OXIDATION BEHAVIOR OF CU-NB MICROCOMPOSITES

The oxidation behavior of Cu-15vol.%Nb microcomposites was studied in air over the temperature range from 600 to 900-degrees-C. The pattern of scaling and subsurface oxidation was examined by optical metallography, scanning electron microscopy, X-ray diffraction and microprobe analysis. In this two-phase microcomposite in which the second phase takes the form of fine filaments spaced less than 1-mu-m apart, the oxidation morphologies in the longitudinal and transverse directions were distinct. In the transverse direction, an external scale consisting of Cu2O and CuO was formed with a niobium oxide inner layer. In the longitudinal direction, preferential oxidation along the fine, high aspect ratio niobium filaments was found underneath the external copper oxide scale. Thermogravimetric analyses show the kinetics to be parabolic. The activation energy for oxidation parallel to the niobium filament direction measured 45.8 kcal mol-1. The oxidation behaviors of Cu- 15vol.%Ta and Cu-15vol.%Cr microcomposites were also investigated and compared with that of Cu-15vol.%Nb. The effect of alloy composition, microstructure of the two-phase mixture and temperature on the oxidation behavior was also considered.