Table of contents

The convection and macrosegregation predicted by a simulation of a directionally solidified binary alloy (Pb - 23.2 wt%Sn) are presented, and the simulated macrosegregation is compared with the macrosegregation in an experimental casting. The casting was solidified at a rate of and a thermal gradient of approximately ; such thermal gradients are much greater than those previously simulated. The calculated results showed channels at the vertical casting surfaces and segregated internal pockets in the mushy zone, in agreement with the observation of freckles in the experimental casting.

A three-dimensional grain model, in which the grains are represented by regular truncated octahedra, has been developed to study probabilistic time-dependent intergranular failure in polycrystalline arrays. In this model, grain boundary facets are assumed to fail randomly in time, as a function of the facet normal stress. A simple approximate method for calculating the load shed by failed facets and a reasonable choice of failure criterion complete the model. This leads to a conceptually simple, but computationally complex, model capable of handling assemblages consisting of relatively large numbers (> 5000) of grains. The predicted scatter in the times-to-failure and the variation in number of failed facets with time are in quite reasonable agreement with available experimental data.

Hydride precipitation and dissolution in a zirconium matrix have been simulated numerically using a finite element technique. The evolution of the accommodation energy associated with such phase transformations has been evaluated for isotropic and anisotropic misfitting hydrides in an infinite stress-free body. It was found that there is an optimal hydride shape for which the accommodation energy required for hydride precipitation or dissolution is a minimum. It was also found that the accommodation energy gap (i.e. the free energy gap) that exists between hydride precipitation and dissolution is smaller than the value previously calculated.

We have developed an empirical EAM potential for magnesium by fitting to ab initio forces (the `force matching' method) and experimental data. The database includes many different structures, including bulk, cluster, liquid and several defect structures. The potential fit to the forces database, which has 2201 forces generated using a local orbital pseudopotential method, is good. This new EAM potential gives good results for various bulk structural properties.

The effects of multiple internal reflections within a laser weld joint as functions of joint geometry and processing conditions have been characterized. A computer-based ray tracing model is used to predict the reflective propagation of laser beam energy focused into the narrow gap of a metal joint for the purpose of predicting the location of melting and coalescence to form a weld. Quantitative comparisons are made between simulation cases. Experimental results are provided for qualitative model validation. This method is proposed as a way to enhance process efficiency and design laser welds which display deep penetration and high depth-to-width aspect ratios without high powered systems or keyhole mode melting.

The SCF MO LCAO method in the valence approach with neglect of diatomic differential overlap (NDDO) is used to study the effect of atomic hydrogen on the silicon lattice relaxation in the nearest vicinity of a vacancy. It is shown that hydrogen atoms are localized mainly as second-nearest neighbours of the vacancies on the Si - Si bond, which results in a significant extension of the vacancy region. The potential barrier height and its dependence on the vacancy charge state were calculated for Frenkel pair annihilation with a hydrogenated vacancy in the cases of hydrogen localization inside and outside the vacancy. The results substantiate a model of enhanced annihilation of Frenkel pairs in hydrogenated crystalline Si.