Stacking Fault Tetrahedra Research Articles

On the stability of nanovoids in fcc metals and the influence of hydrogen

Atomic-scale simulations are used to examine the stability of nanometric vacancy clusters in variety of face centered cubic metals (Au, Ag, Al, Cu and Ni), according to their shape, their size and the presence of hydrogen. It is shown that whilst in most of metals the stacking fault tetrahedron is the most stable defect in absence of hydrogen, cavities becomes more stable when the concentration of hydrogen is sufficiently large to cover the internal surfaces. The (100) surfaces are found to be the less energetic, which favors vacancy clusters with cuboid shape in the metals concerned here. The simulations are shown to agree well with theoretical predictions provided that the theory is parametrized with surface and bulk energies conformed to the atomistic models. The affinity between hydrogen and cuboid cavities has also been confirmed by the computation of hydrogen solubility, which is neatly enhanced in the vicinity of these cavities. Our study paves the way towards the comprehensive atomistic-based predictions for micro-structure in hydrogen-rich metals.

Comparative analysis of irradiation-stimulated hardening in the austenite and ferrite phases of F321 stainless steel

F321 stainless steel is renowned for its outstanding mechanical properties and corrosion resistance, making it a crucial structural material in light water reactors and high-temperature gas reactors. With irradiation (IR), F321 stainless steel undergoes microstructural alterations, such as the formation of IR-stimulated defects (e.g., stacking fault tetrahedrons (SFTs), dislocation loops (DLs), and clusters), resulting in IR hardening. This study compares the IR-stimulated hardening in the austenitic and ferritic phases of F321 stainless steel through both experimental methods and numerical simulations. In the austenitic phase, the size and number density of IR defects (DLs and SFTs) were determined, which were proportional to the IR dose. In the ferritic phase, IR-induced DL parameters were quantified, and the effects of IR on spinodal decomposition were analyzed. The interaction processes and mechanisms between dislocations and IR defects in the austenitic phase were evaluated using molecular dynamics (MD) simulations, leading to the development of a model for IR defect-induced hardening strength. Additionally, an MD data-driven crystal-plasticity-finite-element-model (CPFEM) was established. The nano-indentation hardness of the austenitic and ferritic phases was measured and calculated through both experimental methods and CPFEM simulations. The results show that the nano-indentation hardness of the ferritic phase is reduced compared to the austenitic phase, due to a lower density of IR defects. Understanding these effects provides deeper insights into how IR influences the mechanical and microstructural properties of F321 steel, which is essential for predicting the material's long-term performance and reliability.

Effect of crystallographic orientation on mechanical properties of Al0.25CoCrFeNi high entropy alloy under tension

The deformation behavior of single-crystal materials is closely related to the crystallographic orientation. However, there are few simulations and experimental studies on the effect of crystallographic orientation on the tensile properties of single-crystal AlxCoCrFeNi high entropy alloys (HEAs). Molecular dynamics simulations were used to analyze the mechanical properties and structural variations of Al0.25CoCrFeNi HEAs with different crystallographic orientations under tension. Prior to yielding, elastic softening occurred in [111], [110] and [11̅0]-oriented HEAs, except for [100]-oriented HEA. During plastic deformation, [100]-oriented HEA had high strength and flow stress due to the low Schmid factors of leading partial dislocations and multiple activatable slip systems. [111]-oriented HEA also showed high strength and flow stress, which were attributed to the low Schmid factors of the activatable slip systems and the existence of incomplete stacking fault tetrahedrons. Both [110]- and [11̅0]-oriented HEAs showed low strength, but the presence of the secondary increased stress suggested they had good plasticity. The difference between them was that [110]-oriented HEA was easier to form hexagonal close-packed lamellae and experienced secondary yielding at larger strain. This suggested that different crystallographic orientations in the non-tensile direction also affected the mechanical properties.

