Deformation twinning in nanocrystalline materials

Data on the experimental structure of small particles is reviewed, the emphasis being an attempt to correlate experimental information with theoretical models. First, a general discussion of some of the controlling factors is presented, primarily equilibrium shapes of small particles, the effect of surface stresses, kinetics, and the role of chemisorption and the substrate. Experimental techniques for obtaining information about small particles are then described, primarily electron microscopy approaches. Experimental data on the static structure of small particles is then reviewed, both single crystals and the many, complicated twinned structures in face-centred cubic materials. An overview is then given of some of the more recent results on dynamic phenomena in small particles. Finally, a general model merging thermodynamic and kinetic factors is presented to attempt to rationalize the available data, followed by a brief discussion.

Experimental studies of small particle structures

Polycrystalline materials with nanometer-sized grains, termed nanocrystalline materials, can be formed by crystallizing completely amorphous solids under proper heat treatment conditions. The crystallized nanocrystalline materials exhibit some unique structural characteristics and novel properties which are fundamentally different from those of the conventional coarse-grained polycrystalline materials. This article reviews the present state of the art in this field. The current status of research and developments on the nanocrystallization, microstructure and properties of the materials will be summarized. Comparisons of structural characteristics and properties are made between the crystallized nanocrystalline materials and those prepared by other methods. Further considerations of the development and applications of this new class of materials will also be presented.

Nanocrystalline metals crystallized from amorphous solids: nanocrystallization, structure, and properties

Review on superior strength and enhanced ductility of metallic nanomaterials

Nanomechanics of Hall-Petch relationship in nanocrystalline materials

A comprehensive review is given on experimental studies of small particles with fivefold symmetry accompanied by an in-depth theoretical description of their characteristics and computer modeling. The cases of uniform and nonuniform deformations (disclination model), stability and relaxation of elastic stresses in pentagonal particles and needle-like crystals, models of their formation are discussed.

Pentagonal Symmetry and Disclinations in Small Particles

In this paper the size effect of lattice-dislocation stability in nanocrystals (NC's) caused by a strong interaction of dislocations with interfaces is analyzed. When the nanocrystallite size l becomes less than the characteristic length \ensuremath{\Lambda}, a substantial dislocation redistribution in the nanocrystal may occur. This length is estimated to be dozens of nanometers and depends on the character of interfaces: coherent or incoherent (slipping), on a difference of elastic moduli of NC phases, etc. Simple analytical estimations of \ensuremath{\Lambda} on the basis of exact calculations have been obtained.

Size effects of dislocation stability in nanocrystals

A generalization of the Hall-Petch relationship is proposed. The generalized relationship takes account of the contributions from intergrain sliding, generation of lattice dislocations, and influence of disclination-like defects. From this approach, a critical size corresponding to a maximum of the Hall-Petch size dependence is obtained. The value of the critical size essentially depends on the state of boundaries and it explains contradictory results for the microhardness of nanocrystals (NCs), since grain-boundary sliding is facilitated in unrelaxed nanocrystals and constrained in aged ones.

On the yield stress of nanocrystals

On the basis of continuous elastic models relaxation channels of elastic stresses inside the pentagonal needle-like nanoparticles (PNNPs) are considered. Six models are analyzed: 1. creation of a dislocation inside the PNNP; 2. opening of a gap in the PNNP; 3. creation of a disclination bound with the PNNP surface; 4. splitting of the pentagonal axis; 5. growth of monocrystalline region around the PNNP axis; 6. displacement of the pentagonal axis towards the periphery of the PNNP. An energetic balance and the possibility of realization for each of these channels are discussed.



Auf der Grundlage elastischer Kontinuumsmodelle werden Relaxationskanale der elastischen Spannungen im Inneren von pentagonalen nadelformigen Nanopartikeln (PNNP) untersucht. Sechs Modelle werden analysiert: 1. Bildung einer Versetzung im PNNP, 2. Offnung einer Lucke im PNNP, 3. Bildung einer an der PNNP-Oberflache gebundenen Disklination, 4. Aufspaltung der pentagonalen Achse, 5. Wachstum eines einkristallinen Bereiches um die PNNP-Achse, 6. Verschiebung der pentagonalen Achse zur Peripherie der PNNP. Ein Energieausgleich und die Moglichkeit fur die Realisierung jedes dieser Kanale werden diskutiert.

V. G. Gryaznov

Papers

Pentagonal Symmetry and Disclinations in Small Particles

Size effects of dislocation stability in nanocrystals

On the yield stress of nanocrystals

Channels of Relaxation of Elastic Stresses in Pentagonal Nanoparticles

Size effect of dislocation stability in small particles and microcrystallites