Perfect Dislocations Research Articles

Mechanical Load-Induced Atomic-Scale Deformation Evolution and Mechanism of SiC Polytypes Using Molecular Dynamics Simulation.

Silicon carbide (SiC) is a promising semiconductor material for making high-performance power electronics with higher withstand voltage and lower loss. The development of cost-effective machining technology for fabricating SiC wafers requires a complete understanding of the deformation and removal mechanism. In this study, molecular dynamics (MD) simulations were carried out to investigate the origins of the differences in elastic–plastic deformation characteristics of the SiC polytypes, including 3C-SiC, 4H-SiC and 6H-SiC, during nanoindentation. The atomic structures, pair correlation function and dislocation distribution during nanoindentation were extracted and analyzed. The main factors that cause elastic–plastic deformation have been revealed. The simulation results show that the deformation mechanisms of SiC polytypes are all dominated by amorphous phase transformation and dislocation behaviors. Most of the amorphous atoms recovered after completed unload. Dislocation analysis shows that the dislocations of 3C-SiC are mainly perfect dislocations during loading, while the perfect dislocations in 4H-SiC and 6H-SiC are relatively few. In addition, 4H-SiC also formed two types of stacking faults.

Phase field modeling of dislocations and obstacles in InSb

We present a phase-field dislocation dynamics (PFDD) model informed by first-principle calculations to elucidate the competitive dislocation nucleation and propagation between the glide and shuffle sets in InSb diamond cubic crystal. The calculations are directly informed with generalized stacking fault energy curves on the (111) slip plane for both the “glide set,” with the smaller interplanar spacing, and the “shuffle set,” with the larger interplanar spacing. The formulation also includes elastic anisotropy and the gradient term associated with the dislocation core. The PFDD calculations show that under no stress the equilibrium structure of screw glide set dislocations dissociates into Shockley partials, while those of the shuffle set dislocations do not dissociate, remaining compact. The calculated dislocation core widths of these InSb dislocations agree well with the measured values for other semiconductor materials, such as Si and GaN. We find that a shuffle set dislocation emits from a dislocation source at an applied stress about three times smaller than that needed to emit leading and trailing partials successively on the glide set plane. Once the partial dislocations in the glide set are emitted, they propagate faster than the shuffle set perfect dislocation at the same stress level.

Radiation-induced segregation on dislocation loops in austenitic Fe-Cr-Ni alloys

Radiation-induced segregation of alloying elements to crystallographic defects is commonly observed in irradiated austenitic stainless steels. The interaction between solutes and radiation-induced defects changes the physical distribution of solutes and thus affects the formation and growth of defects. The change of the microstructure consequently affects the mechanical properties of the material. A qualitative and quantitative understanding of the interaction between solutes and defects is desirable to better predict the service lifetime of nuclear materials. We used atom probe tomography to measure the distribution of solutes at dislocation loops in 304L stainless steel, irradiated with 2 MeV protons up to 1.5 displacements per atom at 373 and 633 K. No segregation at dislocation loops was found in samples irradiated at 373 K, whereas Ni and Si enrichment and Cr depletion were detected at dislocation loops irradiated at 633 K. The experimentally observed perfect and faulted dislocation loops in vacancy and interstitial types were reproduced by molecular dynamics (MD). A hybrid MD/Monte Carlo method was used to predict the redistribution of alloying atoms at all possible types of dislocation loops in face-centered cubic Fe-Cr-Ni alloys at the same irradiated temperatures (373 and 633 K). The simulations show that, at both temperatures, Cr clusters were formed and distributed randomly, and Ni atoms enriched or depleted at interstitial or vacancy dislocation loops, respectively. The change of solute concentration reaches the highest at the edge of the loop. Ni profiles exhibit characteristic behavior in terms of the stress field of the loops: tension inside of vacancy loops showing depletion of Ni atoms compared with compression inside of interstitial loops showing enrichment of Ni atoms. In addition, the stress field is reduced after solute redistribution. The absence of alloying segregation observed in experiments at a lower temperature (373 K) is explained by a rate theory model: Low-temperature irradiation requires significantly longer irradiation time to see the same amount of segregation as at high temperatures because of the extremely low diffusion of vacancies at low temperatures.