Atomic-scale wear behavior of two-phase TiAl alloys by vibrational horizontal friction

To investigate the influence of vibrational horizontal friction on the microscopic distortion of γ/α2 biphasic TiAl alloys, a molecular dynamics approach was used for simulation by analyzing the mechanical properties, atomic displacements, shear strains, and defect distributions of the substrate in vibrational horizontal friction and conventional friction. The friction force and coefficient of friction in vibrational horizontal friction were found to be less than conventional friction. Vibrational horizontal friction has a shallower depth of abrasion marks and a larger wear area compared to conventional friction. At the same time, this γ/α2 phase boundary changes the direction of motion of the atom so that they move in the same orientation as parallel to the interfacial boundary. Vibrational horizontal friction has a larger strain concentration area than conventional friction. The existent γ/α2 interface impedes the strain transmission so that some of the stresses are concentrated near the interface, and the γ-phase accumulates a large proportion of dislocations, which leads to work-hardening of the material. The slip of the horizontal stacking faults in the vibrational horizontal friction releases internal stresses. The formation of structures such as stacking fault tetrahedrons has a significant strengthening effect on the material.

Atomic simulations of crack propagation in Ni-Al binary single crystal superalloy with a central crack

Nickel (Ni)-based single-crystal superalloys are of great importance in the aircraft industry due to their excellent mechanical properties, and cracks as unavoidable defects may affect the mechanical performances of materials dramatically. In this paper, large scale molecular dynamics (MD) simulations are carried out to understand the deformation mechanisms of Ni-based single crystal with a central crack under tension. Here, the effects of matrixes (γ, γ′ and γ/γ′), strain rates (1 × 109 s−1 ∼ 3 × 109 s−1) and temperatures (300 K∼900 K) on the role of crack propagation are considered. It is observed that dislocations and slip systems in the γ′ model are concentrated near the crack, resulting in the rapid expansion of dislocation, which leads to the fastest crack growth speed and early fracture. While the crack propagation rate of γ and γ/γ′ models are relatively slow, due to the combined action of the Lomer-Cottrell lock and stacking fault tetrahedron structure and Stair-rod dislocation, which hinders crack propagation. In addition, deformation at increased strain rates and/or reduced temperatures, lead to superior yield stress and Young′s modulus for models with a central crack at γ/γ′ interface. On the other hand, high temperature and high strain rate will promote crack propagation in the γ phase, and the higher the strain rate and/or temperature, the faster the crack propagation speed will be. These results will enrich our understanding on the crack propagation and evolution mechanisms in Ni-based single crystal superalloy.

Atomistic-level analysis of nanoindentation-induced plasticity in arc-melted NiFeCrCo alloys: The role of stacking faults

Concentrated solid solution alloys (CSAs) have attracted attention for their promising properties; however, current manufacturing methods face challenges in complexity, high costs, and limited scalability, raising concerns about industrial viability. The prevalent technique, arc melting, yields high-purity samples with complex shapes. In this study, we explore nanoindentation tests at room temperature where arc-melted samples exhibit larger grain sizes, diminishing the effects of grain boundaries on the results. Motivated by these findings, our investigation focuses on the atomistic-level exploration of plasticity mechanisms, specifically dislocation nucleation and propagation during nanoindentation tests. The intricate chemistry of NiFeCrCo CSA influences pile-ups and slip traces, aiming to elucidate plastic deformation by considering both pristine and pre-existing stacking fault tetrahedra. Our analysis scrutinizes dynamic deformation processes, defect nucleation, and evolution, complemented by stress–strain and dislocation densities–strain curves illustrating the hardening mechanism of defective materials. Additionally, we examine surface morphology and plastic deformation through atomic shear strain and displacement mappings. This integrated approach provides insights into the complex interplay between the material structure and mechanical behavior, paving the way for an enhanced understanding and potential advancements in CSA applications.

