MICROSTRUCTURAL STABILITY OF STRESSED LAMELLAR AND FIBER COMPOSITES

Microstructural stability can be modified by the presence of internal or applied stresses. We employ a linear stability analysis to examine the effect of stresses on the interface diffusion controlled morpholog ical stability of lamellar (plate-like) and fibrous (rod-like phase) e utectic microstructures. These stresses can be either due to misfit st rains and/or externally applied loads. For misfitting plates, the nomi nally flat plate-matrix interface is unstable with respect to the grow th of perturbations with wavelengths greater than a critical wavelengt h, provided that the plates are elastically stiffer than the surroundi ng matrix. In contrast, for stresses generated by externally applied l oads, the flat plate-matrix interface is always unstable as long as th e plate modulus and the matrix modulus are different. In addition, the present analysis shows that misfit strains can either counteract or e nhance the destabilizing influence of applied loads, depending on the elastic properties of the plate and the matrix and the volume fraction of the two phases. For misfitting rods, the nominally cylindrical rod -matrix interface in an isotropic matrix is unstable with or without e lastic effects. However, the elastic effects can decrease (increase) t he maximally unstable wavelength over that predicted by curvature effe cts alone provided that the rods are elastically stiffer (softer) than the surrounding matrix. Stresses generated by externally applied load s, on the other hand, always lead to a decrease in the instability wav elength compared with the Rayleigh instability wavelength. Stability d iagrams are presented which identify the material properties and opera ting conditions required to maintain a stable interface in these eutec tic microstructures. (C) 1997 Acta Metallurgica Inc.