In-situ TEM observation of the ITB evolution in aluminum deformation twinning

Deformation twinning is another important phenomenon besides dislocation slip during the plastic deformation of metals and alloys. In this work, the plastic deformation behavior of coarse-grained aluminum with high stacking fault energy was studied by in-situ tensile TEM. The results show that deformation twins appear at the crack tip under continuous applied stress, and the thickness of deformation twin increases with the appearance of incoherent twin boundaries (ITBs). Further analysis shows that Frank partial dislocation in deformation twins is decomposed into perfect dislocation and Shockley partial dislocation under the action of applied stress. These perfect dislocation and Shockley partial dislocation can slip along the crystal planes and react to form the dislocation combinations that constitute the ITB. The results of this work provide a new way for researchers to understand the role of ITBs on deformation twins and the formation of dislocation combinations in ITBs.

Molecular dynamics simulation for plastic deformation mechanisms of single crystal diamond during nanoindentation

ABSTRACT The elastic-plastic deformation mechanism of single crystal diamond under spherical nanoindentation was emphasised using molecular dynamics (MD). The benchmark tests to determine the empirical potential that accurately describe the interactions between diamond atoms were completed. Tensile strength of diamond at different temperatures was studied under the empirical potential preferentially selected by benchmark tests. The nanoindentation process of the diamond is subsequently carried out on the surface (001). Plastic deformation behaviour of brittle diamond was found, such as amorphisation, phase transformation, graphitisation and dislocation evolution were analysed and discussed using combined methods. The simulation results show that the plastic deformation behaviour of the diamond material is also evident, even though diamond is a brittle material with high strength and hardness. ‘Pop-in’ event of the P-h curve is the turning point from elastic to plastic deformation. The elastic stage can be divided into the pure elastic and the quasi-elastic deformation stages. The plastic deformation behaviour of diamond is dominated by a combination of phase transformation, graphitisation and dislocation nucleation. It is also found that the primary dislocation type is 1/2<110> perfect dislocation, which was distributed in the area of large stress concentration below the indenter.

Micro-defects evolution of (Al0.33Cr0.21Fe0.28Zr0.18)O2 and (Al0.33Cr0.2Mn0.09Fe0.22Zr0.16)O2 induced by hydrogen ions irradiation

The micro-defects evolution of (Al 0 . 33 Cr 0 . 21 Fe 0 . 28 Zr 0.18 )O 2 and (Al 0 . 33 Cr 0 . 2 Mn 0 . 09 Fe 0 . 22 Zr 0.16 )O 2 thin films irradiated with low energy (60 eV) hydrogen ions (H + ) is investigated through Transmission Electron Microscopy (TEM) and the slow positron Doppler broadening spectroscopy (DBS) technique in this work. The results revealed that some perfect dislocation loops with Burgers vector b = a/2<1 1 0> were observed as the quaternary (Al 0 . 33 Cr 0 . 21 Fe 0 . 28 Zr 0.18 )O 2 irradiated with a fluence of 5.2 × 10 22 cm −2 . The micro defects are mainly vacancy (V), hydrogen vacancy complex (i.e. H–V complex), and vacancy dislocation loops. However, after the fluence increased to 1.2 × 10 23 cm −2 , the phase of (Al 0 . 33 Cr 0 . 21 Fe 0 . 28 Zr 0.18 )O 2 transformed from FCC structure to amorphous, suggesting that the weak ionic bonds between the cations with O. The defects mainly exist as large amounts of hydrogen bubbles, some H–V complex and few voids. As the addition of little Mn element, the phase of quinary (Al 0 . 33 Cr 0 . 2 Mn 0 . 09 Fe 0 . 22 Zr 0.16 )O 2 was unstable and easily produce precipitate under the irradiation temperature. And the micro-defects in (Al 0 . 33 Cr 0 . 2 Mn 0 . 09 Fe 0 . 22 Zr 0.16 )O 2 irradiated with 5.2 × 10 22 cm −2 mainly exist as V, H–V complex and some vacancy clusters. While the fluence increase to 1.2 × 10 23 cm −2 , the defects mainly exist as H–V complex and hydrogen bubbles. • The defects evolution of the two HEOs induced by H + irradiation behave differently. • The ionic bonds between the cations with O in (Al 0 . 33 Cr 0 .. 21 Fe 0 . 28 Zr 0.18 )O 2 are weak. • As the addition of Mn, the phase of (Al 0 . 33 Cr 0 . 2 Mn 0 . 09 Fe 0 . 22 Zr 0.16 )O 2 was unstable. • Hydrogen diffuse into both the surface and inner direction of films.

