Microcantilever bending experiments in NiAl - Evaluation, size effects, and crack tip plasticity

For a better understanding of the local fracture behavior of semi-brittle materials, we carried out bending experiments on notched microcantilevers of varying sizes in the micrometer range using NiAl single crystals. Smaller and larger beams were milled with a focused ion beam in the so-called "soft" < 110 > and "hard" < 100 > orientation and were tested in situ in a scanning electron microscope and ex situ with a nanoindenter, respectively. The measurements were evaluated using both linear-elastic fracture mechanics and elastic-plastic fracture mechanics. The results show that (i) the fracture toughness is in the same range as the macroscopically determined one which is around 3.5 MPa root m for the soft orientation and around 8.5 MPa root m pm for the hard orientation, that (ii) there is a strong influence of the anisotropic behavior of NiAl on the fracture toughness values, and that (iii) the J-integral technique is the most accurate quantification method.