Plastic Deformation in Nanocrystalline Materials

The present book deals with plastic deformation processes in the so-called nanocrystalline materials, a special type of deformed solid-state nanostructure which comprises a significant fraction of the general class of nanostructures. Typical examples of deformed solid-state nanostructures are bulk nanocrystalline metals and alloys, nanolayered heterostructures, nanocrystalline thin filins and coatings, etc. [6–8]. The most common feature of these structures is that they consist of small particles or layers which are separated by specific interfaces and have a characteristic size from 1 to 100 nanometers (1 nm = 10⁻⁹ in). The bottom limit of this range corresponds to some 2–4 interatomic distances in a solid, while the top limit is quite symbolic. In our previous book [8], we subdivided solid-state nanostructures into the ordered and disordered ones. For example, bulk nanocrystalline metals and alloys as well as nanocrystalline thin films may be classified as disordered nanostructures because the size of their nanograins (nanocrystallites) is a random quantity, not to mention the randomness of their shape, inhomogeneity in the chemical composition and defect structure across the sample. In contrast, due to peculiarities in their fabrication and function, high-quality nanolayered heterostructures may be classified as ordered nanostructures.

Most mechanical properties of nanocrystalline materials (NCMs) are essentially different from those of conventional coarse-grained polycrystals. NCMs exhibit extremely high strength and good fatigue resistance [224–226], as desired for numerous applications. Most NCMs show low tensile ductility which limits their practical utility. At the same time, several examples of substantial ductility and even superplasticity have been reported. In particular, nanocrystalline ceramics commonly exhibit superplasticity at lower temperatures and faster strain rates than their coarse-grained counterparts [227, 228].

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.

The key features of NCMs (i.e., high density of grain boundaries and triple junctions with their generic defects like grain boundary dislocations and disclinations) which mainly determine their mechanical properties, may stimulate, under special conditions, the generation and development of a rotational mode of plastic flow. This mainly concerns NCMs fabricated under highly non-equilibrium conditions like ball milling and severe plastic deformation. In the present chapter, we consider different models of rotational plastic deformation in nano- and polycrystalline materials, when this deformation is realized by conservative motion of partial disclination dipoles capturing or issuing lattice dislocations or by the formation of multipole configurations of partial grain boundary disclinations leading to rotation of grains.

The processes developing in the late stages of plastic deformation in conventional (i.e., non-nanocrystalline in their initial state) crystalline materials are characterized by defect structures which have passed over some intermediate steps in their evolution [56, 263–266, 381, 389, 430–440]. Their principal difference from the defect structures inherent in the initial stages of plastic deformation is the mainly disclinational (rotational) character of the defect configurations (misorientation bands, fragment boundaries, high-angle grain boundaries of deformation origin, etc.) observed in experiments, which may be considered as carriers of rotational plasticity. At this late stage of plastic deformation in metallic materials, there exist two general paths for further evolution of the defect structures. The first path is the generation of microcracks in the regions of highest stress concentration, leading to fracture of the material. The second alternative is a transition to a nanocrystalline and/or amorphous structure of the metal [441, 442]. Obviously, the second path for structural evolution allows the deformed sample to achieve larger steps of plastic deformation than the first and this explains why it is very promising for metal-forming technologies. Nowadays, this approach is widely used in various techniques for fabricating NCMs and amorphous alloys (e.g., ball milling and mechanical alloying of powders [14], equal-channel angular pressing [28,443], etc.). In the 1980s, probably the first amorphous—nanocrystalline composites [36,37] were obtained under the combined action of high pressure and intensive shear.

We have considered theoretical models of plastic flow in NCMs, paying special attention to the abnormal Hall—Petch effect, localization of deformation, crossover to rotational modes of plasticity, amorphization and generation of micro- and nanocracks. On the basis of the results presented, one can draw the following general conclusions: