Nanocrystalline Structure

[Keri Gamrath]

Ananocrystalline (NC) material is a polycrystalline material with a crystallite size of only a few nanometers. These materials fill the gap between amorphous materials without any long range order and conventional coarse-grained materials. Definitions vary, but nanocrystalline material is commonly defined as a crystallite (grain) size below 100 nm. Grain sizes from 100 to 500 nm are typically considered "ultrafine" grains.

Nanocrystalline materials can be prepared in several ways. Methods are typically categorized based on the phase of matter the material transitions through before forming the nanocrystalline final product.

Solid-state processes do not involve melting or evaporating the material and are typically done at relatively low temperatures. Examples of solid state processes include mechanical alloying using a high-energy ball mill and certain types of severe plastic deformation processes.

Nanocrystalline metals can be produced by rapid solidification from the liquid using a process such as melt spinning. This often produces an amorphous metal, which can be transformed into an nanocrystalline metal by annealing above the crystallization temperature.

The exceptional yield strength of nanocrystalline metals is due to grain boundary strengthening, as grain boundaries are extremely effective at blocking the motion of dislocations. Yielding occurs when the stress due to dislocation pileup at a grain boundary becomes sufficient to activate slip of dislocations in the adjacent grain. This critical stress increases as the grain size decreases, and these physics are empirically captured by the Hall-Petch relationship,

where σ y \displaystyle \sigma _y is the yield stress, σ 0 \displaystyle \sigma _0 is a material-specific constant that accounts for the effects of all other strengthening mechanisms, K \displaystyle K is a material-specific constant that describes the magnitude of the metal's response to grain size strengthening, and d \displaystyle d is the average grain size.[6] Additionally, because nanocrystalline grains are too small to contain a significant number of dislocations, nanocrystalline metals undergo negligible amounts of strain-hardening,[5] and nanocrystalline materials can thus be assumed to behave with perfect plasticity.

Due to the large amount of interfacial energy associated with a large volume fraction of grain boundaries, nanocrystalline metals are thermally unstable. In nanocrystalline samples of low-melting point metals (i.e. aluminum, tin, and lead), the grain size of the samples was observed to double from 10 to 20 nm after 24 hours of exposure to ambient temperatures.[5] Although materials with higher melting points are more stable at room temperatures, consolidating nanocrystalline feedstock into a macroscopic component often requires exposing the material to elevated temperatures for extended periods of time, which will result in coarsening of the nanocrystalline microstructure. Thus, thermally stable nanocrystalline alloys are of considerable engineering interest. Experiments have shown that traditional microstructural stabilization techniques such as grain boundary pinning via solute segregation or increasing solute concentrations have proven successful in some alloy systems, such as Pd-Zr and Ni-W.[7]

While the mechanical behavior of ceramics is often dominated by flaws, i.e. porosity, instead of grain size, grain-size strengthening is also observed in high-density ceramic specimens.[8] Additionally, nanocrystalline ceramics have been shown to sinter more rapidly than bulk ceramics, leading to higher densities and improved mechanical properties,[5] although extended exposure to the high pressures and elevated temperatures required to sinter the part to full density can result in coarsening of the nanostructure.

While the synthesis of nanocrystalline feedstocks in the form of foils, powders, and wires is relatively straightforward, the tendency of nanocrystalline feedstocks to coarsen upon extended exposure to elevated temperatures means that low-temperature and rapid densification techniques are necessary to consolidate these feedstocks into bulk components. A variety of techniques show potential in this respect, such as spark plasma sintering[9] or ultrasonic additive manufacturing,[10] although the synthesis of bulk nanocrystalline components on a commercial scale remains untenable.

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We discuss the grain-size measurements made during shock compression using in situ x-ray diffraction. Our experiments have shown unambiguously that single-crystal iron shock loaded above 13 GPa along the [100] direction will transform from the ambient α phase (bcc) to a highly ordered polycrystalline ϵ phase (hcp). Here, we present a detailed shape analysis of the diffraction peaks using a modified Warren-Averbach method to quantify the microstructure of shock-compressed high-pressure iron. The ϵ phase was determined through this method to have grain sizes between 2 and 15 nm, in reasonable agreement with results from large-scale molecular-dynamics simulations. We conclude that single-crystal iron becomes nanocrystalline in shock transforming from α to ϵ phase.