The role of stacking fault tetrahedra on void swelling in irradiated copper

A long-standing and critical issue in the field of irradiated structural materials is that void swelling is significantly higher in face-centered cubic-structured (fcc) materials (1% dpa−1) as compared to that of body-centered cubic-structured (bcc) materials (0.2% dpa−1). Despite extensive research in this area, the underlying mechanism of the difference in swelling resistance between these two types of materials is not yet fully understood. Here, by combining atomistic simulations and STEM imaging, we find stacking fault tetrahedra (SFTs) are the primary cause of the high swelling rate in pure fcc copper. We reveal that SFTs in fcc copper are not neutral sinks, different from the conventional knowledge. On the contrary, they are highly biased compared to other types of sinks because of the SFT-point defect interaction mechanism. SFTs show strong absorption of mobile self-interstitial atoms (SIAs) from the faces and vertices, and weak absorption of mobile vacancies from the edges. We compare the predicted swelling rates with experimental findings under varying conditions, demonstrating the distinct contributions of each type of sink. These findings will contribute to understanding the swelling of irradiated structural materials, which may facilitate the design of materials with high swelling resistance.

Introducing vacancy defects by rapid quenching in FCC metal for deep learning micrograph dataset

Accurate detection of crystal defects from microstructure micrographs is essential for evaluating the mechanical and functional properties of metallic materials. Emerging deep learning techniques have great potential for application in crystal defects detection, but the excellent detection accuracy requires domain knowledge and micrograph datasets. Here we target to detect and distinguish nanoscale dislocation loops (DLs) and stacking fault tetrahedrons (SFTs) in transmission electronic microscope (TEM) micrographs. The micrograph datasets of crystal defects derive from copper prepared by rapid quenching and post-annealing. Applying standard U-Net architecture, the maximum Mean Intersection over Union (mIOUmax) of 3-class model is 54.29%, while it is 73.81% and 74.67% of DL 2-class and SFT 2-class model. Results suggest that excellent detection accuracy of deep learning model could be achieved by learning features from specific defect datasets. These findings provide a new strategy for accurate detection of crystal defects in the field of material engineering.

First-principles study of stability of small vacancy clusters in aluminum

The vacancy cluster is one of the most common defects in aluminum alloys. Using density-functional theory-based first-principles calculations, the energetics of small vacancy clusters in aluminum, including voids, stacking fault tetrahedra (SFT), and vacancy platelets on the {111} plane, was investigated. It is found that the significant difference in monovacancy formation energies reported in literatures is mainly related to their used exchange-correlation functions. On average, LDA is the most reliable approximation for the monovacancy formation energies in Al, followed by PBE, PBEsol, PW91, and AM05. The results in this work confirm that the divacancy in Al is indeed energetically unfavorable. Moreover, vacancy clusters with a size smaller than five in any form are unstable against their corresponding number of isolated monovacancies. The SFT is the most stable form of most small vacancy clusters, followed by the void and vacancy platelet. These results are helpful to understand the experimentally observed size distributions of vacancy clusters in Al.

Mixing behavior of Ti–Al interface during the ultrasonic welding process and its welding strength: Molecular dynamics study

In this study, we conducted molecular dynamics simulations to investigate the mechanical mixing and deformation behavior of hcp Ti/fcc Al bimetal formed by ultrasonic welding (UW). To analyze the effect of the interface shape, we considered sixteen sinusoidal interfaces of various heights and spatial periods along with the flat interface. Mechanical mixing between Ti and Al occurs mainly in the vibrational loading direction, while it is suppressed in the interface-normal direction, as the loading direction lies within the slip planes of both the hcp and fcc structures. The degree of mechanical mixing depended on the shape of the interface. According to the simulation results, mechanical mixing becomes active as the sinusoidal height increases, and the spatial period decreases because of the enlarged interface areas. During the bonding process, phase transformation is observed at the sinusoidal interface; hcp Ti is converted to fcc Ti as misfit dislocations formed at the interface glide as Shockley partials on the slip plane owing to the applied vibrational loading. A simple shear test was performed to analyze the welding strength. Although sinusoidal Ti/Al interfaces can have a welding strength that is higher than that of a flat interface, we found that the welding strength was not closely related to the degree of mechanical mixing. Rather, the welding strength was affected by the interaction between a wall of misfit dislocations, stacking fault tetrahedra, and lattice dislocations generated near the interface during the UW process.