Effects of grain size and protrusion height on the surface integrity generation in the nanogrinding of 6H-SiC

The size and protrusion height of abrasive grains of a grinding wheel are not uniform, which can greatly influence the surface integrity generation of a ground component. The purpose of this paper is to explore such effects on the deformation and material removal mechanisms of 6H-SiC subjected to nanogrinding with the aid of molecular dynamics simulations. It was found that a nanogrinding process can fall into one or more of the following regimes with respect to material removal, i.e., no-wear, adhering, ploughing and cutting regimes, depending on both the depth of grain cut and size of abrasive grains. The plastic deformation in 6H-SiC starts with surface amorphization under the grinding stresses, followed by the activation of partial and perfect dislocations in the shuffle set of the material’s basal plane.

Fine core structure and spectral luminescence features of freshly introduced dislocations in Fe-doped GaN

Dislocations introduced by Vickers tip microindentation of an a-plane free-standing semi-insulating Fe-doped GaN halide vapor phase epitaxy (HVPE) crystal were investigated by means of cathodoluminescence and scanning transmission electron microscopy techniques. Detailed combined analyses of both spectral properties and the core structure of the introduced a-screw dislocations revealed that Fe-doped GaN exhibit not only dislocation-bound emission at ∼3.35 eV of perfect a-screw dislocations previously found in such kind of samples but also luminescent bands at 3.1–3.2 and 3.3 eV due to dissociated a-screw dislocations and extended dislocation nodes previously observed only in low-resistance n-GaN. For the first time, all these luminescent bands were observed together in the same sample. Structural studies revealed the coexistence of the dislocations with the dissociated and the perfect core as well as with extended dislocation nodes, thus establishing a correlation between previously observed luminescence bands and a fine dislocation core structure.

Influence of carbon and oxygen impurities on the migration rate of &lt;110&gt; tilt boundaries in austenite

The effect of impurity carbon and oxygen atoms on the migration rate of the tilt boundaries with the misorientation axis &lt;110&gt; in γ-Fe with fcc crystal lattice was studied by the method of molecular dynamics. Dependences of energy of the considered boundaries and rate of their migration at a temperature of 1600 K on the misorientation angle were obtained. The migration rate of &lt;110&gt; tilt boundaries under the same conditions turned out an order of magnitude lower than the migration rate of &lt;111&gt; and &lt;100&gt; boundaries, which is primarily due to the relatively low energy of &lt;110&gt; boundaries. In addition, the low-angle &lt;110&gt; tilt boundaries are unique compared to other tilt boundaries – grain­boundary dislocations in them are ordinary perfect edge dislocations with even cores that do not contain jogs periodically located on them as in &lt;111&gt; and &lt;100&gt; boundaries. The introduction of impurity carbon and oxygen atoms led to a significant decrease in migration rate of the grain boundaries. The binding energies of impurity carbon and oxygen atoms with grain-boundary dislocations in the austenite were calculated. The obtained values correlate well with the dependences of migration rate of &lt;110&gt; boundaries on the impurities concentration. Effect of impurities on migration of the boundaries in austenite turned out to be stronger than in the previously studied nickel and even more so in silver, which can be explained by the relatively low value of the electronegativity of iron atoms in comparison with nickel and silver. A higher value of the binding energy with dislocations in austenite and, accordingly, a greater effect on the migration rate of grain boundaries were obtained for carbon atoms.

