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Ab initio molecular dynamics simulations are performed to investigate the atomic structures and dynamics of Ni

Nanotechnology offers the potential for the creation of new products, but also the possibility to upgrade conventional technologies. As nanotechnologies become increasingly embedded in products and processes, their integration with the micro-scale is becoming critically important. This has many challenges both in understanding the fundamental scientific principles that underlie the integration, and also in the industrial realisation of components that exploit nanoscale phenomena. As an example, nanocrystalline diamond layers grown by CVD on appropriate substrates—silicon single crystals—combine the remarkable properties of conventional diamond, such as extreme hardness and wear resistance, a surface roughness in the range of a few nanometers and a negligibly small coefficient of friction of about 1 %. The films and layers consist of ultra small equiaxed grains of typically ~10 nm diameter with a semi-transparent appearance and a remarkable stability up to elevated temperatures. As such, they exhibit a considerably improved lifetime and reliability of tooling components and others. Moreover, by a sophisticated combination of photolithographic techniques and efficient reactive ion etching (RIE) processes established in microelectronics and silicon based microfabrication complex shaped diamond microparts and components can be designed and fabricated. A number of different applications, such as high precision lubrication- and wear-free microcomponents, micromechanical sensors and bio-applications are discussed.

Mechanical attrition, mechanical alloying and other methods of extreme plastic deformation (high pressure torsion, equal channel angular pressing) have been developed as versatile alternatives to other physical and chemical processing routes in preparing nanophase materials. Here several examples are discussed including the deformation-induced nanophase formation in powder particles, in thin-foil sandwich structures and at the surface of alloys exposed to friction-induced wear, leading to the formation of nanocrystals and, in some cases, amorphous nanostructures. This opens exciting perspectives in preparing nanostructured materials with a number of different interface types in terms of structure (crystalline/crystalline, crystalline/amorphous) as well as atomic bond (metal/metal, metal/semiconductor, metal/ ceramic etc.). It is expected that the study of nanostructure formation by mechanical means in the future not only opens new processing routes for a variety of advanced nanophase materials but also improves the understanding of technologically relevant deformation processes on a nanoscopic level.

When heating over 300--800 K in a calorimeter, ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}{\mathrm{BH}}_{x},$ $xl~5,$ microcrystals desorb the H atoms in six irreversible endothermic signals. They appear at temperatures ${T}_{p}\ensuremath{\sim}340,$ 439 (doubly degenerate), 567, 605, and 653 K at the heating rate $\ensuremath{\beta}=15\mathrm{K}/\mathrm{m}\mathrm{i}\mathrm{n},$ following the modified Kissinger relation $\mathrm{ln}{(T}_{p}/\ensuremath{\beta}{)=(E}_{a}{/R)T}_{p}^{\ensuremath{-}1}+\mathrm{const},$ with $R$ the gas constant. The 340 signal has an extremely weak intensity at a slow heating, $\ensuremath{\beta}l~40\mathrm{K}/\mathrm{m}\mathrm{i}\mathrm{n},$ because the H atoms in the involved interstitial site in the sample slowly tunnel to neighboring higher-transition-energy sites. The thermogram at 439 K is the most prominent. It contains two signals which could be resolved by selective isothermal desorptions. These six different identified thermal signals are assigned to the desorptions of the H atoms from six specific $4c,$ $16k2,$ $16k1,$ $4e,$ $8j2,$ and $8j1$ crystallographic interstitial sites (between the Fe atoms) in the sample, characterized by six different activation energies ${E}_{a}$ between 48 and 123 kJ/mol, taking into account their H occupancies ${n}_{i}(\mathrm{H}).$ The distribution of partial enthalpies in the thermal signals (at $\ensuremath{\beta}=15\mathrm{K}/\mathrm{m}\mathrm{i}\mathrm{n}$) determines ${n}_{i}(\mathrm{H})\ensuremath{\cong}0,$ 6, 5, 2, 5, and 2 H atoms in the respective sites per ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}{\mathrm{BH}}_{x},$ $x\ensuremath{\sim}5,$ crystal unit cell with a total of $4x=20\mathrm{H}$ atoms. It is found that a significant portion of the thermally excited H atoms in these sites, in the process to the desorption, redistributes over neighboring sites of modified energies to keep the H atoms at high temperatures. The redistribution reaction is exothermic. It has been observed separately in $8j2$ and $8j1$ sites at 616 and 684 K (at $\ensuremath{\beta}=50\mathrm{K}/\mathrm{m}\mathrm{i}\mathrm{n}$) in a partially H-desorbed sample in the lower-transition-energy sites by heating it at 583 K. A local redistribution of the interatomic distances and/or the electronic charges occurs within the lattice following the thermal desorption of the H atoms. It results in a monotonically decreasing exothermic (structural relaxation) signal of the rate of the change of the enthalpy, $(\ensuremath{\partial}H/\ensuremath{\partial}{t)}_{T},$ with time $t,$ after the desorption at $t=0,$ following the primary endothermic desorption signal. The results are discussed with simulations of related processes.

