ELECTRON-MICROSCOPY OBSERVATION OF AN INSITU CU-NB COMPOSITE

An in situ composite comprised of niobium filaments in a nearly pure copper matrix, produced by wire drawing a cast Cu-20vol.% Nb ingot, has been examined in the as-drawn and heat-treated conditions using scanning and transmission electron microscopy. In the as-drawn state the strength of the composite is dependent on the spacing of the filaments in accord with a Hall-Petch relationship. Heat treatment of the composite at temperatures above 300°C results in a substantial decrease in strength which current models have attributed to a reduction in the dislocation density. However, observation of the microstructure of samples which had been heat treated in the range 300-600°C reveals that the dislocation density remains relatively constant with increasing treatment temperature while the morphology of the niobium filaments changes from being thin and flat in cross-section to coarse cylinders. The temperature at which the morphological change becomes significant correlates with a drop in the electrical resistivity of the composite, the measurement of which has proven to be a sensitive tool for determining temperatures at which microstructural variations occur.