Atomistic structure and thermal stability of dislocation loops, stacking fault tetrahedra, and voids in face-centered cubic Fe

Irradiation-induced hardening and changes to mechanical properties can often lead to material degradation and can limit the operation of reactor components. In addition, changes to mechanical properties can be a precursor to other material environmental degradation. Dislocation loops, stacking fault tetrahedra (SFT), and voids are major irradiation-induced lattice defects that occur in structural materials and are often observed post-irradiation. However, these defects are often considered only as obstacles to dislocation motion, with hardening assumed to be dependent on the habit plane and Burgers vector of the defect. The stability and nature of dislocation loops and SFTs with temperature are often overlooked, and this may play a crucial role in the mechanical properties of structural materials. We combine high-resolution transmission electron microscopy (HR-TEM) and atomistic simulations to determine the atomistic configurations and thermal stability of dislocation loops and SFTs. Dislocation dissociations are observed in both vacancy and interstitial-type perfect dislocation loops as well as in vacancy-type Frank loops, whereas interstitial-type Frank loops remain stable and are the least influenced by temperature. Triangle-shaped intrinsic stacking faults are formed at the edges of Frank vacancy loops, and a direct transformation from triangle-shaped Frank vacancy loops to SFTs is observed in atomistic simulations. SFTs are energetically more stable than Frank vacancy loops based on formation energy calculations in pure face-centered cubic Fe, which may explain why vacancy-type Frank loops are infrequently observed experimentally. The motivation for this study is to develop a framework for the stability and nature of irradiation defects as a function of temperature, which will be subsequently used for more complex (alloy) systems.

Transfer of dislocation slip through grain boundaries in metal-graphene nanocomposites

A model is suggested that describes the onset of plastic deformation in metal-graphene nanocomposites. Within the model, the plastic deformation of a deformed metal-graphene nanocomposite is realized through the generation of perfect or partial dislocation loops. These loops nucleate at grain boundaries (GBs) of the metallic matrix and expand across grains. The expanded dislocation loops promote the nucleation and expansion of secondary dislocation loops in adjacent grains, thereby realizing dislocation transmission across GBs. Within the model, we have calculated the critical stresses, τc1 and τc2, for the generation of a secondary perfect or partial dislocation loop and for the bypass of the graphene inclusion by the secondary loop. The critical stress τc2 increases with an increase in the graphene concentration and with a decrease in the thickness of the inclusion. This implies that the addition of graphene can raise the critical stress for the expansion of the secondary dislocation loops and thus increase the yield strength of metals.

A Scalable Network Model for Electrically Tunable Ferroelectric Domain Structure in Twistronic Bilayers of Two-Dimensional Semiconductors.

Moiré structures in small-angle-twisted bilayers of two-dimensional (2D) semiconductors with a broken-symmetry interface form arrays of ferroelectric (FE) domains with periodically alternating out-of-plane polarization. Here, we propose a network theory for the tunability of such FE domain structure by applying an electric field perpendicular to the 2D crystal. Using multiscale analysis, we derive a fully parametrized string-theory-like description of the domain wall network (DWN) and show that it undergoes a qualitative change, after the arcs of partial dislocation (PD) like domain walls merge (near the network nodes) into streaks of perfect screw dislocations (PSD), which happens at a threshold displacement field dependent on the DWN period.

