Accelerated aging of metal matrix composites

Accelerated aging of Metal Matrix Composites (MMCs) with precipitation hardening matrices has often been attributed to an increase in the dislocation density in the immediate vicinity of the reinforcement. The plastic zone is generated due to differential thermal contraction of the matrix and the reinforcement during cooling from the solutionizing temperature. In this work, a finite element approach was used to model the geometry of the plastic zone and the expended plastic work in the matrix and relate them to material parameters and reinforcement morphology. The information obtained was then used to relate the degree of accelerated aging to the state of plastic strain in the matrix. To identify the relative contributions of dislocations and the matrix residual stress field to accelerated aging, transmission electron microscopy and differential scanning calorimetry were done on 6061 Al-SiC MMCs. A theoretical model was developed to predict the rate of silicon clustering (the first step in the aging of 6061 Al) in the residual stress field around fibers in the MMC. The results were compared with the kinetics of clustering on an array of edge dislocations. It was concluded that for a given matrix dislocation density, there is a critical range of reinforcement sizes below which a residual stress mechanism dominates and above which a dislocation mechanism dominates. To determine the dislocation density distribution dependence of aging in MMCs, theoretical Avrami-type precipitation curves were generated assuming dislocation-density-dependent nucleation only and nucleation and growth. Curves were generated for both uniform dislocation density (to model cold work) and a dislocation density gradient (to model MMCs). These theoretical results were compared to pseudo-isothermal precipitation curves generated by differential scanning calorimetry of monolithic 6061 Al and 10 v/o SiC reinforced 6061 Al. It was found that precipitation in the MMC starts earlier and ends later than in the unreinforced alloy with the same amount of plastic work as the composite. The reaction rate order for {dollar}betaspprime{dollar} precipitation was also determined from the calorimetric studies. These experimental results were then rationalized in terms of the results of the theoretical calculations.