CREEP STRENGTHENING IN A DISCONTINUOUS SIC-AL COMPOSITE

High-temperature strengthening mechanisms in discontinuous metal matri x composites were examined by performing a close comparison between th e creep behavior of 30 vol pet SiC-6061 Al and that of its matrix allo y, 6061 Al. Both materials were prepared by powder metallurgy techniqu es. The experimental data show that the creep behavior of the composit e is similar to that of the alloy in regard to the high apparent stres s exponent and its variation with the applied stress and the strong te mperature dependence of creep rate. By contrast, the data reveal that there are two main differences in creep behavior between the composite and the alloy: the creep rates of the composite are more than one ord er of magnitude slower than those of the alloy, and the activation ene rgy for creep in the composite is higher than that in the alloy. Analy sis of the experimental data indicates that these similarities and dif ferences in creep behavior can be explained in terms of two independen t strengthening processes that are related to (a) the existence of a t emperature-dependent threshold stress for creep, tau(0), in both mater ials and (b) the occurrence of temperature dependent load transfer fro m the creeping matrix (6061 Al) to the reinforcement (SiC). This findi ng is illustrated by two results. First, the high apparent activation energies for creep in the composite are corrected to a value near the true activation energy for creep in the unreinforced alloy (160 kJ/mol e) by considering the temperature dependence of the shear modulus, the threshold stress, and the load transfer. Second, the normalized creep data of the composite fall very close to those of the alloy when the contribution of load transfer to composite strengthening is incorporat ed in a creep power law in which the applied stress is replaced by the effective stress, the stress exponent, n, equals 5, and the true acti vation energy for creep in the composite, Q(c), is equal to that in th e alloy.