On the calculation of the matrix-reinforcement interface diffusion coefficient in diffusional relaxation of composite materials at high temperatures

Metal or intermetallic matrix composites reinforced with discontinuous fibers exhibit creep strength which is superior to the unreinforced matrix strength due to the constraint imposed by the fibers on the deformation of the matrix. However, experimental measurements indicate that at temperatures higher than approximately half of the melting temperature of the matrix the composite creep resistance is limited, and in some cases the strengthening imparted by the reinforcements is completely lost even when no damage accumulation or debonding occurs at the matrix-reinforcement interface. Slip of the matrix over the reinforcement or diffusional mass transport along the matrix-reinforcement interface have been shown to be possible mechanisms responsible for the loss of strengthening. Current model predictions for the effect of the diffusional relaxation mechanism are based on a diffusion coefficient for the mass transport along the matrix-reinforcement interface. This diffusion coefficient is unknown, hard to quantify and no experimental measurements exist. In this paper a methodology for the calculation of the interface diffusion coefficient is presented on the basis that long range diffusional mass transport occurs with free slip of the matrix over the reinforcement, and that diffusional relaxation is needed in addition to slip for the composite strength to be knocked down to levels around and below that of the pure matrix. The procedure combines experimental measurements of the composite creep strength as a function of applied strain rate and temperature along with corresponding strength predictions from finite element calculations.