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The influence of stress on void growth at high temperatures through its effect on the thermal emission of vacancies from internal sources has been included in numerical solutions of swelling for several years. In this paper we formulate this stress effect into an analytical expression which describes both the incubation and subsequent growth of cavities in an irradiated material under stress.

The influence of dislocation behaviour on void formation and growth has been studied in the High Voltage Electron Microscope (HVEM). Variation of the interstitial loop density has been used to control the concentration of biased linear sinks available for point defects, and thereby the vacancy excess required for void growth. The interstitial loops were nucleated by neutron irradiation at 50°c and a good correlation was obtained between the loop concentration and swelling by electron irradiation at 400°c. Void development was closely related to the detailed microstructural changes that occurred immediately upon electron irradiation. A minimum foil thickness is necessary for void growth in the HVEM, otherwise the surfaces dominate, and the dislocation density always remains low. Voids do not grow if the dislocation density is less than 109 lines/cm2.

The microstructure of mild steel pressure vessel plate and weldments has recently been examined by transmission electron microscopy (TEM) to characterize certain numerous small (<1,000 Å) matrix precipitates and relate them to the bulk mechanical properties of the steel. The TEM examination was undertaken using in-line energy dispersive (EDAX) spectrometry. In view of the strong influence of Cu on radiation hardening and embrittlement in steels1particular emphasis was placed on the identification of any copper-bearing precipitates within the grains. We report here the first observation of discrete particles of copper sulphide, positively identified as Cu1.8S (Digenite I), and found only in the matrix of the mild steel weld specimens. In spite of having a similar copper content to the plate, the precipitation of copper sulphide uniquely in the welds is thought to arise from the high temperature oxidizing environment experienced during welding. This favours the formation of Cu1.8S in the presence of soluble copper from the less stable FeS commonly found in sulphur-bearing steels. The importance of this observation in relation to the mechanical properties of the steel is briefly discussed.