Strengthening mechanisms in Al_2O_3/SiC nanocomposites

摘要

A strengthening mechanism merely arising from internal (residual) microstresses due to thermal expansion mismatch is proposed for explaining the high experimental strength data measured in Al_2O_3/SiC nanocomposites. Upon cooling, transgranular SiC particles undergo lower shrinkage as compared to the surrounding matrix and provide a hydrostatic "expansion" effect in the core of each Al_2O_3 grain. Such a grain expansion tightens the internal Al_2O_3 grain boundaries, thus shielding both weakly bonded and unbonded (cracked) grain boundaries. It is shown that the shielding effect by intragranular SiC particles is more pronounced than the grain-boundary opening effect eventually associated with thermal expansion anisotropy of the Al_2O_3 grains, even in the "worst" Al_2O_3-grain cluster configuration. Therefore, an improvement of the material strength can be found. However, a large stress intensification at the grain boundary is found when intergranular SiC particles are present, which can produce a noticeable wedge-like opening effect and trigger grain-boundary fracture. The present model enables us to explain the experimental strength data reported for Al_2O_3/SiC nanocomposites and confirms that the high strength of these materials can be explained without invoking any toughening contribution by the SiC dispersion.