Abstract
Owing to the presence of the fibres which provide a homogeneous reinforcement, it has been possible to combine, in the whole nickel-base matrix of a unidirectionally solidified composite, fatigue life and fatigue strength conditions which cannot be attained in an ordinary material except in high-stress concentration regions where the observation is usually impossible. Taking advantage of this ideal material, two complementary observation techniques, the high-voltage electron microscope (1 MV) and the field-emission gun scanning electron microscope, have not only revealed deformation microstructures such as dislocation multipoles which may be responsible for vacancy production, but also some unusual aspects of the fatigue fracture surfaces which may be most probably attributed to the presence of a supersaturation of vacancies and to their further condensation. These last results, which were obtained at room temperature, may be correlated with the observations performed on ordinary materials but at medium temperatures, about 0.5 Tm, in which case the condensation of excess vacancies may lead to the formation of cavities, not only grain boundary cavities, but more essentially cavities homogeneously distributed inside the grains (Arnaud, Le Hazif and Martin 1985, Bhanu Sankara Rao, Vijayalakshmi, Valsan, Mannan and Rodriguez 1986, 1989). The present investigation thus not only confirms the mentioned results but moreover suggests that the role of vacancies in fatigue is not restricted to cyclic hardening: at least in some particular materials and under specific conditions, point defects may be responsible for fatigue failure.