Deformation behaviour of TWIP steels: Constitutive modelling informed by local and integral experimental methods used in concert

This article addresses the influence of stacking fault formation on the deformation and strain hardening behaviour of steels showing the twinning-induced plasticity effect. The role of the twin nucleation process was demonstrated on the results of mechanical testing, microstructure analyses, and constitutive modelling that were performed exemplarily for a high-alloy austenitic steel X2CrMnNi16-7-10. The microstructural evolution of the steel was studied in situ by acoustic emission measurements and post mortem by X-ray diffraction, and by scanning and transmission electron microscopy. The acoustic emission data provided information about the movement of partial and perfect dislocations and about the onset and extent of the deformation-induced twinning. The X-ray diffraction quantified lattice strains caused by the microstructure defects, and revealed averaged dislocation densities and stacking fault probabilities. The scanning electron microscopy delivered grain sizes and dislocation densities and unravelled the relationship between local grain orientation and the governing deformation mechanism. Transmission electron microscopy confirmed the validity of the microstructure model that was used for evaluation of the X-ray diffraction data, and visualized the interplay between individual deformation mechanisms. For the description of the mechanical response of the TWIP steel, a constitutive model was formulated, and supplied with the entirety of the experimental data. This approach helped to reduce the number of the free parameters of the constitutive model. Even with less free parameters, the model was able to describe the strain hardening of the steel reliably. It was found that, in contrast to dislocation motion, deformation twinning contributes to strain only marginally. The major impact of twinning on the enhanced plasticity arises from the reduction of the dislocation mean free path and the associated dislocation-related strain hardening. Based on the experimental data and model predictions, general guidelines for austenitic steel design were established. • Stacking fault mediated twinning studied in detail using complementary methods. • Quantification of the deformation mechanisms by acoustic emission measurement • The amount of twin shear itself contributes only slightly to the overall plasticity. • Modelling reveals: Increased strain hardening due to dislocation multiplication

Defect cluster configurations and mobilities in α-Zr: A comparison of the BMD19 and M07 interatomic potentials

Preferred defect cluster configurations and diffusion behavior are calculated with the recently published BMD19 interatomic potential for α-zirconium and compared with results calculated from the M07 potentials. Both potentials predict that small SIA clusters form configurations contained entirely within a single basal plane, while large SIA clusters form perfect dislocation loops on first-order prismatic planes with Burgers vector b→=13<112¯0>. The M07 potentials predict stable void formation for vacancy clusters while the BMD19 potential accurately indicates a preference for planar configurations on either first-order or second-order prismatic planes. Small basal vacancy clusters are found to form faulted pyramidal structures and only transform to faulted c-loops at larger sizes. In evaluating the defect diffusivity, single vacancies and interstitials are both found to exhibit anisotropic diffusion within the basal-plane. Small SIA clusters are found to migrate exclusively in 2-D within a single basal plane with the BMD19 potential but migrate in 3-D with the M07 #3 potential. Small vacancy clusters are either unstable or immobile with the M07 #3 and #2 potentials, while these clusters exhibit appreciable mobility either along the c-axis or in a quasi-isotropic manner with the BMD19 potential. The apparent difference in the diffusional anisotropies of defect clusters, rather than point defects, could be a critical component for improving predictive capabilities for mesoscale modeling of microstructural evolution in α-zirconium.

Effects of Cr/Ni ratio on physical properties of Cr-Mn-Fe-Co-Ni high-entropy alloys

