Mechanical properties of nanocrystalline materials

Mechanical alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.

Mechanical Alloying And Milling

Nanostructured materials: basic concepts and microstructure☆

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

50 Years of Image Analysis

Nanocrystalline fcc metals have been synthesized by mechanical attrition. The crystal refinement and the development of the microstructure have been investigated in detail by x-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. The deformation process causes a decrease of the grain size of the fcc metals to 6–22 nm for the different elements. The final grain size scales with the melting point and the bulk modulus of the respective metal: the higher the melting point and the bulk modulus, the smaller the final grain size of the powder. Thus, the ultimate grain size achievable by this technique is determined by the competition between the heavy mechanical deformation introduced during milling and the recovery behavior of the metal. X-ray diffraction and thermal analysis of the nanocrystalline powders reveal that the crystal size refinement is accompanied by an increase in atomic-level strain and in the mechanically stored enthalpy in comparison to the undeformed state. The excess stored enthalpies of 10–40% of the heat of fusion exceed by far the values known for conventional deformation processes. The contributions of the atomic-level strain and the excess enthalpy of the grain boundaries to the stored enthalpies are critically assessed. The kinetics of grain growth in the nanocrystalline fcc metals are investigated by thermal analysis. The activation energy for grain boundary migration is derived from a modified Kissinger analysis, and estimates of the grain boundary enthalpy are given.

Structural and thermodynamic properties of nanocrystalline fcc metals prepared by mechanical attrition

Computer simulation of 3-D grain growth using a phase-field model

Bulk ZnO samples, epitaxially grown ZnO layers, and ZnO nanostructures frequently exhibit a characteristic emission band at $3.31\text{\ensuremath{-}}\mathrm{eV}$ photon energy whose origin is controversially discussed in the literature. Partly, this omnipresent band is ascribed to $(e,{A}^{0})$ transitions of conduction band electrons to acceptors, which are abundant in relatively high concentrations but have not positively been identified. The band is, in particular, often reported after intentional $p$-type doping of ZnO, preferentially with group V species. In the present work, we study the $3.31\text{\ensuremath{-}}\mathrm{eV}$ band by low-temperature cathodoluminescence (CL) with high spatial resolution, by scanning electron microscopy, and by transmission electron microscopy (TEM). Line shape analyses at different temperatures give clear evidence that the band originates from an $(e,{A}^{0})$ transition where the acceptor binding energy is $(130\ifmmode\pm\else\textpm\fi{}3)\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. The $3.31\text{\ensuremath{-}}\mathrm{eV}$ luminescence is exclusively emitted from distinct lines on sample surfaces and cross sections representing intersections with basal planes of the wurtzite hexagons. Correlating monochromatic CL images with TEM images, we conclude that the localized acceptor states causing the $3.31\text{\ensuremath{-}}\mathrm{eV}$ luminescence are located in basal plane stacking faults.

Stacking fault related 3.31 − eV luminescence at 130 − meV acceptors in zinc oxide

Highly supersaturated nanocrystalline FexCu100−x alloys (10≤x≤95) have been prepared by mechanical alloying of elemental crystalline powders The development of the microstructure is investigated by x‐ray diffraction, differential scanning calorimetry, and transmission electron microscopy The results are compared with data for ball‐milled elemental Fe and Cu powders, samples prepared by inert gas condensation, and sputtered films The deformation during milling reduces the grain size of the alloys to 6–20 nm The final grain size of the powders depends on the composition of the material Single‐phase fcc alloys with x≤60 and single‐phase bcc alloys with x≥80 are formed even though the Fe‐Cu system exhibits vanishingly small solid solubilities under equilibrium conditions For 60≤x≤80, fcc and bcc solid solutions coexist The alloy formation is discussed with respect to the thermodynamic conditions of the material The role of the large volume fraction of grain boundaries between the nanometer‐sized cryst

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Carl E. Krill

Papers

Structural and thermodynamic properties of nanocrystalline fcc metals prepared by mechanical attrition

Computer simulation of 3-D grain growth using a phase-field model

Stacking fault related 3.31 − eV luminescence at 130 − meV acceptors in zinc oxide

Mechanically driven alloying and grain size changes in nanocrystalline Fe-Cu powders

Modelling the influence of grain-size-dependent solute drag on the kinetics of grain growth in nanocrystalline materials