Preface
Nanostructured materials (or nanomaterials) are built up from microstructural units (e.g., grains) having smaller size than 100 nm at least in one dimension of space. This class of materials includes thin layers that have thicknesses smaller than 100 nm, while their lateral dimensions are usually several centimeters, which means that the criterion for nanostructured materials fulfills only in one dimension. Nanotubes also pertain to nanomaterials as their diameter is several nanometers, i.e., they satisfy the criterion in two dimensions. Crystalline nanoparticles and bulk materials having grain sizes smaller than 100 nm are also members of this class; however, these materials are also called as nanocrystalline materials since their microstructural unit (the crystalline grain or particle) is smaller than 100 nm in all directions. It is noted that in some cases the size of microstructural unit depends on the method used for studying the microstructure. For instance, in severely deformed metallic materials the grain size measured by electron microscopy is usually several hundreds of nanometers while the crystallite size obtained from the broadening of X-ray diffraction peak profiles is smaller than 100 nm. This apparent dichotomy is attributed to the fact that in these materials the crystallite size corresponds rather to the subgrain size. Therefore, these specimens are often classified as ultrafine-grained (UFG) materials for which the average grain size is between 100 and 1000 nm but the term of nanomaterials is also used for these samples as the subgrain size is smaller than 100 nm.
In 1990's, nanomaterials have become a focal point of materials science due to their unique physical, chemical, and mechanical properties that destine these materials to novel and promising applications. The small dimension of the grains or particles in nanomaterials and their specific processing methods affect their defect structure (vacancies, dislocations, disclinations, stacking and twin faults as well as grain boundaries) that has a significant influence on the properties of these materials. The knowledge of the relationships between the production methods, the lattice defects, and the physical properties of nanomaterials is very important not only to understand the specific phenomena occurring when the grain size is very small but also from the point of view of practical applications of these materials. Knowing these correlations, the functional properties of nanomaterials can be tailored by tuning the defect structure via an appropriate selection of the processing conditions. This process can be referred to as “defect structure engineering”.
This book aims to synthesize the existing knowledge of lattice defects formed in processing and subsequent plastic deformation of nanomaterials and their effect on functional properties. The first edition of this work was published in 2012 under the title “Defect structure in nanomaterials”. The great success of that book triggered the publication of this second and extended edition. The eight chapters in the first edition were improved and completed with four additional chapters, resulting in a doubled volume of the book. In the second edition a separate chapter is devoted to the characterization methods of lattice defects. In addition, four chapters deal with the influence of defect structure on functional properties of nanomaterials. These include the electrical resistivity, the diffusivity as well as the mechanical and hydrogen storage properties. Accordingly, the title of the second edition was changed to “Defect structure and properties of nanomaterials” which reflects these improvements. The information in this book is organized and presented in the form that is hopefully beneficial for a wide audience: materials scientists, engineers as well as lecturers, and students at universities. This book contains 12 chapters. In the following a brief description of each chapter is given.
Chapter 1 reviews the processing methods of bulk nanomaterials and nanocrystalline particles. These processing methods include “top-down” procedures that produce nanomaterials by severe plastic deformation (SPD), and “bottom-up” synthesis methods of nanoparticles where the particles are built up from individual atoms or from their clusters. Another route for production of nanostructured powders is milling, which is a “top-down” nanopowder processing technique. The powders obtained by either “top-down” or “bottom-up” methods can be sintered into bulk nanomaterials. The most frequently used consolidation techniques, e.g., hot isostatic pressing and spark plasma sintering, are described in this chapter. Finally, the details of electrodeposition of nanostructured thin films and nanocrystallization of bulk amorphous materials are presented.
In Chapter 2, the characterization methods of crystal lattice defects, such as vacancies, dislocations, planar faults, and grain boundaries are overviewed. These include direct methods, such as transmission electron microscopy and electron backscatter diffraction imaging, as well as indirect methods, such as X-ray line profile analysis (XLPA), electrical resistometry or positron annihilation spectroscopy. In the latter methods, first only a fingerprint of the defect structure—a data series—is obtained and then the parameters of the lattice defects are extracted by analyzing these data. Special attention is paid to XLPA since the majority of the results presented in this book are obtained by this technique. The most important advantages of this method are (1) the easy sample preparation, (2) its nondestructivity, and (3) the good statistics of the resulted microstructural parameters compared with microscopic methods. A comparison of the different methods is also presented.
