A Theoretical Investigation of Accelerated Aging in Metal-Matrix Composites

Abstract

Accelerated aging of precipitation hardening alloys reinforced with particulates or fibers has often been attributed to an increase in dislocation density in the immediate vicinity of the reinforcement. The plastic zone is generated during cooling from the solutionizing temperature due to a mismatch in thermal expansion coefficients of the matrix and the reinforcement. In this work, continuum mechanics and finite element approaches were used to develop models to calculate the plastic strain and the expended plastic work in the matrix in terms of the solutionizing temperature, matrix yield strength and elastic con stants, and the reinforcement volume fraction. The information obtained was then used to relate the degree of accelerated aging with the state of plastic strain of the matrix. It was assumed that the plastic zones from adjacent reinforcements are non-interacting and that the reinforcement is incompressible. Aging curves were generated for a monolithic 6061 Al alloy and fiber composites with 10v/o and 30v/o SiC. It was found that composites with a higher volume fraction of SiC age faster. Aging data generated for the monolithic alloy strained plastically after quenching were compared to those for the composites to deter mine the reliability of the model in determining the degree of accelerated aging, based on materials parameters and reinforcement morphology.