Physical properties of ten single-phase FCC CrxMn20Fe20Co20Ni40-x high-entropy alloys (HEAs) were investigated for 0 ≤ x ≤ 26 at%. The lattice parameters of these alloys were nearly independent of composition while solidus temperatures increased linearly by ∼30 K as x increased from 0 to 26 at.%. For x ≥ 10 at.%, the alloys are not ferromagnetic between 100 and 673 K and the temperature dependencies of their coefficients of thermal expansion and elastic moduli are independent of composition. Magnetic transitions and associated magnetostriction were detected below ∼200 K and ∼440 K in Cr5Mn20Fe20Co20Ni35 and Mn20Fe20Co20Ni40, respectively. These composition and temperature dependencies could be qualitatively reproduced by ab initio simulations that took into account a ferrimagnetic ↔ paramagnetic transition. Transmission electron microscopy revealed that plastic deformation occurs initially by the glide of perfect dislocations dissociated into Shockley partials on {111} planes. From their separations, the stacking fault energy (SFE) was determined, which decreases linearly from 69 to 23 mJ·m−2 as x increases from 14 to 26 at.%. Ab initio simulations were performed to calculate stable and unstable SFEs and estimate the partial separation distances using the Peierls-Nabarro model. While the compositional trends were reasonably well reproduced, the calculated intrinsic SFEs were systematically lower than the experimental ones. Our ab initio simulations show that, individually, atomic relaxations, finite temperatures, and magnetism strongly increase the intrinsic SFE. If these factors can be simultaneously included in future computations, calculated SFEs will likely better match experimentally determined SFEs.

Enhancing the strength-ductility trade-off in a NiCoCr-based medium-entropy alloy with the synergetic effect of ultra fine precipitates, stacking faults, dislocation locks and twins

In this work, multi strengthening/deformation mechanisms were introduced into a Ni-rich Ni40Co32Cr20Al3.5Ti3Nb1.5 (at%) medium-entropy alloy. After thermo-mechanical procedures, the microstructure is characterized by ultra fine γ' particles (2–18 nm) and short stacking faults. The alloy has superior mechanical properties with gigapascal yield strength, excellent ductility and strong work hardening ability. The initial high strength is caused by the blocking effect of ultra fine γ' particles, dislocation locks (Lomer-Cottrell locks), solute atoms and grain boundaries to dislocation movement. TEM observations at different strains indicate that the increasing number of stacking faults, Lomer-Cottrell locks and deformation twins are the main reasons for the improvement of work hardening ability during deformation. Twinning and dislocation slip, including perfect dislocations and partial dislocations, are the main deformation mechanisms. Our findings show that the coordination of multi strength/toughing mechanisms may be an effect way to enhance the strength-ductility trade-off.

Graphene Oxide-Induced Substantial Strengthening of High-Entropy Alloy Revealed by Micropillar Compression and Molecular Dynamics Simulation.

Plastic deformation mechanisms at micro/nanoscale of graphene oxide-reinforced high-entropy alloy composites (HEA/GO) remain unclear. In this study, small-scale mechanical behaviors were evaluated for HEA/GO composites with 0.0 wt.%, 0.3 wt.%, 0.6 wt.%, and 1.0 wt.% GO, consisting of compression testing on micropillar and molecular dynamics (MD) simulations on nanopillars. The experimental results uncovered that the composites exhibited a higher yield strength and flow stress compared with pure HEA micropillar, resulting from the GO reinforcement and grain refinement strengthening. This was also confirmed by the MD simulations of pure HEA and HEA/GO composite nanopillars. The immobile <100> interstitial dislocations also participated in the plastic deformation of composites, in contrast to pure HEA counterpart where only mobile 1/2 <111> perfect dislocations dominated deformation, leading to a higher yield strength for composite. Meanwhile, the MD simulations also revealed that the flow stress of composite nanopillar was significantly improved due to GO sheet effectively impeded dislocation movement. Furthermore, the mechanical properties of HEA/1.0 wt.% GO composite showed a slight reduction compared with HEA/0.6 wt.% GO composite. This correlated with the compositional segregation of Cr carbide and aggregation of GO sheets, indicative of lower work hardening rate in stress-strain curves of micropillar compression.

