Strain fields at interfaces of Al-based metal matrix composites

Owing to the different thermal expansion coefficients of the phases involved, stresses are induced in metal matrix composites (MMCs). In the present study, the strain fields in MMCs consisting of an Al-based alloy containing 12wt.% Si and 4wt.% (Cu, Mg, Ni, Fe), reinforced by Fiberfrax® short fibers, are investigated by large-angle convergent beam electron diffraction (LACBED). Strain fields used to simulate the LACBED patterns are adjusted to experimental LACBED images. For these simulations the dynamical scattering theory is used. While thermally induced stresses in the Al matrix are reduced by a large number of dislocations near the fibers, these stresses are maintained in the Si precipitates. The strain fields are not homogeneously distributed around the fibers. Inhomogeneities near the fiber–matrix interfaces, 10–50 nm in extent, cause local stress concentrations. These inhomogeneous internal stresses can lead to plastic deformation of the Al matrix, to cracking of the fibers and to detachment at the matrix–fiber interfaces. The sign of the strains in the Si precipitates depends on the properties and the arrangement of the surrounding phases, whereas compressive strains perpendicular to the fiber interfaces prevail in the Al grains.