The effect of migrating Σ3 {112} incoherent twin boundary on radiation tolerance of nanotwinned copper

Nanotwinned face-centered cubic metals possess superior radiation tolerance; however, the underlying mechanism of alleviating radiation damage is still under debate. In the present study, the effect of Σ3{112} incoherent twin boundary (ITB) on the radiation tolerance of nanotwinned copper was systematically investigated by the molecular dynamic simulation method. The simulation results reveal that both stacking fault tetrahedron and interstitial dislocation loop can be easily absorbed into the migrating ITB. The two types of defects occupy two different types of atom layers of the ITB and move in the opposite direction of each other, resulting in their annihilation, thus enhancing the radiation tolerance of nanotwinned copper. Another interesting finding is that the migration of ITB can be accelerated rather than retarded by both interstitials and vacancies. The above simulation results were further investigated in terms of defect formation and migration energies. The present work provides a deep insight into the radiation tolerance mechanisms of nanotwinned metals.

Atomic insight into tribological behavior of AlCoCrFeNi high entropy alloy at various nanoscratching conditions

High-entropy alloys (HEAs) have garnered significant interest in recent years due to their exceptional properties. Among the HEAs, the AlCoCrFeNi HEA has been extensively studied due to its outstanding thermal stability and mechanical properties. However, limited attention has been given to its tribological behavior under high temperature and high velocity conditions. In this study, we first investigate the effect of temperature, scratching velocity, and scratching depth on the tribological behavior of AlCoCrFeNi HEA at the atomic level employing molecular dynamics method. The AlCoCrFeNi HEA exhibits good wear resistance at high temperature due to fewer dislocations to nucleation and slip. Regardless of a significant increase in scratching velocity, there is little difference in the wear volume, indicating that the AlCoCrFeNi HEA still maintains excellent wear resistance even under extremely high scratching velocity. The stacking fault tetrahedron is generated in the AlCoCrFeNi HEA under the large scratching depths of 1.5 nm and 2.0 nm, implying that the wear resistance of AlCoCrFeNi HEA would be improved after the nanoscratching with a relatively large scratching depth. These findings are highly significant for expanding the application potential of AlCoCrFeNi HEA under extreme conditions and provide profound insights into the tribological behavior of the HEA.

Effect of twin boundary spacing on the mechanical properties of nano-columnar crystalline Cu-Ni alloy

ABSTRACT Nanotwinned exist in crystals as coherent interfaces with low interfacial energy, which can not only improve the strength of metal materials, but also increase the ductility. In this manuscript, we have performed molecular dynamics simulations of the mechanical properties of a nano-columnar crystalline Cu-Ni alloy with different twin boundary spacing. It is found that the model without twin has a stacking fault tetrahedron composed of stair-rod dislocations, which results in a small change in dislocation density at the later stage of deformation, and the average stress after yielding is lower than that of the model with twin. During the deformation process, with the increase of Other atoms, the dislocation slip barrier is enhanced, the tensile strength is increased, and the yield phenomenon is delayed, which is more obvious with the decrease of twin boundary spacing. The dislocation density decreases with the decrease of the spacing of the twin boundary, and the dislocation segments become shorter. When the twin boundary spacing is 0.625 nm, the tensile strength is increased by about 71% compared with the model without twin structure.

