Magnesium sheet for automotive applications is very attractive due to its light weight. The poor formability of magnesium and its alloys at room temperature, however, has limited the applications of these alloys. For this reason, at present, magnesium must be formed at elevated temperatures. This study investigates the hot deformation and fracture characteristics of Mg-1wt% Zn alloys containing a range of Al and Mn levels. Hot-rolled specimens were tensile tested over a range of strain rates and temperatures. Strain rate versus flow stress diagrams plotted on log-log scales revealed a transition in deformation mechanisms as a change in slope (the 'stress exponent'). Specifically, non-uniform deformation (i.e. necking) is observed at high strain rates, while uniform deformation is observed at low rates. This transition is accompanied by a change in fracture mechanism from dimpled rupture at high strain rates to cavitation and cavity interlinkage at low strain rates. Specimens which had a stress exponent of ∼2 and which failed through uniform deformation showing interlinked cavities have been associated with the grain boundary sliding (GBS) deformation mechanism. Specimens which had a stress exponent of ~5 and which failed through necking showing a dimpled fracture surface have been associated with the dislocation creep deformation mechanism. Increasing aluminum appears to somewhat favour the GBS regime as indicated by a slightly decreasing stress exponent. Manganese also appears to favor the GBS regime, since the onset of cavitation appears at higher strain rates compared to alloys without Mn.

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The segregation of a number of impurity elements to grain boundaries in reactor pressure vessel steels, under both thermal and irradiation conditions, have been observed to cause embrittlement. In low alloy steels, the embrittlement has been associated with small additions of phosphorus to alloys, an impurity element that lowers the cohesive strength of grain boundaries, thereby permitting brittle, intergranular fracture to occur more easily. Conversely, carbon additions to the same steel alloys have been shown to increase the grain boundary cohesiveness, thereby reducing the propensity for the alloy to fail in an intergranular manner. An increased understanding of the behaviour of these alloys under typical reactor service conditions is therefore sought after. Experimental grain boundary segregation data is available for long-term thermally aged material, and theoretical models exist which can reasonably predict the magnitude and temperature dependence of impurity element segregation. Isothermal ageing-induced segregation, known as equilibrium segregation, has been predicted using a variety of analytical models, that can predict the effect of alloying elements that both interact during segregation and that segregate competitively. However, grain boundary segregation data for irradiated material is scarcer, primarily owing to the difficulty of dealing with radioactive samples, but also due to the relative scarcity of material itself. Theoretical models, based on thermal non-equilibrium types of segregation, currently exist but are somewhat limited in their approach, since they only predict segregation in binary alloys. These models have been extended in this Thesis to predict the behaviour of ternary alloy systems. Comparison with currently available experimental results has shown that these modifications have resulted in a more accurate prediction of the segregation behaviour of these impurity elements. In addition, the effect of thermally induced segregation has been incorporated into theoretical models to predict the behaviour of M234 type precipitates under long term thermal ageing conditions in austenitic stainless steels. These predictions have also been compared to experimentally observed precipitation behaviour in a number of alloys and have been found to show close agreement.

This research is concerned with identifying and characterizing four solid-state precipitation reactions in the W-Ni-Fe system that can be induced by appropriate heat treatments. Previous work in this area is reviewed, and a general overview of the research on this system that may, directly or indirectly, further the understanding of microstructure/mechanical property relationships in the W-Ni-Fe heavy alloy is presented. The many metallographic and analytical techniques that have been employed in the course of this investigation are also briefly reviewed. The specific precipitation reactions studied in the 90W-5Ni-5Fe heavy alloy are listed and briefly described. Interfacial precipitation - interfacial precipitates at the W-andgamma; and andgamma;-andgamma; boundaries were found to be andeta;-carbides. This morphology is expected to severely embrittle the alloy. Matrix-phase precipitation - discontinuous precipitation of W was observed in the matrix region. W-W grain boundary precipitation - W-W grain boundary allotriomorphs (andgamma;-phase) were identified and characterized. W-phase precipitation hardening - fine scale precipitation in the W-phase was found to strengthen the alloy. Deformation prior to aging significantly increased the rate of precipitate nucleation. TEM contrast analysis showed the precipitates to be plate-like with a {100} habit plane, and the strain in the W lattice normal to the precipitates was found to be tensile in nature. Matrix-composition alloys were manufactured to simulate the andgamma;-phase in the 90W-5Ni-5Fe alloy. Metallographic observations of these alloys furthered the understanding of reactions 1) and 2) above. These results are discussed in terms of (a) current theories of solid-state precipitation and (b) the influence of the various morphologies on mechanical properties.

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