The presence of brittle reinforcements and perhaps of metal oxides, however, tends to reduce their ductility and fracture toughness. Continuing development may reduce some of these deficiencies. The properties of materials reinforced with whiskers depend strongly on their orientation. Randomly oriented whiskers produce an isotropic material. Processes such as extrusion can orient whiskers, however, resulting in anisotropic properties.
Whiskers also reduce ductility and fracture toughness.
MMCs reinforced with aligned fibers have anisotropic properties. They are stronger and stiffer in the direction of the fibers than perpendicular to them. This is one of the major advantages of MMCs over PMCs, which can rarely be used without transverse reinforcement. Because the modulus and strength of metal matrices are significant with respect to those of most reinforcing fibers, their contribution to composite behavior is important. The stress-strain curves of MMCs often show significant nonlinearity resulting from yielding of the matrix.