Testing strength and fracture toughness of hard second phases at micron scale
Local mechanical properties LMP 2015, November 12-14, 2014, Stará Lesná, Slovakia. Many alloys and composites combine a ductile matrix with discrete reinforcing phases, such as the ceramic fibers or particles that are used in metal matrix composites. It is well understood that the quality of the particles strongly influences strength and fracture toughness of the composite material; however, the strength of the particulate reinforcing phases is seldom quantified because it is challenging to measure. We present two newly developed methods that probe, respectively, the local strength and the fracture toughness of such microscopic phases using NextelTM 610 nanocrystalline alumina fibers produced by 3MTM, used here as testbench material. The strength measurement method involves FIB machining of a wide rectangular notch within the fiber that is afterwards tested in compression via nanoindenter. This produces high bending tensile stresses opposite to the notch end, which eventually cause failure. The fracture toughness measurement method consists of carving microscopic cantilever beams within the fiber, which contain a triangular shaped chevron-notch, using focused ion beam micromilling. For such a beam, when loaded via nanoindenter in bending, a crack initiates at the apex of the triangle, and subsequently propagates stably in Mode I until it reaches a critical length at which unstable crack-growth begins. Both test methods are complemented with three-dimensional finite element simulation of each tested sample.