahigh YS was mainly due to the strengthening from ultra-fine recrystallized grains (0.61 μm) with grain boundary segregation of solute Al and Ca atoms. The growth of DRXed grains was inhibited mainly by the co-segregation of Al atoms and Ca atoms, nanosized Al2Ca and β-Mn particle phases at the grain boundaries. Plastic instability occurred under tensile loading in the ultrafine-grained AXM070304 alloy, which may be due to the fact that the grain boundary segregation of Al atoms and Ca atoms requires a high energy barrier for dislocation emission. Once the tensile stress reaches the peak value, the mobile dislocation density increases suddenly.

Microcrack initiation and propagation in 3 C-SiC ceramic based on molecular dynamics nano-drilling

The evolutionary mechanism of microcracking has been the focus of research in materials processing. The experimental calculation and analysis of microcracking are inevitably difficult due to the limitations of the controllability of its emergence and expansion and the existing observation techniques. In this paper, an experimental model of nano-drilling of 3 C-SiC ceramic parts is constructed using molecular dynamics simulations to achieve real-time observation of the drilling process at the atomic scale, and the microcrack evolution behavior at different depths during the drilling process is discussed, and the microcrack initiation and propagation laws of 3 C-SiC ceramic drilling processing are proposed. The results show that when drilling reaches a certain depth, the compressive stress generated by drilling leads to a prominent region in the plastic deformation zone below the drill bit, where the atomic spacing becomes larger and the workpiece material splits, forming a microcrack eruption zone. The expansion direction of the microcrack is related to the dislocation slip direction, and it expands continuously with the continuous emission of dislocations at the crack tip. The influence of the special evolutionary behavior of the dislocation ring on the microcrack expansion during drilling is focused on, and it is found that the trend of microcrack expansion changed due to the creation of the dislocation ring, and the limit of microcrack expansion is significantly correlated with the dislocation ring evolutionary behavior. With the drilling simulation, the new microcrack expansion showed a periodic occurrence pattern with the special evolutionary behavior of dislocation rings.

Research on the influence of stress wave on crack and chip formation mechanism in radial ultrasonic vibration assisted grinding

The propagation of stress waves in the ultrasonic vibration grinding zone was studied and analyzed in this paper and the microscopic mechanism of ultrasonic vibration assisted grinding is clarified by combining the method of finite element analysis (FEA). The creation of an experimental platform for ultrasonic vibration assisted grinding allowed for a comparative experimental evaluation of radial ultrasonic vibration assisted grinding (RUVAG) and conventional grinding (CG). Experiments analysis show that: (a) Compared with conventional grinding, the alternating impact of stress waves in ultrasonic vibration grinding is beneficial to the propagation of microcracks and the removal of materials. Meanwhile, there is no burn on the machined surface, so the quality of the machined surface is better. (b) Under the alternating and superimposed action of stress waves, dislocation emission becomes difficult due to the suppressed thermal activation process. Therefore, the plastic deformation of the material is suppressed and reduced in the grinding zone, and the material is more likely to break than during conventional grinding. Therefore, compared with conventional grinding, both cracks and chips are easier to form in it..

Compositive role of refractory element Mo in improving strength and ductility of face-centered-cubic complex concentrated alloys

Complex concentrated alloys (CCAs) with a face-centered-cubic (FCC) structure exhibit remarkable mechanical properties, introducing the expansion of compositional space in alloy design for structural materials. The formation of a single solid-solution phase is enabled by configuring various 3d-transition elements, while doping other elements even of a small portion generally leads to the formation of brittle intermetallic compounds. Herein, we demonstrate through a systematic investigation of single FCC (CoNi)100-xMox alloys that a wide range of refractory element Mo can simultaneously improve the strength and ductility while sustaining the solid-solution structure. The addition of Mo with a larger atomic size than those of 3d-transition elements introduces severe lattice distortion in the FCC lattice and causes grain-boundary segregation enriched by Mo atoms. In addition, increasing Mo content effectively reduces the stacking fault energy (SFE). The increased lattice distortion with Mo content enhances the solid-solution strengthening of the alloys. Besides, along with reduced SFE and stabilization of the dislocation emission site by grain-boundary segregation, this elevated solid-solution strengthening increases grain-boundary strengthening, reaching a yield strength of ∼1 GPa. Moreover, the reduction of SFE with increasing Mo results in the transition of dislocation substructures and the refinement of deformation twins, allowing for enhanced strain-hardening capability and thus ∼1.3 GPa tensile strength and ∼50% ductility. Such compositive and synergetic effects of refractory element Mo enable the CCAs with a single FCC solid solution to overcome the strength and ductility trade-off.