Misfit Accommodation in a Quenched and Aged Al-Cu Alloy with Silicon Particles

Quenched and aged specimens of the Al-1.3 at. pct Cu-19.1 at. pct Si alloy were studied by X-ray diffraction. Since this alloy contains a high volume percentage (~20 vol pct) of second- phase Si particles, it is regarded as a model for a metal matrix composite (MMC). During isothermal aging of the solid-quenched Al-1.3 at. pct Cu-19.1 at. pct Si alloy, the Cu and Si atoms precipitate. This causes the Al-rich phase lattice parameter to increase from a value lower to a value higher than the lattice parameter of pure unstrained aluminum. Due to thermal misfit after quenching from heat-treatment temperatures, all lattice parameters are influenced by re- sidual stresses. A model describing the elastic/plastic accommodation of a misfitting spherical inclusion in an infinite matrix is adapted for the case of misfitting inclusions in a finite matrix. This model describes the measured lattice parameter shifts of the Si phase reasonably well. Comparison of the model for elastic accommodation and the model for elastic/plastic accom- modation with measured stresses shows significant discrepancies for the low-temperature range (ΔT < 200 K). These discrepancies may be related to the volume effect of defects (dislocations, vacancies) created in the plastic zone.