Nanostructured materials and composites present a promising class of engineering materials with structural length scales well below one micrometer. This microstructure characteristics results in properties and property combinations that are often enhanced or even unique compared to the polycrystalline counterparts. It has attracted enormous interest in basic research as well as in the application oriented engineering sciences or, more recently, even in public media, although - in the historical perspective and even disregarding the obvious coincidence of structure formation on a nanometer scale and basic synthetic chemistry - nanostructured materials have been around for a very long time. Conceptually, nanocrystalline (or nanostructured) materials are described as bulk materials consisting of single crystallites of nanometer dimensions that are connected by grain boundaries and form a three-dimensional (macroscopic) nanocrystalline solid. After almost three decades of intensive research, a wealth of insight has been gained on all aspects of materials science of this new class of materials and research on nanostructured metallic materials has also produced enhanced insight into the microstructure–property relation of non-metallic materials or into the structures and properties of defects of the crystal lattice, e.g. the grain boundaries or the triple junctions between grain boundaries, because of the abundance of these defects at small grain sizes. Due to the continuing high interest in nanocrystalline materials it is inherently impossible to cover the full range of research on these materials or to present a full coverage even of a selected range of subtopics. Thus, the present chapter is meant to cover a range of interconnected topics in the area of nanocrystalline materials that, to the author’s belief, represent best the aspects that fall under the chapter topic of “physical metallurgy of nanocrystalline materials”.

Physical Metallurgy of Nanocrystalline Metals

Publisher Summary This chapter deals with functional nanostructured materials. New routes for nanostructure formation are presented by sequentially combining different non-equilibrium processing pathways that are based, for example, on rapid cooling or continuous strain energy input. There are several aspects concerning the thermodynamic stability and the interface diffusion of nonstrustured materials that are drastically affected by the presence and atomic structure of heterophase interfaces as well as by the selection of synthesis and processing pathways. The modification of phase stabilities and phase diagrams for nanostructured materials as well as the seemingly inherent presence of high-mobility interfaces is described in the chapter. The observed behavior, such as a shift of the melting temperature either to higher or to lower temperature depending solely on the atomic structure of the interface or the presence of interfaces with diffusivities that are several orders of magnitude higher than usual (again apparently depending simply on the atomic-scale structure of the interface), highlights the future potential and the newly available range of opportunities for property modification that is uniquely offered by the class of nanocrystalline materials.

Functional Nanostructured Materials – Microstructure, Thermodynamic Stability and Atomic Mobility

Initial powder mixtures of Cu, Fe and Co are exposed to severe plastic deformation by high-pressure torsion to prepare solid solutions. A broad range of compositions is investigated, whereas this study aims at the synthesis of soft magnetic materials and therefore at the formation of a homogeneous and nanocrystalline microstructure. For intermediate ferromagnetic contents, high-pressure torsion at room temperature yields single-phase supersaturated solid solutions. For higher ferromagnetic contents, two consecutive steps of high-pressure torsion deformation at different temperatures yield the desired nanocrystalline microstructure. Depending on the Co-to-Fe-ratio, either a single-phase supersaturated solid solution or a nanocomposite forms. The composite exhibits an enhanced magnetic moment, indicating the formation of a (Fe,Co)-alloy upon severe plastic deformation. Soft magnetic properties are verified for large Co-to-Fe-ratios and this microstructure is found to remain stable up to 400 °C.

Sampling the Cu–Fe–Co phase diagram by severe plastic deformation for enhanced soft magnetic properties

The nanocrystallization-induced structural evolution of the intergranular amorphous phase in a Fe78B13Si9 alloy was investigated by X-ray diffraction (XRD) measurements, transmission electron microscopy (TEM), and positron annihilation spectroscopy. Crystallization occurs at 773 K, where nanocrystallites of α-Fe with an average grain size of a few tens of nanometers are formed in an amorphous matrix. With increasing annealing temperature up to 973 K, the average grain size increases up to ∼80 nm. In the as-prepared sample corresponding to an amorphous precursor, more than 90% of the positrons are localized at vacancy-sized free volumes dominantly surrounded by Fe atoms and other positrons are trapped by microvoids. Along with the appearance of nanocrystallites and their growth due to annealing, the concentration of microvoids is increased in the intergranular amorphous phase.

Nanocrystallization-induced structural evolution of intergranular amorphous phase in Fe78B13Si9 alloy

71Ge tracer diffusion was studied to gain insight into the atomistic transport processes underlying the formation of magnetic anisotropy in nanocrystalline soft-magnetic Fe73.5Si13.5B9Nb3Cu1. The interfacial diffusion characteristic was determined by the residual intergranular amorphous phase, which gives rise to a strongly reduced interface diffusivity compared with grain boundaries in metals. Ge diffusion in the nanocrystallites, which is considered to characterize Si self-diffusion, is much slower than Fe diffusion owing to the D03 order of the Fe3Si nanocrystallites. Slow Si diffusion in the nanocrystallites is identified as the rate-controlling process for the generation of the field-induced magnetic anisotropy.

Diffusion and induced magnetic anisotropy in nanocrystalline Fe73.5Si13.5B9Nb3Cu1

Iron tracer diffusion was studied in soft-magnetic nanocrystalline Fe90Zr7B3 without any influence of porosity, relaxation, or grain growth. The interfacial diffusion characteristics differ substantially from grain boundaries in metals due to the presence of an intergranular amorphous phase. The reduced diffusivity in thin amorphous layers compared to in the initial amorphous phase indicates the effect of confinement. The indication of a second, fast interfacial diffusion path is found and quantitatively analyzed within the framework of a two interface-type model.

Interface diffusion and amorphous intergranular layers in nanocrystalline Fe90Zr7B3

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Interface diffusion and amorphous intergranular layers in nanocrystalline Fe90Zr7B3