Damage and fracture of particle-reinforced metal matrix composites

Particle Reinforced Metal Matrix Composites (PRMMCs) are produced by high-pressure infiltration of ceramic particle beds. Matrix materials are either pure Al or Al-4,5%Cu, and the reinforcement is made either of angular or of polygonal alumina particles. The resulting composites feature a high volume fraction (40-60%) of uniformly distributed reinforcement. These materials accumulate damage during deformation, in the form of particle cracking or matrix voiding; localization of damage induces final failure. Our work aims towards a better understanding of the link between this damage build-up and fracture. Mechanical tests are conducted on smooth or notched tensile, smooth compressive, as well as smooth or double-notched 4-pt bend bars. This large set of different specimen geometries allows a wide variation of the stress state and of the size of the stressed volume. Small bending specimens show the largest strain at failure and tensile specimens the lowest, which indicates a scale-dependence of fracture. Double-notched specimens are employed to observe damage just prior to failure: when one notch fails, the other is close to failure. Metallographic cuts of the unbroken ligament reveal a large stable process zone in pure aluminium composites, whereas failure behaviour is unstable in alloyed matrix composites. The final aim of this study will be the establishment of a micro-mechanically based model that describes how the accumulation of damage induces failure in these materials.