Localization of Plastic Flow in Nanocrystalline Materials

In general, plastic deformation in NCMs can be spatially homogeneous or localized in narrow shear bands. Evidence of plastic flow localization has been observed in NCMs tested in tension [244], compression [257], fatigue [323], and microhardness [324, 325]. For instance, during microhardness testing, nanostructured TiB₂ films have exhibited plastic flow which has been realized in either the homogeneous or inhomogeneous regime [324, 325]. In the latter case, their plastic behaviour has been very similar to the well-known phenomena of superplasticity which is quite common for fine-grained metals, intermetallics and ceramics under certain specific temperature and strain rate conditions [326–332]. The effect of superplasticity in NCMs has also been detected experimentally [227, 228, 333–338]. It has been well established [326–332] that grain boundary sliding is mainly responsible for the superplasticity.