Direct formation of novel Frank loop and stacking-fault tetrahedron complex

Stacking-fault tetrahedron (SFT) is one kind of typical three-dimensional vacancy defect in quenched, deformed or irradiated face-centered cubic (FCC) metals, which can seriously degrade the mechanical properties of materials. It's generally believed that high stacking fault energy (SFE) is unfavorable for the formation of SFTs. Here, we report the first in-situ investigation of irradiation-induced formation of novel Frank loop-SFT complexes in Pd with extremely high SFE. Our findings reveal a new mechanism that vacancy clusters rearrange directionally to form SFTs due to the ambient stress deviations and compressive stress fields induced by interstitial Frank loops. Continuous hydrogen implantation will lead to a synergistic growth of the complex, while direct interaction between the Frank loop and SFT under thermal effect can cause the complex to disappear. These results uncover a unique formation mechanism for SFT and provide a new perspective for understanding nano-defects in high SFE metals.

Nano-tribological behavior of CuCoCrFeNi high-entropy alloys at cryogenic temperature: A molecular dynamics study

High-entropy alloys exhibit great potential for cryogenic applications. This study investigates the nano-scratching behavior of CuCoCrFeNi high-entropy alloy at a cryogenic temperature (77 K) using molecular dynamics. Results show that compared with the single-grain model, the average friction coefficient (AFC) increases for all three polycrystalline models with different grain sizes d, but the anti-wear property can be improved by 28.5%, when grain size d = 10.7 nm. The smaller friction on the scratching surface of the single-grain model (AFC is 15.5% less than that of the model with d = 8.2 nm), which makes the overall temperature rise lower compared to that of the polycrystalline models. However, due to the stress concentration released when a complete stacking fault tetrahedron is produced, the single-grain model cannot significantly harden the surface and subsurface to a greater degree. In the polycrystalline models, dislocations are blocked at grain boundaries (GBs). However, the introduction of GBs changes the von Mises stress distribution. Finally, an attempt was made to reveal the role of yield pressure H3/E2 (H—hardness, E—elastic modulus) in friction-reducing and anti-wear properties.

Defective Graphene Effects on Primary Displacement Damage and He Diffusion at a Ni–Graphene Interface: Molecular Dynamics Simulations

Ni–graphene nanocomposites with high-density interfaces have enormous potential as irradiation-tolerant materials applied in Gen-IV reactors. Nevertheless, the mechanism wherein the intrinsic and/or irradiation-induced defects of graphene affect the irradiation tolerance of the composites remains poorly understood. Here, we investigate the effects of the two types of defective graphene on the displacement damage and He diffusion of the composites, respectively, using atomistic simulations. The introduction of the intrinsic defects of graphene has a significant effect on the Ni lattice structure near the Ni–graphene interface, especially showing that after displacement cascades, the number of defects gradually increases with the increase in graphene-defective size due to the formation and growth of stacking fault tetrahedra. The existence of the irradiation-induced defects of graphene does not diminish the ability of the interface to trap He atoms/clusters and even may be maintained or improved, mainly reflected in the fact that many isolated He atoms and small clusters can gradually migrate toward the interface and the fraction of He within the interface is up to 37.72% after 1 ns. This study provides an important insight into the understanding of the association relationships of defective graphene with the irradiation tolerance of composites.

Mechanisms of helium nanobubble growth and defect interactions in irradiated copper: A molecular dynamics study

It is well-established that mechanical behavior of the materials is significantly affected under irradiation. This is often due to the growth of nanobubbles that changes the microstructure of the irradiated materials. In this work, growth of helium nanobubbles with different sizes and helium to vacancy (He/V) ratios in copper is investigated using molecular dynamics (MD) simulations. Our simulations reveal that the nanobubble growth is accommodated by nucleation of various types of defects such as: stacking fault octahedron (SFO), dislocation network, punched-out dislocation loop, stacking fault tetrahedron (SFT) etc. around the nanobubbles. We also unveil the mechanisms of formation as well as punching-out of the dislocation loops, and further show that such defects are activated beyond a threshold of bubble size and He/V ratio. In final, we propose an equation of state (EOS) for predicting the pressure of the nanobubbles inside copper from the input bubble size, He/V ratio, and temperature.