Chapter 3 presents the evolution of defect structure during processing of nanostructured materials by SPD. It is shown that at high imposed strain in SPD-processing, the grain size reaches its minimum value and the dislocation density gets saturated. Although there is no strict correlation between the grain size and the dislocation density, the higher dislocation density is usually associated with smaller grain size. The saturation values of the dislocation density and the grain size are strongly influenced by (1) the homologous temperature, the pressure, and the strain rate applied in SPD, (2) the solid solution alloying, (3) the second phase particles, and (4) the degree of dislocation dissociation (i.e., the stacking fault energy). It is revealed that the vacancy concentration in metals processed by SPD at room temperature is as high as the equilibrium value at the melting point and the majority of vacancies are clustered. The lattice defects formed during SPD (e.g., dislocations) facilitate precipitation, thereby influencing the phase composition of the as-processed UFG alloys. In addition, due to the Gibbs–Thomson effect the volume fraction of secondary phase nanoparticles is influenced by the energy of interfaces and the size of particles.
In Chapter 4, the effect of low stacking fault energy (SFE) on defect structure in nanomaterials is summarized. The low value of SFE leads to a large degree of dislocation dissociation into partials that hinder strongly the cross-slip and climb of dislocations. As a consequence, a relatively large dislocation density develops during SPD of low SFE metallic materials. Additionally, the low SFE is accompanied by a small value of twin boundary energy resulting in a significant twinning activity during plastic deformation that alters grain refinement mechanisms. Among the pure face-centered cubic (fcc) metals, silver has the lowest SFE. Therefore, the effect of processing conditions and impurity content on dislocation density and twin-fault probability in UFG and nanocrystalline Ag is revealed and discussed in detail. In addition, the lattice defect structure in UFG alloys with low SFE is also studied. It is shown that the reduction of grain size increases the splitting distance between partials and the probability of occurrence of twinning in UFG and nanocrystalline materials.
Chapter 5 reviews the type and densities of lattice defects formed during processing of nanoparticles and consolidation of bulk nanomaterials from nanopowders. The evolutions of arrangement, edge/screw character and density of dislocations, and grain size during milling of metallic powders are presented. The minimum grain size achievable by milling at room temperature is correlated to the melting point of metals. The formation of dislocations, disclinations, and twin faults in nanoparticles produced by bottom-up methods are discussed. The effects of the initial powder particle size and the consolidation conditions on the defect structure in sintered metals, diamond, and ceramics are discussed. When blends of nano- and coarse-grained powders are sintered, the defect structure in the nanocrystalline fraction of the consolidated material is strongly influenced by the amount of the coarse-grained powder component and vice versa.
In Chapter 6, the lattice defects in nanocrystalline thin films and multilayers are overviewed. For electrodeposited Ni the organic additives increase the dislocation density and twin fault probability as well as reduces the grain size. For magnetron sputtered Cu foil relatively small dislocation density and large twin boundary probability were revealed. Subsequent rolling at room temperature yielded an increment in the dislocation density and a reduction of the twin fault probability due to untwinning. It was found that the substrate orientation has a deterministic effect on the average twin fault spacing in sputtered Ag film. In Cu–Nb multilayer processed by magnetron sputtering at room temperature, the stresses induced by the lattice mismatch between the Cu and Nb phases were relaxed by formation of misfit dislocations with high density and Burgers vector parallel to the interface of the layers. The influence of He ion implantation on the lattice defect structure in different multilayers is also discussed.
Chapter 7 overviews the defect-related mechanical properties of nanomaterials. The influence of small grain size on plastic deformation mechanisms, strength, and ductility is discussed. The smaller the grains size in fcc metals, the higher the activity of twinning at the expense of dislocation glide, while hexagonal close-packed materials behave contrarily. The relationship between the dislocation structure and the yield strength is studied in details. Above the grain size of ∼20 nm, the decrease of grain size is accompanied by an increase of yield strength and a decrease of ductility. The loss of ductility can be moderated by the incorporation of coarse-grains into the nanocrystalline matrix. A combination of large strength with good ductility can be achieved with nanograins containing high density of twin boundaries. Below the grain size of ∼20 nm, the yield strength is found to decrease with the reduction of grain size or remains unchanged within the experimental error. The possible explanations of this inverse Hall-Petch behavior are discussed in details.
