Probing the local strength and toughness of microscopic hard second phases in metals and composites

Many engineering alloys and composites combine a ductile matrix with discrete microscopic reinforcing phases, e.g. silicon particles in aluminium alloys, carbides in steels or ceramic particles in metal matrix composites. It is well understood that the mechanical properties of such alloys and composites are strongly influenced by those of their reinforcing particles; yet not much is known of the nature and size of strength-limiting flaws within such microscopic second phases. To investigate the strength and fracture toughness of these microscopic particles directly, we have developed test methods that combine focused ion beam (FIB) milling with microtesting techniques and bespoke finite element simulation. Local strength is probed by micromachining individual reinforcing particles in such a way that tensile stresses are produced in a volume of material free of FIB and polishing artifacts upon the application of a load via nanoindenter. Fracture toughness is measured by the chevron-notch fracture test technique adapted for microscopic samples. Testing methods are established using nanocrystalline alumina fibers as the testbench material, so as to compare data thus obtained with known fiber properties from tensile tests on long sections of the fibers. These test methods are currently being transposed towards the microscopic testing of more irregularly shaped and anisotropic brittle second phases, with the objective to measure their local mechanical properties and identify microstructural flaws that govern the strength of such particles, using in-depth electron microscopy characterization combined with microtesting.