[en] The generation and motion of crystalline defects during plastic deformation are critical processes that determine the mechanical properties of a crystal. The types of defect generated are not only related to the symmetry of a crystal but also associated with the symmetry-breaking process during deformation. Proposed here is a new mathematical framework to capture the intrinsic coupling between crystal symmetry and deformation-induced symmetry breaking. Using a combination of group theory and graph theory, a general approach is demonstrated for the systematic determination of the types of crystalline defect induced by plastic deformation, through the construction of a crystal deformation group and a deformation pathway graph. The types of defect generated in the deformation of a face-centered cubic crystal are analyzed through the deformation pathway graph and compared with experimental observations.

[en] Ordering and self-organization are critical in determining the dynamics of reaction-diffusion systems. Here we show a unique pattern formation mechanism, dictated by the coupling of thermodynamic instability and kinetic anisotropy. Intrinsically different from the physical origin of Turing instability and patterning, the ordered patterns we obtained are caused by the interplay of the instability from uphill diffusion, the symmetry breaking from anisotropic diffusion, and the reactions. To understand the formation of the void/gas bubble superlattices in crystals under irradiation, we establish a general theoretical framework to predict the symmetry selection of superlattice structures associated with anisotropic diffusion. Through analytical study and phase field simulations, we found that the symmetry of a superlattice is determined by the coupling of diffusion anisotropy and the reaction rate, which indicates a new type of bifurcation phenomenon. Our discovery suggests a means for designing target experiments to tailor different microstructural patterns.

[en] The clustering properties of near Zwicky clusters are studied by using the two-point angular correlation function. The angular correlation functions for compact and medium compact clusters, for open clusters, and for all near Zwicky clusters are estimated. The results show much stronger clustering for compact and medium compact clusters than for open clusters, and that open clusters have nearly the same clustering strength as galaxies. A detailed study of the compactness-dependence of correlation function strength is worth investigating. (author)

[en] A monolithic fuel design based on a U-Mo alloy has been selected as the fuel type for conversion of the United States High-Performance Research Reactors (HPRRs). A 2015 post-irradiation examination (PIE) report showed accelerated swelling in U-10Mo fuels at fission densities much lower than previously observed. This PIE report showed a large amount of compositional banding, or regions of low Mo content adjacent to regions of high Mo content, with low Mo content typically along grain boundaries. Lower Mo content can lead to phase decomposition from the gamma U-Mo body-centered cubic phase to the alpha U phase as well as an earlier onset of recrystallization. Thus, the phenomenon of Mo depletion at grain boundaries is an important factor in the accelerated swelling behavior of U-Mo fuel. However, the physical origin of Mo depletion at grain boundaries is still unclear. In this work, molecular dynamics simulations have been performed to calculate the grain boundary and surface energies of body-centered cubic (bcc) U, bcc Mo and alloys of U-Mo from 600 K to 1200 K. It is observed that the lower grain boundary energy of bcc U, compared to bcc Mo, provides the driving force for Mo depletion at grain boundaries. This driving force diminishes with increasing temperature, but is not eliminated. This information can be utilized as inputs to higher length scale modeling methodologies and provide specification guidance to fabricators.

[en] Precipitate microstructure in the B2 parent phase is known to have profound impacts on the properties of NiTi-based high temperature shape memory alloys (HTSMAs), including the martensitic transformation (MT) start temperature $M_s$, temperature- and stress-hysteresis, work output, dimensional stability and functional fatigue resistance. In order to understand the underlying mechanisms and hence to optimize aging heat treatments to achieve desired properties, we systematically investigate both the mechanical and chemical effects associated with nanoscale coherent precipitates on the behavior of MT. Using NiTi-Hf HTSMA as an example, we first study the equilibrium shape and stress and strain fields of an H-phase precipitate as a function of its size. We then determine quantitatively the elastic interaction energy between a precipitate and a nucleating martensitic particle consisting of either a single variant or multiple self-accommodating variants. In the meantime, we calculate the variation of concentration field around an H-phase precipitate during its growth. Finally, we quantify and compare the effects of the spatially inhomogeneous stress and concentration fields around an H-phase precipitate on $M_s$. The results indicate that the former is the dominant factor for long aging times while latter is the dominant factor for short aging times. Since the model predicts $M_s$ as a function of aging temperature and time, it can aid the design of aging heat treatment schedule to achieve desired $M_s$.