A herringbone nanostructured Mn-rich Heusler Mn50Ni50-Sndelta (8 - 9) alloy exhibits tailored magnetocaloric properties in the martensite and ferro-paramagnetic transitions concur in a narrow temperature window. In a Sn --Ni substitution 8 - 9, the martensite (M)-- austenite (A) transition up-lifts adequately well above the room temperature - 310.5 K in the DSC thermogram and magnetization scanned with temperature. A noninterrupted heating following a cooling in DSC at a given rate gives a smaller enthalpy change deltaH(M-- A) - deltaH(M --A approximately equal to 282 mJ/g (deltaC(P)(M-- A) - deltaC(P)(M --A) approximately equal to 0.025 mJ/g-K in the heat capacity), i.e., the M-A transition process lacks a complete reversibility. Warming a zero-field cooled sample retains lower magnetization (sigma) at low fields B, e.g., by 58% over the field cooled value at 5 mT, wherein merely low field magnetic susceptibility imparts the magnetization process. A reversible thermal hysteresis thus the transition traces in cooling and heating. The field diminishes difference in two sigma-values progressively, e.g., only - 12% lasts at 5 T. The two curves bifurcate below 160 K (B-5 mT) and the gap grows exponentially over lower temperatures before sigma(M-- A) gets steady near 60 K in a superparamagnetic (SPM) behavior. The SPM feature (follows the Langevin model) below a paramagnetic regime begins (or = 250 K) before a ferromagnetic A-state lines-up the successive transitions. Temperature and frequency dependence ac and dc susceptibilities describe the surface spins dynamics.

The equilibrium viscosity of the Pd43Ni10Cu27P20 bulk metallic glass-forming liquid was measured over a wide temperature range from the equilibrium supercooled liquid state to the glass transition region using parallel-plate rheometry and three-point beam bending. Based on the measured viscosity data, the fragility of this liquid was quantitatively determined. The Pd43Ni10Cu27P20 alloy, despite exhibiting the best glass-forming ability reported thus far, is relatively fragile compared with other bulk glass-forming liquids, such as Vit 1 and Vit 4.

The specific heat and the related thermodynamic functions of highly undercooled liquids are essential in analyzing the nucleation kinetics during crystallization and glass formation from the liquid or solid state. However, for metals and alloys the measured data are very scarce and in general limited to low-melting point systems. For alloys which exhibit higher melting points, such data generally do not exist. Thus, in order to measure specific heat of metals and alloys at elevated temperatures a new modulation method is being developed. The basic principle for this measurement and preliminary data on solid samples are discussed.

Strain-induced cementite dissolution is a well-documented phenomenon, which occurs during the cold plastic deformation of pearlitic steels. Recently new results that can shed additional light on the mechanisms of this process were obtained thanks to atom probe tomography investigations of pearlitic steel deformed by highpressure torsion (HPT). It was shown that the process of cementite decomposition starts by carbon depletion from the carbides due to defect motion; once enough carbon is robbed from the carbide it is thermodynamically destabilized resulting in rapid break-up. Additionally, it was shown that the carbon atoms do not really dissolve in the ferrite but that they segregate to the dislocations and grain boundaries of nanocrystalline ferrite.

The physical properties of metal oxides are diverse, including semiconductors or insulators (ZnO, ZrO2, TiO2), good metals (RuO2), and metal-insulator systems (VO2) in terms of the electron band structure and variation of electrical resistivity (σ) as a function of temperature (T), i.e., thermal coefficient of σ, expressed as In terms of distributing the magnetic spins (of total value S) in the metal cations via O2− anions, a metal oxide behaves to be paramagnetic (C2O3), S ≠0, or diamagnetic (YBa2Cu3O7 and other ceramic superconductors1–5) with S=06–8 Depending on the relative strength of the spin-ordering over thermal effects, a paramagnetic oxide often behaves as a ferrimagnet (γ-Fe2O3 or Fe3O4) or antiferromagnet (FeO or MnO2). Metallic CrO2, with a Curie temperature T C =390 K, is the only ferromagnet in this class. Schwarz9 used local-spin-density-approximation (LSDA) band theory to predict that the S-moment would be the full 2µB required by Hund’s rules for Cr4+ (3d2) state in CrO2. The Fermi levelE F lies in a partly filled (metallic) band for the majority (up-spin)electrons, but for minority (down) spins lies in a semiconductor like energy bandgap aqueous precursor solution of CrO3 or in general a Cr6+ -compound. In the proposed polymer precursor method, the Cr6+ cations ultimately are reduced to Cr4+ cations as soon as admixing to a polymer, whichreacts with Cr6+ to form a metal ion-polymer complex capping in part of the polymer molecules.