Comparative study of vacancy cluster formation in pure Ni, CoCrNi, and CoCrFeNi with a CoCrFeMnNi multicomponent system

It is well known that the CoCrFeMnNi equiatomic high-entropy alloy (HEA) exhibits excellent irradiation resistance. It is very difficult to form vacancy clusters in this alloy. In the present study, vacancy cluster formation was investigated for three (CoCrNi) and four element medium-entropy alloys (MEAs) in a CoCrFeNi system with the same crystal structure as that of a CoCrFeMnNi HEA. Stacking fault tetrahedra (SFTs), which are a type of vacancy cluster, were observed clearly in both MEAs, which were deformed at high speed, after annealing at 373 K. For comparison, Ni and 316 L stainless steel (SS316L) were also subjected to high-speed deformation experiments. SFTs with high density were clearly observed in the Ni sample after annealing at 773 K, but these were not observed in the SS316L sample. The SFT density was significantly lower in MEAs than that in pure Ni. This result indicates that the CoCrNi and CoCrFeNi MEAs also exhibit good irradiation resistance similar to that of SS316L. Further, calculation results based on the first principles indicate that the binding energies of di-, tri-, and tetra-vacancy clusters in the CoCrNi and CoCrFeNi MEAs, are positive. However, the attractive energy of the tri-vacancy cluster in the CoCrFeMnNi HEA is negative.

Atomistic simulation on the generation of defects in Cu/SiC composites during cooling

The coefficient of thermal expansion (CTE) mismatch between the reinforcement and the matrix results in thermal residual stresses and defects within metal-matrix composites (MMCs) upon cooling from the processing temperature to ambient temperature. The residual stresses and thermally induced defects play an important role in the mechanical properties of MMCs, it is critical to understand the mechanism of defect formation and evolution. This study provides atomistic simulations to reveal the generation of thermal residual stresses, dislocation and incomplete stacking fault tetrahedron (ISFT) during cooling in the idealized Cu/SiC composites. We found that dislocations are generated explosively in a certain temperature range during cooling, which results in a non-linear relationship between dislocation density and temperature. The combined effect of the stresses induced by CTE mismatch and the thermodynamic state of the metal leads to the rapid generation of dislocations. The Shockley partial and the highly stable stair rod are the two dominant dislocation structures. The immobile stair-rod dislocations and the highly stable ISFTs formed in the initial high temperature stage inhibit further development of plastic deformation. The present results provide new insights into the defect formation mechanism and the dislocation strengthening mechanism of MMCs caused by thermal mismatch between constituents.

Comparison of formation and evolution of radiation-induced defects in pure Ni and Ni–Co–Fe medium-entropy alloy

High-entropy alloys (HEAs) and medium-entropy alloys (MEAs) have attracted a great deal of attention for developing nuclear materials because of their excellent irradiation tolerance. Herein, formation and evolution of radiation-induced defects in NiCoFe MEA and pure Ni are investigated and compared using molecular dynamics simulation. It is observed that the defect recombination rate of ternary NiCoFe MEA is higher than that of pure Ni, which is mainly because, in the process of cascade collision, the energy dissipated through atom displacement decreases with increasing the chemical disorder. Consequently, the heat peak phase lasts longer, and the recombination time of the radiation defects (interstitial atoms and vacancies) is likewise longer, with fewer deleterious defects. Moreover, by studying the formation and evolution of dislocation loops in Ni–Co–Fe alloys and Ni, it is found that the stacking fault energy in Ni–Co–Fe decreases as the elemental composition increases, facilitating the formation of ideal stacking fault tetrahedron structures. Hence, these findings shed new light on studying the formation and evolution of radiation-induced defects in MEAs.