[en] Wrought Mg–Zn–Zr alloys with a strong basal texture usually exhibit highly anisotropic plastic behavior, resulting in premature fracture during deformation at room temperature. In the present study, we prepared a Mg–1Zn–0.2Zr (ZK10) alloy sheet by multi-pass hot rolling and annealing, tailoring the basal texture and reducing the plastic anisotropy with the addition of dilute Ca. Microstructural characterizations of the tensile-deformed alloys show that pyramidal c+a> slip and $\left\{10\overline{1}2\right\}$ twinning are more easily formed in the Ca-containing alloy compared with those in the ZK10 alloy. The weakened basal texture involving Ca addition leads to promoted activation of $\left\{10\overline{1}2\right\}$ twinning, without significant impact on the Schmid factor distribution of pyramidal c+a> slip. Simulation results of the visco-plasticity self-consistent model further indicate that Ca reduces the ratio of critical resolved shear stress for pyramidal c+a> slip and $\left\{10\overline{1}2\right\}$ twinning versus basal a> slip, leading to a significant improvement in ductility. This work may provide a new strategy to develop wrought Mg alloys with enhanced ductility.

[en] Highlights: • Point defect diffusion effectively tuned by U-Mo composition • Strongly correlated defect migration, mediated by minor atoms • Interstitial 3D migration via the major atoms, preferring $〈110〉$ configuration • Highly mobile vacancies, and interstitial anti-clustering in U-rich systems Defect dynamics constitutes the foundation for describing microstructural evolution in any material systems for nuclear applications, including body-centered cubic $\gamma$-U, Mo, and their alloys. However, defect properties and evolution, and the impact of a large atomic size mismatch between U and Mo atoms on defect dynamics have not been elucidated. In this work, we use molecular dynamics to extensively examine composition-dependent defect behavior in U-Mo alloys and the pure metals. It has been found that point defect migration is strongly correlated and mediated by minor atoms via preferential paths in alloys. Interstitial dumbbells migrate three-dimensionally through the major atoms with a preferred $〈110〉$ configuration. Vacancies are less mobile than interstitials, but become comparable (one order of magnitude difference in diffusivity) in U-rich systems. Overall, compared with the pure metals, defect diffusivity can be tuned up or down based on the alloy composition. Finally, interstitial clustering is found to be unfavorable in U-rich systems, as opposed to Mo which exhibits an efficient formation of interstitial-type dislocation loop with 1D diffusion mode. These findings not only provide necessary input to high-fidelity meso-scale simulations of microstructural evolution in these systems, but also have important implications towards explaining radiation effects influenced by the dimensionality and rates of defect diffusion.

[en] Although the temperature window of helium ion irradiation for gas bubble superlattice (GBS) formation was found to be in the range of approximately 0.15–0.35 melting point in literature, the thermal stability of He GBS has not been fully investigated. This work reports the experiment using an in-situ heating holder in a transmission electron microscope (TEM). A 3.0 mm TEM disc sample of Mo (99.95% pure) was irradiated with 40 keV He ions at 300 °C to a fluence of 1.0E+17 ions/cm², corresponding to a peak He concentration of approximately 10 at.%, in order to introduce He GBS. In-situ heating was conducted with a ramp rate of ∼25 °C/min, hold time of ∼30 min, and temperature step of ∼100 °C up to 850 °C (0.39T_m homologous temperature). The result shows good thermal stability of He GBS in Mo with no noticeable change on GBS lattice constant and ordering. The implication of this unique and stable ordered microstructure on mechanistic understanding of GBS and its advanced application are discussed.

[en] The present paper focuses on introducing the sorts, stages and grades of nuclear accidents, especially analyzing the radiation protection measures that should be taken at different phases of nuclear facility emergency. The difficulties, which might be encountered in the implementation of radiation protection measures, the risk existing and the cost to be paid were discussed. Some methods for quick and rational selection of effective protection measures were provided, which could be applied as optimization references for knowing the hurt of nuclear accident, the rule of development and emergency rescues actions. (authors)

[en] Self-organized patterns, realized in non-equilibrium processes, have been widely observed in physics and chemistry. As a powerful tool to create far-from-equilibrium environments, irradiation produces a variety of types of defects, which can self-organize through physical interactions and chemical reactions. Such a process becomes complicated especially when both thermodynamics and kinetics play critical roles in pattern formation. Here, we explore the formation and self-organization mechanism of void superlattices in metals and alloys under irradiation through phase field modeling and simulations. For the first time, three different formation mechanisms of void superlattices are clearly distinguished according to their thermodynamic origin and reaction kinetics. It is discovered that the characteristic length and symmetry of an emerging superlattice is determined by the interplay of the thermodynamic driving force and the kinetic anisotropy of the system. Through parametric study, the effects of kinetic coefficients, such as atomic mobility and irradiation dose rate, on the nucleation, growth, coarsening, coalescence, and ordering of voids are systematically investigated. The theoretical model developed in this work may provide guidelines for designing desired self-organized microstructures under irradiation.