The Influence of Grain Boundaries on Crystal Structure and Tensile Mechanical Properties of Al0.1CoCrFeNi High-Entropy Alloys Studied by Molecular Dynamics Method

The mechanical properties of high-entropy alloys are superior to those of traditional alloys. However, the key problem of finding a strengthening mechanism is still challenging. In this work, the molecular dynamics method is used to calculate the tensile properties of face-centered cubic Al0.1CoCrFeNi high-entropy alloys containing Σ3 grain boundaries and without grain boundary. The atomic model was established by the melting rapid cooling method, then stretched by the static drawing method. The common neighbor analysis and dislocation extraction algorithm are used to analyze the crystal evolution mechanism of Σ3 grain boundaries to improve the material properties of high-entropy alloys during the tensile test. The results show that compared with the mechanical properties Al0.1CoCrFeNi high-entropy alloys without grain boundary, the yield strength and Young’s modulus of a high-entropy alloy containing Σ3 grain boundary are obviously larger than that of high-entropy alloys without grain boundary. Dislocation type includes mainly 1/6&lt;112&gt; Shockley partial dislocations, a small account of 1/6&lt;110&gt; Stair-rod, 1/2&lt;110&gt;perfect dislocation, and 1/3&lt;111&gt; Hirth dislocations. The mechanical properties of high-entropy alloys are improved by dislocation entanglement and accumulation near the grain boundary.

Role of stabilization heat treatment inducing γ′-γ″ co-precipitates and grain boundary η phase on tensile and creep behaviors of Inconel 706

The effects of the stabilization conditions on the microstructure evolution in Ni-Fe-based superalloy Inconel 706 were investigated. In addition, the relationship between the microstructure and tensile and creep properties of the alloy was investigated. Stabilization at 800–840 °C resulted in the precipitation of the η phase at the grain boundaries. With an increase in the stabilization temperature, the η phase fraction increased. Non-compact γ′-γ″ co-precipitates were also formed at 800–840 °C, and their size was the largest when the stabilization was carried out at 840 °C. After double aging, the sample stabilized at 840 °C showed a cuboidal γ′ phase covered entirely with γ″ films, i.e. compact γ′-γ″ co-precipitates were observed along with a coarsened lamellae η phase. The double-aged sample stabilized at 800 °C (S80) exhibited the highest yield strength because of the presence of fine γ′ and γ″ precipitates. The double-aged samples stabilized at 810 and 840 °C (S81 and S84, respectively), exhibited similar yield strengths. Isolated stacking faults were generated in the γ′ particles of the S81 and S84 samples during the tensile deformation, indicating the high mobility of perfect dislocations in the γ-matrix. After creep at 650 °C/650 MPa, S81 and S84 exhibited higher creep strain at a given time than S80. This higher creep strain can be attributed to the synergistic effect of the increased dislocation mobility and η phase fraction, which resulted in strain accumulation at the grain boundaries. The anti-phase boundary shearing observed only in the S84 sample contributed to the strengthening of its γ-matrix and increased its creep rupture life.

Dislocation Loops in Proton Irradiated Uranium-Nitrogen-Oxygen System

In this study, we investigated the type of dislocation loops formed in the proton-irradiated uranium-nitrogen-oxygen (U-N-O) system, which involves uranium mononitride (UN), uranium sesquinitride (α-U 2 N 3 ), and uranium dioxide (UO 2 ) phases. The dislocation loop formation is examined using specimens irradiated at 400°C and 710°C. Based on the detailed transmission-based electron microscopy characterization with i) the morphology-based on-zone and ii) the invisibility-criterion based two-beam condition imaging techniques, only a single type of dislocation loop in each phase is found: a/2⟨110⟩, a/2⟨111⟩, or a/3⟨111⟩ dislocation loops in UN, α-U 2 N 3 , and UO 2 phases, respectively. Molecular statics calculations for the formation energy of perfect and faulted dislocation loops in the UN phase indicate a critical loop size of ∼ 6 nm, above which perfect loops are thermodynamically favorable. This could explain the absence of faulted loops in the experimental observation of the irradiated UN phase at two temperatures. This work will enhance the understanding of irradiation induced microstructural evolution for uranium mononitride as an advanced nuclear fuel for the next-generation nuclear reactors.