The influence of microstructural scale on the matrix deformation and fracture in aluminum/magnesia-alumina spinel composites

The flow stress was measured of model composites constructed from magnesium aluminate spinel particles dispersed in a matrix of pure aluminum and aluminum alloys.; The size and volume percent of spinel was varied in order to study how the flow behavior is influenced by the interparticle spacing, defined as the edge to edge aluminum spacing between particles. The volume percent of the spinel ranged from 10 to 50 v/o, particle size from 1 to 15 {dollar}mu{dollar}m and interparticle spacing from 0.7 to 14.2 {dollar}mu{dollar}m. In compression, the initial flow stress and the maximum flow stress of the composites ranged from 76 to 760 MPa, and broadly followed an inverse square root dependence with interparticle spacing.; In-situ strain measurements quantified the development of plastic strain gradients as the composite is stressed near the initial flow stress. Larger aluminum matrix regions, with less spinel available to constrain plastic flow, experienced higher strains compared to the average overall strain. Slip lines were observed in the larger aluminum grains, whereas they were absent near the spinel particles or smaller matrix regions. In tension, cracks grew from these slip lines causing both loss of strain hardening and subsequent failure. Compression specimens can also experience slip within the aluminum regions but catastrophic crack growth will be suppressed. This enables the composite to strain harden to a higher level compared to tensile specimens. Hence, Al/spinel compression flow stresses are higher than in tension and the difference increases at higher strain levels.; Since the composites had good ductility in compression, the change in dislocation substructure was examined by transmission electron microscopy. In the deformed samples, low angle boundaries were observed, features which were not seen in the undeformed samples. Low angle boundaries are required to satisfy slip compatibility adjacent to the interface. A new model proposes that the Burgers vector deficit between slip vectors of dislocations in the slip plane and dislocations that accommodate sliding across the metal-ceramic interface, leads to the formation of low angle boundaries. Reasonable agreements both in magnitude of the flow stress and its functional dependence on the interparticle spacing, are obtained.