Measuring the local strength of alumina reinforcements
Ceramic particles in metal matrix composites, or hard second phases in alloys, often serve as reinforcements, raising its stiffness, hardness and strength. Their efficiency depends on their strength, a quantity that is statistically distributed over microscopic volumes and seldom quantified because it is, with the exception of long fibres, difficult to measure. We combine focused ion beam (FIB) milling methods with micro-testing techniques and finite element simulation to probe the strength of alumina fibres or particles that are used to reinforce aluminium. We present results from microscopic in situ tests conducted on Nextel ™ 610 nanocrystalline alumina fibres produced by 3M and used to reinforce continuous aluminium matrix composite wires. After exposing alumina fibres from the composite wire, we first use FIB milling to machine a wide rectangular notch in the fibre together with a line of load application along its top surface. Then, the notched fibre is loaded until failure by applying compressive force via a nanoindenter equipped with a flat tip: tensile stresses are thus created in the material, by bending under compressive loading. The deformation of each notched fibre sample is modelled using second-order elastic finite element analysis in order to obtain the three-dimensional stress state in the bent ligament prior to failure. The resulting strength distribution data are then used, knowing the local fracture toughness of the material, to estimate the nature and size distribution of local defects that govern the microscopic in-situ strength of the ceramic reinforcement. It is found that this defect population differs from that which governs the macroscopic tensile strength distribution of the fibres. We conclude with a discussion of the transposition of this methodology towards quantification of the microscopic strength of irregularly shaped ceramic particles or second phases that have potential for the reinforcement of aluminium.