The present article reviews the current status of research and development on the structure and properties of nanocrystalline materials. Nanocrystalline materials are polycrystalline materials with grain sizes of up to about 100 nm. Because of the extremely small dimensions, a large fraction of the atoms in these materials is located at the grain boundaries, and this confers special attributes. Nanocrystalline materials can be prepared by inert gas-condensation, mechanical alloying, plasma deposition, spray conversion processing, and many other methods. These have been briefly reviewed.

A clear picture of the structure of nanocrystalline materials is emerging only now. Whereas the earlier studies reasoned out that the structure of grain boundaries in nanocrystalline materials was quite different from that in coarse-grained materials, recent studies using spectroscopy, high-resolution electron microscopy, and computer simulation techniques showed unambiguously that the structure of the grain boundaries is the same in both nanocrystalline and coarse-grained materials. A critical analysis of this aspect and grain growth is presented.

The properties of nanocrystalline materials are very often superior to those of conventional polycrystalline coarse-grained materials. Nanocrystalline materials exhibit increased strength/hardness, enhanced diffusivity, improved ductility/toughness, reduced density, reduced elastic modulus, higher electrical resistivity, increased specific heat, higher thermal expansion coefficient, lower thermal conductivity, and superior soft magnetic properties in comparison to conventional coarse-grained materials. Recent results on these properties, with special emphasis on mechanical properties, have been discussed.

New concepts of nanocomposites and nanoglasses are also being investigated with special emphasis on ceramic composites to increase their strength and toughness. Even though no components made of nanocrystalline materials are in use in any application now, there appears to be a great potential for applications in the near future. The extensive investigations in recent years on structure-property correlations in nanocrystalline materials have begun to unravel the complexities of these materials, and paved the way for successful exploitation of the alloy design principles to synthesize better materials than hitherto available.

Copyright: 2017 Dasan et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: Dr. Beh Hoe Guan have received funding from Ministry of Higher Education Malaysia (MOHE) the project under the Fundamental Research Grant Scheme (FRGS) with cost center number of 0153AB-I50. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Ni-Zn spinel ferrites nanoparticles found to be one of the versatile ferrites from the view of technological applications with high resistivity and low eddy current loss [10]. Zinc ferrite bulk material possesses normal spinel structure with their divalent and trivalent cations are located on the tetrahedral and octahedral sites respectively. On the other hand, Nickel ferrite bulk materials have a spinel structure, where the trivalent cations occupy the A and B sites equally while all the divalent cations move to the octahedral site. Generally, this type of ferrites is called as a hopping semiconductor, where their conducting properties are the thermally activated hopping of electrons from one to another cation [10]. There is numerous attempt have been made in order to enhance the qualities of ferrites by incorporating the same suitable nonmagnetic/diamagnetic impurities with different valence state at the A and B sites includes Copper [11, 12], Manganese [13], Praseodymium [14], Lanthanum [15], Neodimium [16] ion and etc [17]. Lanthanum is known as the second most abundant and lightest rare earth element (REE) in the lanthanide series. This silvery white mineral found in monazite and bastnasite ores. Lanthanum possesses distinct quality as compared to other REE such as simple electronic spectra which is helpful for experimental analysis; it has the highest boiling point and lowest vapour pressure at its melting point; and at atmospheric pressure lanthanum is the only superconducting REE [18]. Therefore, lanthanum is demand for some important application includes a) used in the manufacture of expensive glasses as lanthanum imparts a high refractive index to the glass [19]; b) used in NiMH batteries that are currently used in almost all hybrid-electric vehicles [20, 21]; c) Lanthanum rich compounds are used in alloy and fluid cracking catalyst for petroleum refinery industry [22]

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