Table of contents

A two-dimensional model was developed according to infiltration-induced structural changes in C/SiC composites and physicochemical phenomena involved in isothermal chemical vapour infiltration (ICVI) process. The mathematical model was implemented to simulate the densification behaviour of the C/SiC component of a small-scale thruster liner for rocket engine. The calculated results show that infiltration efficiency is high at first and then decreases dramatically, which is in agreement with the corresponding experimental results. The correspondence between calculated results and experimental data implied that this mathematical model is reasonable and feasible for characterizing the densification behaviour of C/SiC composites in the ICVI process. The dependence of densification behaviour of C/SiC composites on infiltration temperature has also been investigated. Calculation results show that the densification rate increases while density uniformity of overall composites decreases evidently with elevated temperature.

A new approach based on an artificial neural networks (ANN) model was used to model the ageing behaviour of an Al–Mg–Si alloy. A systematic combination of hardness measurements, transmission electron microscopy (TEM), image analysis and the ANN method was used to correlate the key precipitate parameters with the age-hardening response. The ageing behaviour of AA6022 during isothermal heating was characterized by hardness measurements and the structural evolution was studied by TEM. To distinguish the precipitate morphology at each stage of ageing, an image analysis algorithm capable of capturing orientation gradient, nearest neighbour distances, number density, shapes and size of precipitates was developed. A parametric study was performed to identify the significance of each precipitate parameter, and then the most important parameters were used to train the ANN model. The model combines the most important precipitate parameters including volume fraction, shape, size and distance between precipitates. It was found that the model is able to successfully predict the age hardening behaviour of AA6022 in both deformed and undeformed conditions.

The purpose of this study is the assessment of the conductance of deposited atomic chains. The structures considered are linear chains of covalent and metallic atoms, i.e. As and Ag, deposited onto monolayer steps of the Si(100) surface. The study is based on the extended Debey–Hückel theory, used for the evaluation of the electronic structure and conductance, and the calculations analyse the relationship between the structural parameters of steps, their binding and adsorption energies and the conductance. This evaluation shows that the binding strength of the deposited chains is perceptibly enhanced above that of the free-standing chains and the cohesion of the steps is not lowered by the deposited structures. While the length of the chain has no effect on the total electronic structure, its composition and the adsorption sites are important and a functional relationship between these quantities and the adsorption energy is clearly observed. The central result of the calculations is that the conductance of the entire step and of the deposited chain have a dependence on these structural parameters similar to those of the binding and adsorption energies and their behaviour is parallel to that of the characteristic energies.

Compressive mechanical behaviours of closed cell Al-alloy foams produced by melt based and powder metallurgical methods were investigated by a finite element model composed of multiple unit lattices. The unit lattice consists of spherical and cubic sections possessing a thickness ratio between them. A Gaussian distribution of the relative density among the lattices and the random allocation of lattices were implemented in the model to address the structural heterogeneity. The constitutive relation for the lattice material was determined by a nondestructive instrumented sharp indentation test on the cell wall. The simulated compressive stress–strain curves for the ductile Al–Si–Ca foam were found to be in good agreement with the experimental ones over the entire strain range while the resistance of brittle Al–Si–Cu–Mg foam to the deformation turned out to be lower than that predicted by the model after about 0.50 strain. The discrepancy between experimental and simulation results for Al–Si–Cu–Mg foam in high strain range was associated with the disintegration of cell walls broken by the large deformation.

We have investigated the melting behaviours of disordered and symmetric gold clusters (Au_N, N = 54–56) by means of molecular dynamics simulations. We have found that there is no single isolated lowest energy structure for this size of Au clusters. Instead there are many nearly degenerate disordered low lying structures. The melting behaviours of these disordered structures showed that the melting occurs as a gradual process in which initially, behaviours of the surface and the inner atoms are quite different from each other, and they do not mix until the beginning of the melting. On the other hand, the symmetric forms of the Au_N (N = 54–56) present different melting behaviours from those of the corresponding disordered structures. Their melting occurs suddenly, i.e. over a very short temperature interval. During the heating of these symmetric forms no phase changes occur until the melting temperature at which melting occurs as a collective motion of all the atoms in the cluster. On the other hand, the phase changes in the disordered structures take place as a result of both collective motions of all the atoms in the cluster, and as local displacements of the atoms.

A phase-field approach is used to describe electro-deoxidation, i.e. direct electrochemical reduction of solid oxides in molten salts. Based on this description, temporal evolutions of chemical and electric potential fields are simulated numerically in one and two dimensions of a hypothetical system. The results of the simulations illustrate three distinct stages of electro-deoxidation, including a relatively rapid initial stage in which the reduction process propagates at the surface of cathode. This initial stage of 'surface reduction', if present, can influence the process kinetics substantially. The existence of the initial stage is shown to depend most significantly on a specific topological property of the modelled cell, namely on the connectivity of the internal boundaries between pairs of model entities, i.e. cathode, current collector and electrolyte. The preliminary results also suggest the overall kinetics of electro-deoxidation, within the examined range of parameters, to be diffusion controlled and thus independent of the initial value of the electric conductivity of oxide.

The methods of numeric modelling within the framework of molecular dynamics were used for calculating the interactions of nanostructural elements. A method offered is based on the pairwise static interaction potential of nanoelements that is built up with the help of the approximation of the numerical calculation results using the method of molecular dynamics. Based on the potential of the pairwise interaction of the nanostructure elements, which takes into account forces and moments of forces, the method for calculating the nano-elements interaction, the ordering and self-organizing processes has been developed. The dependence of the ultimate stress limit of a monodisperse powder nanocomposite on the diameter of the constituent nanoparticles has been calculated. The investigation results of the self-organization of the system consisting of two or more particles are presented and the analysis of the equilibrium stability of various types of nanostructures has been carried out.

In this paper, a superposition method is proposed to compute the stress field due to a dislocation loop in a heterogeneous thin film–substrate system. The problem is decomposed into two sub-problems: the stresses due to a dislocation loop in an infinitely extended bi-material space and the stresses induced by equivalent tractions on the free surface. The area-integral expression of the former is further reformulated in a line-integral form. Taking advantage of the Galerkin potential method and the Fourier transform, this paper then derives the elastic field in a heterogeneous thin film–substrate system due to surface loading. Numerical results demonstrate that both the free surface and the material heterogeneity have great influence on the stress field due to a dislocation loop.

The present paper examines the interface fracture of a bicrystal niobium/alumina specimen using a cohesive modelling approach. Crystal plasticity theory has been used to model the single crystalline niobium. The effect of different cohesive law parameters, such as cohesive strength and work of adhesion, has been studied. The cohesive strength is found to have a profound effect on the crack growth resistance and fracture energies as compared with the work of adhesion. The cohesive model parameters are identified by validating the finite element analyses results with experiments. Theoretical interlink between the local adhesion capacity and macroscopic fracture energies has been analysed. The results presented in this work provide an insight into the role of cohesive strength and the work of adhesion in macroscopic fracture is also presented which can be used by experimentalists to design better bimaterials by varying the cohesive strength and the work of adhesion.

The ideal strength of a nano-component, which is the maximum stress of the structure, provides an insight into minute material. We conducted tensile simulations for a cylindrical-shaped Cu nano-wire composed of an atomic chain as a core wrapped around by shell(s) with the structure of (111) layers in a fcc crystal. Young's moduli and the ideal strengths of the wires are less than a single atomic chain and sheet. The mechanical strength of the wire is weakened by the following three factors: (a) changes in electron arrangement caused by combining core and shell; (b) a larger interatomic distance (inherent tensile strain) of the outer shell introduced for combining with the core chain; and (c) weak bonding (gap) on the outer shell caused by the mismatch of atomic layers due to the curvature difference. Factor (a) weakens the bonding of the shell(s) that occupy a greater part of the wire. A 5-1 wire, which consists of a core and a shell, is weakened compared with the single atomic chain and the single sheet due to (a) and (b). A 10-5-1 wire, consisting of a core and two shells, has less strength than a 5-1 wire due to (c) in addition to (a) and (b).

We have performed the first-principles simulations within the local density and pseudopotential approximations to investigate the effects of pressure on the energetics and atomic and electronic structural behaviour of vacancy defects in MgSiO₃ perovskite. The simulations use a supercell containing 160 atoms whose positions are fully optimized for each defective system. Schottky formation energy is shown to increase by a factor of 2.5 over the pressure regime (0 to 150 GPa) studied. The calculated three-dimensional datasets for atomic configuration and electron charge density are visualized in detail. It is shown that these point defects induce substantial distortions in the surrounding atomic and electronic structures, and these distortions remain nearly unchanged with pressure.

An artificial neural network (ANN) model is developed to predict the constitutive flow behaviour of austenitic stainless steels during hot deformation. The input parameters are alloy composition and process variables whereas flow stress is the output. The model is based on a three-layer feed-forward ANN with a back-propagation learning algorithm. The neural network is trained with an in-house database obtained from hot compression tests on various grades of austenitic stainless steels. The performance of the model is evaluated using a wide variety of statistical indices. Good agreement between experimental and predicted data is obtained. The correlation between individual alloying elements and high temperature flow behaviour is investigated by employing the ANN model. The results are found to be consistent with the physical phenomena. The model can be used as a guideline for new alloy development.

Adhesion is a complex and multifaceted phenomenon which is controlled by various factors such as the loading rate, interface toughness, temperature and geometric and molecular properties. The mode of failure of adhesive joints (adhesive or cohesive) is decided through a complex interplay between these factors. In this work, we study the failure under tensile loading of a thin layer of a polymeric material confined between two rigid walls using molecular dynamics simulations. The strength of the interface is controlled by the interaction potential between the polymer and wall atoms. The polymer modelled is a simple linear chain of 'united atoms' having a fixed bond length but contributions to the energy arise from bending and torsion of bonds as well as from non-bonded interactions between the 'united atoms'. The results indicate that even when the adhesion between the wall and the polymer is weak, a short chained polymer is more likely to fail by a mixed adhesive cohesive mode. A long chained polymer, with the same interface strength, fails in a pure adhesive manner. However, when the interface is sufficiently strengthened, the long chained polymer fails cohesively and it can bear a much higher load. The failure mode is somewhat modulated by the rate at which deformation occurs. Moreover, when the polymer is confined such that the spacing between the walls is comparable to the end-to-end distance of the polymer chain, strength of the joint increases significantly. In such a situation, even polymers with weak interfacial adhesion might fail cohesively.

The distributions of the interfacial energy within the composition triangle of the Zr–Ni–Al and Zr–Ni–Al–Cu systems are obtained with the non-structural approach by investigating the effect of enthalpy, entropy and the compositions of liquid and crystal in this paper. The calculation results show that the interfacial energy is highest near the eutectic lines in the 66.6% at.% Zr isopleth of the Zr–Ni–Al–Cu system. There are two regions with higher interfacial energy in the Zr–Ni–Al system: one is in the range x_Zr = 0.40–0.68, x_Ni = 0.21–0.36, the other is in x_Zr = 0.08–0.12, x_Ni = 0.31–0.40. The obtained higher interfacial energy regions are in good agreement with the composition ranges with higher glass-forming ability.