Measuring the local strength and toughness of microscopic hard second phases in alloys and composites

It is well known that the mechanical performance of many metallic alloys or composites is strongly influenced by the local strength of the various embedded second phases that they might contain; yet, surprisingly little hard quantitative data exist to document such local strength values. We have developed test methods that combine focused ion beam (FIB) milling with microtesting coupled with bespoke finite element simulation to measure the strength and fracture toughness of such microscopic particles directly. Local strength is measured by probing, using an instrumented precision nanoindenter, micromachined samples that have been carved into individual reinforcing particles using the FIB. The shape and micromachining processes are systematically designed so as to confine elevated tensile stresses in a volume of the material that has not been altered by FIB micromilling. Testing methods initially established using nanocrystalline alumina fibers as the testbench material are now being transposed towards the microscopic testing of more irregularly shaped and/or anisotropic brittle second phases. A different test procedure has also been developed to probe coarsened silicon in aluminium, as the flat shape of this specific second phase makes another microtesting procedure possible.