Influence of microstructural scale on plastic flow behavior of metal matrix composites

Metal matrix composites fabricated from pure aluminum metal and ceramic particles of magnesium aluminate spinel were deformed in uniaxial compression. The flow stress of the composites ranged from about 70 to 700 MPa, and broadly followed an inverse square root relationship with metal ligament size. The composites had excellent ductility. The change in the dislocation substructure with increasing plastic deformation was examined by TEM. Random dislocation tangles observed in the undeformed specimens were found to have been replaced by low angle sub-boundaries in the deformed specimens. This observation is explained by invoking compatibility between crystal slip in metal ligaments and sliding accommodated by dislocation glide parallel to metal-ceramic interfaces. A model based on this mechanism is developed and compared to the experimental results. The analysis successfully predicts the inverse square root dependence of the flow stress on the metal ligament size, and gives fair agreement with the magnitude of the measured flow stresses.