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

The enthalpies of mixing of liquid FeCoTi alloys have been measured calorimetrically for the section Fe 0.9 Co 0.1 -Ti at a temperature of 1600°C. The liquid Fe 0.9 Co 0.1 -Ti alloys show negative mixing enthalpies, which are more negative than those of liquid binary FeTi alloys because of the strong interaction between the Co and Ti atoms. The enthalpy data have been fitted by a least-squares method to a modified power series. Comparing the results from the calorimetric measurements with those from calculation by means of different algorithms of extrapolation from the binary boundary systems, the so-called Hillert-2 geometry turned out to the most suitable geometry for interpolation of the ternary FeCoTi system. In consequence, by use of this geometry the mixing enthalpy of liquid FeCoTi alloys has been calculated in the entire ternary system. Beyond the concentration range where experiments could be performed, the results of the calculation could be taken as a good approximation.

Mixing enthalpy of liquid FeCoTi alloys

Crystallization from the amorphous state to metastable crystalline Al(6)Fe was induced by thermal annealing of mechanically alloyed Al(90)Fe(10) composite powders (where the composition is given in atomic per cent), which consisted of fcc Al nanocrystals in an amorphous matrix. The microstructural evolution and the transformation kinetics were characterized in detail. A thermodynamic analysis of the transformation was given, showing the nature of the polymorphous crystallization in the amorphous composite. The influence of the pre-existing crystals on crystallization was not morphologically distinctive. The observed linear kinetics law behaviour in combination with an Avrami exponent value of 2.7 +/- 0.1 suggest that the growth of a fixed nuclei population at a constant rate in three dimensions governs crystallization. The activation energy for crystallization was found to be close to the activation energy for the diffusion of Fe atoms in Al.

Amorphous-to-crystalline transformation induced by thermal annealing of a metastable Al90Fe10 composite

A one-dimensionally periodic pentagonal quasicrystal with a superstructure has been found in the Al-Co-Ni system in a sample quenched from 1160oC at the composition Al71.5Co25.5Ni3. Its electron diffraction patterns are characterized by a fivefold rotation axis as a unique symmetry element as well as superstructure reflections similar to those of a related decagonal phase. The presence of this symmetry in electron diffraction patterns originates from strong dynamical scattering effects of electrons in relatively thick regions of the sample, in combination with the unique parallel arrangement of fivefold symmetric atom clusters. Differently, the superstructure reflections are caused by the long-range order of this new state which can be described by a random variant of a Penrose rhomb tiling.

Pentagonal Al-Co-Ni quasicrystal with a superstructure

It is shown that the Socolar tiling, which is quasiperiodic and 12-fold symmetric, can be characterized as the unique tiling which is maximally covered by a suitably pair of clusters. Analogous results can be obtained also for other dodecagonal tilings, among them the shield tiling.

Dodecagonal tilings as maximal cluster coverings

Zur algebraischen Beschreibung temperaturabhängiger Mischungsenthalpien flüssiger Legierungen

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