HCl is a textbook example of a polar covalent molecule, and has a wide range of industrial applications. Inspired by the discovery of unexpected stable sodium and potassium chlorides, we performed systematic ab initio evolutionary searches for all stable compounds in the H–Cl system at pressures up to 400 GPa. Besides HCl, four new stoichiometries (H2Cl, H3Cl, H5Cl and H4Cl7) are found to be stable under pressure. Our predictions substantially differ from previous theoretical studies. We evidence a high significance of zero-point energy in determining phase stability. The newly discovered compounds display a rich variety of chemical bonding characteristics. At ambient pressure, H2, Cl2 and HCl molecular crystals are formed by weak intermolecular van der Waals interactions, and adjacent HCl molecules connect with each other to form asymmetric zigzag chains, which become symmetric under high pressure. In H5Cl, triangular H3+ cations are stabilized by electrostatic interactions with the anionic chloride network. Further increase of pressure drives H2 dimers to combine with H3+ cations to form H5+ units. We also found chlorine-based Kagomé layers which are intercalated with zigzag HCl chains in H4Cl7. These findings could help to understand how varied bonding features can co-exist and evolve in one compound under extreme conditions.

Highlights•Evolutionary algorithm USPEX was performed to find potentially stable vanadium carbides.•The Pnnm-V2C is a newly discovered structure with lower energy than previously reported Pbcn-V2C.•Vanadium rearrangement results in the strengthening of mechanical strength.VC1−x compounds, known as ultra-high temperature ceramics, are able to maintain stability in a range of chemical compositions. Herein, using evolutionary algorithm USPEX, we performed a global search for potentially stable VC1−x compounds. We have discovered two stable compounds (P3112-V6C5 and Pnnm-V2C) and a number of near-ground-state compounds (Fm3¯m-VC, P4332-V8C7, P1¯-V5C4, I4¯3m-V4C3, Cmcm-V3C2, and Cmcm-V3C). The stable Pnnm-V2C is a new structure with lower total energy than experimentally reported α-V2C. Based on these stable and meta-stable VC1−x compounds, we have systemically investigated their structures, mechanical properties, and chemical bonding. We found that Cmcm-V3C2 and Cmcm-V3C display different arrangements of vanadium from other VC1−x compounds. We suggest that such rearrangement of vanadium are able to enhance mechanical strength. Most of VC1−x compounds possess very good mechanical properties, indicating that they have potential to be utilized for structural applications.

•MC grain growth simulation and FEM stress analyses are applied for polycrystals.•The grains and their boundaries affect the heterogeneous stress distribution.•The average stress and the grain size agree well with the Hall–Petch relationship.A two-dimensional numerical model of microstructural effects is presented, with an aim to understand the mechanical performance in polycrystalline materials. The microstructural calculations are firstly carried out on a square lattice by means of a 2-D Monte Carlo (MC) simulation for grain growth, then the conventional finite element method is applied to perform stress analysis of a plane strain problem. The mean grain size and the average stress are calculated during the MC evolution. The simulation result shows that the mean grain size increases with the simulation time, which is about 3.2 at 100 Monte Carlo step (MCS), and about 13.5 at 5000 MCS. The stress distributions are heterogeneous in materials because of the existence of grains. The mechanical property of grain boundary significantly affects the average stress. As the grains grow, the average stress without grain boundary effect slightly decreases as the simulation time, while the one with strengthening effect significantly decreases, and the one with weakening effect increases. The average stress and the grain size agree well with the Hall–Petch relationship.

In the present work, a theoretical model of three-dimensional transient temperature field for C/SiC composite brake discs was established by adopting a finite element method according to the theory of energy transformation and transportation. The variation regularities of transient temperature field and internal temperature gradient of the brake discs were obtained. The effects of initial velocity, deceleration and friction coefficient on the highest temperature of the brake discs were also discussed. The heat energy was mainly concentrated on the layer of friction surfaces. The highest temperature of the brake discs under normal landing, overload landing, and rejected take-off landing condition were 869.7 K, 1037.4 K and 1440.3 K, respectively. Furthermore, the highest temperature of the brake discs increased with the increase of the initial velocity and friction coefficient, but decreased with the increase of deceleration. Comparing simulation predictions with experimental results, it is found that the three-dimensional transient temperature field model was valid and reasonable.Research highlights► A friction model for C/SiC composite aircraft brake disc was firstly established. ► The temperature of the friction surface is higher than that inside of the discs. ► The temperature at rejected take-off landing is extremely as high as 1440.3 K. ► The higher the initial velocity and friction coefficient, the higher the temperature.

Adaptive neural fuzzy inference system (ANFIS) combined with GAs is applied to develop an expert system for designing in situ toughened Si3N4 in this paper. ANFIS combines fuzzy linguistic descriptions with accurate experimental data. GAs have been successfully applied in searching and optimizing very large and varied data spaces. ANFIS models were developed in this paper to predict mechanical properties based on sintering processing or microstructure. And GAs models are developed to predict sintering processing based on mechanical properties or microstructure of this material. A software package was developed to realize the applications mentioned above easily and quickly.

HCl is a textbook example of a polar covalent molecule, and has a wide range of industrial applications. Inspired by the discovery of unexpected stable sodium and potassium chlorides, we performed systematic ab initio evolutionary searches for all stable compounds in the H–Cl system at pressures up to 400 GPa. Besides HCl, four new stoichiometries (H2Cl, H3Cl, H5Cl and H4Cl7) are found to be stable under pressure. Our predictions substantially differ from previous theoretical studies. We evidence a high significance of zero-point energy in determining phase stability. The newly discovered compounds display a rich variety of chemical bonding characteristics. At ambient pressure, H2, Cl2 and HCl molecular crystals are formed by weak intermolecular van der Waals interactions, and adjacent HCl molecules connect with each other to form asymmetric zigzag chains, which become symmetric under high pressure. In H5Cl, triangular H3+ cations are stabilized by electrostatic interactions with the anionic chloride network. Further increase of pressure drives H2 dimers to combine with H3+ cations to form H5+ units. We also found chlorine-based Kagomé layers which are intercalated with zigzag HCl chains in H4Cl7. These findings could help to understand how varied bonding features can co-exist and evolve in one compound under extreme conditions.