Thermal cycling of copper matrix-tungsten fiber composites: A metallographic study

Abstract

Composite specimens, consisting of tungsten fibers embedded in a single crystal copper matrix, were subjected to thermal cycling. Stresses arose in the two components due to the differential in the coefficients of expansion of fiber and matrix; the ones arising in the soft copper matrix were large enough to cause plastic flow. The extent of plastic deformation in the copper matrix, as evidenced by the slip markings, was higher immediately next to the fiber/matrix interface and it increased with the number and amplitude of thermal cycles. For a given magnitude of temperature change, the extent of slip deformation in the matrix was higher for high rate cycling than for slow rate cycling. In the as made condition the dislocation density in the copper matrix was higher close to the fiber/matrix interface than far away from the fiber. On thermal cycling this dislocation density gradient evened out. A well-defined cell structure developed after slow rate cycling between ambient and up to 300°C. The well-defined cell structure was conspicuous by its absence after high rate cycling between ambient and liquid nitrogen temperature. The matrix behavior is discussed in relation to the thermal stresses generated and the initial and final dislocation distributions.