Deep-groove ball bearings have been endurance tested at high stress levels. The purpose of the work was to study the simultaneous generation of residual stresses and occurrence of structural changes owing to cyclic rolling contact fatigue. During the investigation the decomposition of both retained austenite and parent martensite were followed. The residual stresses and the austenite decomposition were measured by an X-ray diffraction technique, using signal averaging. Optical and transmission electron microscopy were used to observe the transformed regions resulting from the martensite decay. Depth profiles of residual stress and of successive structural changes were constructed. The observations are discussed in terms of a qualitative modelfor residual-stress generation under cyclic rolling contact.MT/630

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The effects of sintering time and rate of cooling on the microstructure and current/voltage characteristics of a ZnO varistor material have been investigated. The results provide information about the development of microstructure during sintering and have been interpreted in terms of the role which microstructure plays in the determination of electrical properties. A continuous network of intergranular Bi,O,, which lies mainly along the triple junctions between ZnO grains, can give a substantial contribution to the leakage current. The conductivity of this skeleton depends upon its internal microstructure.

Detailed analysis of the microstructure of grain boundaries, especially triple-grain and multiple-grain junctions, in ZnO varistor materials has been performed using transmission electron microscopy. Different polymorphs of Bi 2 O 3 are shown to exhibit different wetting properties on ZnO interfaces. Recent investigations suggest that the equilibrium configuration consists of crystalline Bi 2 O 3 in the triple-grain and multiple-grain junctions and an amorphous bismuth-rich film in the ZnO/ZnO grain boundaries. The present investigation supports this suggestion for ␦-Bi 2 O 3 and also adds to the microstructural image and wetting properties of ␣-Bi 2 O 3 .

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