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

Nanoparticles are discrete nanometer (10−9 m)-scale assemblies of atoms. Thus, they have dimensions between those characteristic of ions (10−10 m) and those of macroscopic materials. They are interesting because the number of atoms in the particles is small enough, and a large enough fraction of them are at, or near surfaces, to significantly modify the particle’s atomic, electronic, and magnetic structures, physical and chemical properties, and reactivity relative to the bulk material. Nanoparticle surfaces themselves may be distinctive. Particles may be terminated by atomic planes or clusters that are not common, or not found, at surfaces of the bulk mineral. These, and other size-related effects will lead to modified phase stability and changes in reaction kinetics.

What makes a nanoparticle a nanoparticle? Definitions of the size ranges for molecules, nanoparticles, and macroscopic solids must be compound specific. However, a useful upper limit for nanoparticles is the size at which one of its properties deviates from the value for the equivalent bulk material by an amount that is significantly larger than the error of the method used to make the measurement (a few percent). In practice, some characteristic will probably be different enough to warrant description as a “nanoparticle” if it is less than a few tens of nanometers in diameter, and perhaps less than a fraction of a micron in diameter.

Because of the importance of size-dependent property changes to the materials sciences, size-property relationships have been studied in detail for some systems. For example, for semiconductors, size effects become important when the particle diameter is close to the Bohr diameter of excitons in the bulk phase. Generally, semiconductor size quantization effects (relevant for naturally occurring metal sulfides, for example) appear when particles are less than 10 nm in diameter (Vogel and Urban 1997).

Definition of the …

Nanoparticles in the Environment

Calorimetric measurements of the glass transition are presented for metallic glasses, ${\mathrm{B}}_{2}$${\mathrm{O}}_{3}$, and organic polymers with cooling and heating rates ranging over more than three orders of magnitude. Fits of the onset temperatures of the glass transition to a Vogel-Fulcher-type equation give divergence temperatures significantly above the values obtained for Vogel-Fulcher fits to the viscosity. The width of the glass transition extrapolates to a sharp step transition at a finite heating and cooling rate, which is close to the range where the data were taken. The results suggest that the glass transition becomes independent of the experimental time scale in a regime that may be accessible to measurements.

Glass transition on long time scales

Microstructure, fabrication and properties of quasicrystalline Al–Cu–Fe alloys: a review

Aluminum 5083 powder was mechanically milled under liquid nitrogen to achieve a nanocrystalline grain size in the range of 20 to 30 nm. The powder was subsequently sprayed using a nozzle designed with a validated numerical model for cold spray technology. The resulting coatings were evaluated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), micro- and nanoindentation. The TEM analysis shows that the nanocrystalline grain structure of the cryomilled feedstock powder was retained after the cold spray process. A significant increase in hardness from 104 to 261±8 (HV300g) was observed when comparing the nanocrystalline coating with cast, cold-worked, Al 5083. The ability to use cold spray to produce nanocrystalline large deposits was also demonstrated in this work.

https://link.springer.com/content/pdf/10.1007%2Fs11661-005-0099-y.pdf

Cold spray deposition of nanocrystalline aluminum alloys

Magnetothermal analysis (MTA), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD) were employed to investigate the crystallization behavior of amorphous Fe82P18. Three transition stages with distinct microstructures were identified: lamellae of the supersaturated intermetallic compound Fe3P with about 5-nm width and varying composition are the result of the first transition (stage I); the growth of polygonal grains of about 100-nm size from the lamellae marks the second transition (stage II); and in the final stage III, grains of α-Fe are precipitated. The lamellar structure is formed by one-dimensional growth, as concluded from the Avrami exponent n, which was found to lie in the range 1.32≤n≤1.72. Analysis of the transformation kinetics from the characteristic isothermal transformation curves confirmed that the crystallization process depends very sensitively on the heat-treatment temperature.

Crystallization of amorphous Fe82P18

Matching rules for the Penrose tiling can be derived in different ways. One way is the decoration by Ammann bars. In the three-dimensional case, these bars have to be replaced by Ammann planes. The vertices of the P1 (Penrose pentagon) tiling coincide with the vertices of the projections of the icosahedral Penrose lattice along the five-fold axes. The procedures of inflation and deflation of the P1 tiling are well understood. Therefore the deflation tiles of the next generation can be derived in the projection along the five-fold axes. If these tiles are decorated by Ammann planes according to the decoration of the projection and if the obstructions are obeyed, matching rules can be derieved. The different possibilities of decorations in the procedure of deflation of large bricks are enumerated.

Matching rules for the icosahedral penrose tiling derived by inflation-deflation procedures

Electronic properties of dilute Cr in liquid silver

Fe80P20 amorphous alloy, amorphous Fe–P–C, and amorphous Fe–P strengthened with embedded carbon nanotubes were fabricated by the rapid solidification process. Magnetothermal analysis, high-resolution transmission electron microscopy, and x-ray diffraction were employed to investigate the crystallization behavior of these amorphous alloys. Carbon nanotubes embedded in the amorphous matrix increases the apparent crystallization temperature by about 100 K and modifies the crystallization process compared to those of amorphous Fe–P and amorphous Fe–P–C. The role of the added carbon nanotubes will be discussed.

Crystallization behavior of amorphous Fe–P strengthened with embedded carbon nanotubes

Quasicrystals are usually more or less chemically ordered. If there is a long-range order, phasons have to be regarded in the quasilattice as well as in the quasi-sublattices. Depending on the lattice and its chemical order, an atomic flip of a moving phason is connected with the requirement of the conservation of the atomic species or of a change in the site occupation. In this manner, phasons in quasilattices and quasi-sublattices can produce deviations from the ideal stoichiometry obstruct the annealing out of phasons and favour the condensation of phasons in walls and, in consequence, the formation of periodic approximants.

Phasons in chemically ordered quasicrystals

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