In recent years, near-nano (submicron) and nanostructured materials have attracted increasingly more attention from the materials community. Nanocrystalline materials are characterized by a microstructural length or grain size of up to about 100 nm. Materials having grain size of ∼0.1 to 0.3 μm are classified as submicron materials. Nanocrystalline materials exhibit various shapes or forms, and possess unique chemical, physical or mechanical properties. When the grain size is below a critical value (∼10–20 nm), more than 50 vol.% of atoms is associated with grain boundaries or interfacial boundaries. In this respect, dislocation pile-ups cannot form, and the Hall–Petch relationship for conventional coarse-grained materials is no longer valid. Therefore, grain boundaries play a major role in the deformation of nanocrystalline materials. Nanocrystalline materials exhibit creep and super plasticity at lower temperatures than conventional micro-grained counterparts. Similarly, plastic deformation of nanocrystalline coatings is considered to be associated with grain boundary sliding assisted by grain boundary diffusion or rotation. In this review paper, current developments in fabrication, microstructure, physical and mechanical properties of nanocrystalline materials and coatings will be addressed. Particular attention is paid to the properties of transition metal nitride nanocrystalline films formed by ion beam assisted deposition process.

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Nanocrystalline materials and coatings

http://nanomechanics.mit.edu/sites/default/files/documents/kumar_2003.pdf

Deformation of electrodeposited nanocrystalline nickel

Synthesis and mechanical behavior of nanostructured materials via cryomilling

This paper summarizes and reviews the state-of-the-art processing methods, structures and mechanical properties of the metal matrix composites reinforced with ceramic nanoparticles. The metal matrices of nanocomposites involved include aluminum and magnesium. The processing approaches for nanocomposites can be classified into ex-situ and in-situ synthesis routes. The ex-situ ceramic nanoparticles are prone to cluster during composite processing and the properties of materials are lower than the theoretical values. Despite the fact of clustering, ex-situ nanocomposites reinforced with very low loading levels of nanoparticles exhibit higher yield strength and creep resistance than their microcomposite counterparts filled with much higher particulate content. Better dispersion of ceramic nanoparticles in metal matrix can be achieved by using appropriate processing techniques. Consequently, improvements in both the mechanical strength and ductility can be obtained readily in aluminum or magnesium by adding ceramic nanoparticles. Similar beneficial enhancements in mechanical properties are observed for the nanocomposites reinforced with in-situ nanoparticles.

Novel Nanoparticle‐Reinforced Metal Matrix Composites with Enhanced Mechanical Properties

Heterostructured materials are an emerging class of materials with superior performances that are unattainable by their conventional homogeneous counterparts. They consist of heterogeneous zones wi...

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Heterostructured materials: superior properties from hetero-zone interaction

A commercial aluminum alloy, 5083, was processed using a cryomilling synthesis approach to produce powders with a nanostructured grain size. The powders were subsequently degassed, hot isostatically pressed, and extruded. The grain size at each processing step was measured utilizing both X-ray diffraction and transmission electron microscopy (TEM). The mechanical properties of the n-5083 extruded material were determined utilizing ASTM E8-93, Standard Test Methods for Tension Testing of Metallic Materials. This processing technique was found to produce a thermally stable nanostructured aluminum alloy which maintained an average grain size of 30 to 35 nm through several processing steps up to 0.61 T
 mp
 . The thermal stability was attributed to Zener pinning of the grain boundaries by AIN and Al2O3 particles and solute drag of numerous atomic species. The nanostructured 5083 was found to have a 30 pct increase in yield strength and ultimate strength over the strongest commercially available form of 5083, with no corresponding decrease in elongation. The enhanced ductility is attributed to the presence of a few large, single-crystal aluminum grains acting as crack-blunting objects.

Mechanical behavior and microstructure of a thermally stable bulk nanostructured Al alloy

Apfim study of impurities in nanocrystalline Fe-Al alloy

A new approach to preparing tips for atom probe field ion microscopy from powder materials is described. It is especially useful for particle sizes exceeding >0.050 mm. Particles resulting from mechanical alloying of Cu and Ag, and from cryomilling of conventional Ni powders have been prepared this way for field ion imaging and atom probe composition analysis. In both cases, useful information complementing results from transmission electron microscopy and X-ray diffraction was obtained.

A new approach to preparing tips for atom probe field ion microscopy from powder materials

Gas-atomized Ni powders were mechanically milled under liquid nitrogen and subsequently sprayed using a high-velocity oxygen-fuel thermal spraying system. The resultant coatings were evaluated using various analytical methods, including scanning electron microscopy (SEM), transmission electron microscopy, and atom probe field ion microscopy (APFIM). The results indicated that the majority of the grains in nanocrystalline Ni coating were equiaxed with an average grain size of 50 ± 23 nm. SEM elemental dot mapping analysis suggested that oxide particles were distributed along the prior droplet boundaries and interpass boundaries in the coatings. The APFIM analysis indicated that the distribution of oxygen was very inhomogeneous in the nanocrystalline coating samples studied. Mostly small NiO precipitates with a size range of 1–7 nm were found throughout the grains, and the average size of these precipitates was approximately 4 nm. However, large oxide precipitates with a size up to 55 nm were also observed. In addition, increased amounts of oxygen were found at the grain boundaries and at structural defects.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400068333

Microstructural Evolution in Nanocrystalline Ni Coatings

The nature of the steady state reached during ball milling of CuxAg1−x powders (x=35 to 75) is studied as a function of the milling temperature (85K≤T≤503K). The characterization of the powders is performed by using x-ray diffraction, differential calorimetry and atom probe field ion microscopy. A steady-state phase diagram is built. Three-phase coexistence is shown to generally take place at intermediate milling temperatures. Atom probe data reveals that the solid solution stabilized by low milling temperature is nearly random, where as milling at elevated temperatures results in the decomposition of the elements at a lengthscale of 20∼30 nm.

A. J. Melmed

Papers

Mechanical behavior and microstructure of a thermally stable bulk nanostructured Al alloy

Apfim study of impurities in nanocrystalline Fe-Al alloy

A new approach to preparing tips for atom probe field ion microscopy from powder materials

Microstructural Evolution in Nanocrystalline Ni Coatings

Steady State Phase Diagram of Cu−Ag Under Ball Milling: An XRD and APFIM Study