In Chapter 8, the processing methods, the defect structure and the mechanical properties of metal matrix–carbon nanotube (CNT) composites are overviewed. It is revealed that the high dispersity of CNTs in the matrix is the most important criterion for processing composites with high strength and good ductility. The correlation between the flow stress and the dislocation density for Cu-CNT composites suggests that the CNT fragments strengthen the composite rather indirectly via the increase of the dislocation density.
Chapter 9 reviews the influence of lattice defects (vacancies, vacancy clusters, dislocations, planar faults, and grain boundaries) on the electrical resistivity of UFG and nanocrystalline materials. It turned out that the resistivities of vacancies, dislocations, and twin faults are much smaller than that for high-angle grain boundaries (HAGBs), solute atoms, and the intrinsic resistivity at room temperature. For pure metals and equilibrium solid solutions, nanocrystallization by SPD methods yields only a few percentage increase in resistivity while the strength is improved considerably, thereby improving the strength-to-resistivity ratio. The best combination of high strength and good conductivity in alloys is obtained if the strengthening is achieved by grain boundaries and nanosized secondary phase particles while the grain interiors are purified from solute elements. In pure metallic materials an improvement in strength-to-resistivity ratio can be obtained if the majority of HAGBs are substituted by coherent twin faults as the latter interfaces have very low specific electrical resistivity.
In Chapter 10, the most important models, formulas, and methods for the evaluation of diffusion along grain boundaries and dislocations in nanomaterials are overviewed. It was found that the larger the free volume and the energy of grain boundaries, the faster the grain boundary diffusion. In UFG metallic materials processed by SPD, a hierarchical microstructure develops with nonequilibrium and relaxed grain boundaries, which are paths for fast and slow grain boundary diffusion, respectively. As the fraction of boundaries exhibiting fast diffusion is very small, the diffusion rate in nanomaterials is determined by the relaxed boundaries. The diffusivity for these boundaries is very close to that observed for grain boundaries in coarse-grained materials. Therefore, the faster diffusion in nanomaterials compared to coarse-grained counterparts is caused basically by the larger amount of grain boundaries and not by their different quality. In nanomaterials processed by powder metallurgy, the diffusivity for interagglomerate interfaces between particles and pores is several orders of magnitude larger than that for intraagglomerate boundaries. Similar bimodal diffusivity is observed for nanomaterials obtained by crystallization of amorphous materials. Here, the slow and fast diffusion pathways are the amorphous and conventional crystalline interfaces. The diffusion along amorphous boundaries is slower than that for crystalline interfaces.
Chapter 11 reviews the influence of particle size, crystallite size, and lattice defects on hydrogen storage capacity and absorption–desorption kinetics of nanostructured materials. It is shown that the particle and crystallite sizes, and catalyst play two different roles in the sorption process. By decreasing the particle and crystallite sizes, the required diffusion path of hydrogen is drastically reduced that enhances the sorption kinetics significantly. At the same time, catalysts have a promoting effect on dissociation or recombination of hydrogen molecules on the particles' surfaces and they also act as chemisorption sites. The lattice defects (dislocations, stacking faults, and twin boundaries) facilitate the diffusion of hydrogen and the nucleation of hydride phases. It is revealed that large amounts of dislocations and/or planar faults are formed due to stresses induced by phase transformations in hydrogenation and dehydrogenation processes. The effect of defects on hydrogen storage capacity of carbon nanotubes is also reviewed.
Finally, in Chapter 12 the thermal stability of the defect structure in nanomaterials is overviewed. At high temperatures, the activation energy of recovery/recrystallization in pure fcc nanomaterials is close to the activation energy of grain-boundary diffusion. The onset temperature of recovery/recrystallization and the released heat depend on the grain size and the defect density contrary to the activation energy. Some nanomaterials with low SFE and/or low melting point (e.g., Ag or Pb–Sn alloy) tend to recover/recrystallize even during storage at the processing temperature (e.g., at room temperature) that is referred to as self-annealing. The low SFE promotes self-annealing as the recrystallized grains may be easily separated from the matrix by low energy twin boundaries. In addition, the thermal stability of UFG Cu samples processed by SPD and powder metallurgy is compared. The effect of CNT additive on recovery and recrystallization of the UFG Cu matrix is also investigated. Finally, the coarsening of Au nanoparticles and the development of their defect structure during their storage at room temperature is studied.
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