| Summary: | A study has been made of the influence of impurities on the flow stress of melt-grown magnesium oxide single crystals. The state of dispersion of the transition metal impurities chosen -iron, titanium 9 vanadium and chromium - was varied by heat treatment at temperatures up to 2,000oC in different environments? and the associated changes in flow stress measured at room temperature Evidence for impurity redistribution in each case was derived from an application of optical and microwave absorption techniques, combined with direct microstructural observations by electron microscopy a A good correlation has been found between the experimentally observed flow stress and the calculated strengthening by dispersed obstacles identified by these methods. In as-grown crystals the principal defects influencing the flow stress are considered to be in the form of impurity-vacancy pairs and anion-cation vacancy pairs, and it is these defects which govern the initial annealing behaviour. Additional evidence to substantiate these conclusions has been provided by measurements of the temperature dependence of the flow stress, and the recovery of neutron irradiation damage in doped crystals. During low temperature anneals in air vacancy clustering and collapse as dislocation loops has been observed, and correlated with an initial plastic softening behaviour. Impurity oxidation also takes place under these conditions9 leading to an increased concentration of impurity vacancy pairs, and subsequently to the formation of a spinel precipitate dispersion. An explanation for the flow stress variations at low temperatures for long ageing times is proposed in terms of a precipitation hardeninq-overageing behaviour in which precipitates are undeformed in the initial stages of yielding As a result of the high nucleation rate in the supersaturated vacancy environment existing in as-grown crystals, a significant hardening is produced, even for low volume fractions of precipitates In many cases the particle dispersion corresponding to the flow stress maximum has been found to be unresolvable. Nevertheless, a correlation of the flow stress calculated on the basis of an Orowan mechanism with the experimentally observed flow stress is possible by evaluating the precipitate volume fractions indirectly from the E.P.R. and optical results. The particle radii calculated in this way are in good agreement with the observed microstructure over the annealing ranges considered. At elevated temperatures in air the flow stress has been found to be modified by interstitial oxygen diffusion. The centre responsible for the flow stress peak in these conditions is considered to be in the form of an interstitial oxygen ion trapped at an impurity site.
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