In situ mechanical testing of an Al matrix composite to investigate compressive plasticity and failure on multiple length scales

SiC particle reinforced Al matrix composites exhibit high strength, high wear resistance, and excellent high-temperature performance, but can also have low plasticity and fracture toughness, which limits their use in structural applications. This study investigates the plasticity and failure of such a composite on multiple length scales, from strain localization through a complex microstructure to the debonding of individual microparticles from the matrix. Three microscale pillars containing microstructures with different complexities and sizes/volume fraction of SiC particles were used to study the effect of these features on deformation. For the matrix, nanoscale intermetallic precipitates within the Al grains contribute most to the strengthening effect, and the Al grain boundaries are shown to be effective obstacles for preventing strain localization by dominant shear bands and, therefore, catastrophic failure. When shear localization occurs, SiC particles can then debond from the matrix if the shear band and interface are aligned. To investigate whether the interface is a weak point during catastrophic failure, a number of SiC particles were separated from the matrix with direct debonding tests, which yield an interface strength that is much higher than the critical resolved shear stress for a pillar exhibiting both shear localization and interface debonding. Therefore, the matrix–particle interface is ruled out as a possible weak point, and instead, shear localization is identified as the mechanism that can drive subsequent interface debonding.