Microscopic chevron-notch fracture test of hard second phases

It is well understood that the intrinsic mechanical properties of reinforcements and second phases strongly influence those of composite materials and alloys; however, these properties are challenging to determine due to the small size and irregular shape of the particles. We present a method for the measurement of the fracture toughness of micron-sized specimens based on Focused Ion Beam (FIB) milling, combined with micro-mechanical testing techniques and Finite Element (FE) simulation. Microscopic chevron-notched cantilever beams are first machined. These beams are subsequently loaded using a nanoindentation testing apparatus. It is found that the crack initiates at the chevron notch tip under a load that is in most tests lower than that corresponding to the onset of crack instability. Some degree of stable crack growth can thus be observed and meaningful toughness measurements gleaned on these small tested volumes. The measurements are furthermore minimally influenced by milling-induced defects given that, at the onset of unstable growth, most of the crack front is situated away from the FIB-machined surface. Test data are interpreted using compliance calibration curves determined by three‑dimensional FE simulation of each beam, after measurement of its dimensions using electron microscopy. The method is applied to nanocrystalline Nextel ™ 610 alumina fibres, which are used in aluminium-matrix composites. Results are consistent with expected values for the material at hand, suggesting that the technique is reliable despite the small specimen size, and that it can be transposed to other reinforcements. The influence on the test of environmentally induced subcritical crack growth, known to be operative with alumina in air, is also examined.