This is a first systematic report on the synthesis of completely nanocrystalline metals by high-energy deformation processes. Pure metals with body-centered cubic (bcc) and hexagonal close-packed (hcp) structures are subjected to ball milling, resulting in a decrease of the average grain size to ≈9 nm for metals with bcc and to ≈13 nm for metals with hcp crystal structures. This new class of metastable materials exhibits an increase of the specific heat up to 15 pct at room temperature and a mechanically stored energy determined as up to 30 pct of the heat of fusion after 24 hours of high-energy ball milling. The grain boundary energy as determined by calorimetry is higher than the energy for fully equilibrated high-angle grain boundaries.

A solid-state amorphization reaction is observed on hydrided, initially single-crystalline, Fe2Er powder samples (Fe2ErHx, 3.0 < x

The thermophysical properties of interest for casting simulations are melting range, fraction solid, density (thermal expansion), viscosity, specific heat, Gibbs free enthalpies, diffusion coefficients, thermal (electrical) conductivity, surface tension and emissivity. Some of these data can be obtained more or less accurately by conventional methods. High precision measurements on chemically highly reactive melts and fluids at the temperatures of interest require the application of containerless processing using non-contact diagnostic tools. By eliminating the contact between the melt and a crucible accurate surface nucleation control and the synthesis of materials free of surface contamination become possible. Under microgravity conditions, further advantages are expected from the significantly smaller electromagnetic fields needed to stabilize the containerless melts. This was shown in several Spacelab missions in which the results clearly showed an improvement in accuracy over terrestrial measurement,...

Methods of non-contact calorimetry and their application to the measurement of the thermophysical properties of Zr-based liquid metallic alloys are described. These methods were developed for containerless processing in an electromagnetic levitation device under reduced gravity conditions. The experiments were performed in the course of the IML-2 and MSL-1 spacelab missions. Thermophysical properties measured include the specific heat, the enthalpy of fusion, the total hemispherical emissivity, and the effective thermal conductivity. Future applications will be focused on the measurement of thermophysical properties of liquid metallic alloys relevant to industrial processing.

DESPITE the strong interest in melting over the past 100 years, a general theory for the crystal–liquid transition has not been established1. Lattice-instability models, which are either vibrational2, elastic3, isochoric4, defective5 or entropic6 in nature, all predict a melting point somewhat above the experimentally observed thermodynamic melting temperature, with the ultimate stability limit of a superheated crystal being determined by the equality of crystal and liquid entropies4,6; this forces regular melting to be a first-order transition. Here I present a model of melting that is driven by the incorporation into the lattice of randomly frozenin defects. An isentropic condition limits the stability of the crystal as a function of defect concentration; above the glass transition temperature the crystal melts to a liquid, whereas below it 'melting' produces an amorphous solid. This model yields a generic melting diagram with a tunable parameter (defect concentration) that can characterize the static disorder present in solid-state amorphization7–9, the thermodynamic stability of small clusters10 and nanocrystalline materials11, and the frustration present in spin glasses12. The model is also relevant to glacial13, geological14 and stellar-atmospheric15 melting processes.

The nanoindentation technique is a well established method to determine the mechanical properties (hardness, Young’s modulus) of small volumes of materials or thin films. Results obtained by this technique are usually interpreted and discussed with respect to the classical continuum approach. For example, for an ideal elastic-plastic material hardness and Young’s modulus have to be independent of the indentation depth. This behavior has also been observed for nanostructured niobium. The specimens were produced via mechanical attrition resulting in an average grain size of approximately 10 nm. On the microscopic scale the hardness varies between 4 and 7 GPa. This value is 3–6 times higher than that of polycrystalline niobium. Young’s modulus was determined to be 80±30 GPa which despite the large scatter is smaller than values for polycrystalline Nb (95±15 GPa).

The study of metallic melts, their physical properties, and their solidification behaviour is of relevance to understanding of fundamental principles and to metallurgical applications. Many, if not...

The preparation of nanoscaled materials with a particle size in the range of 2 to 10 nm can be achieved by cyclic mechanical deformation processes, by chemical gas phase reactions or a combination of the above. These processes can also result in the destabilization of the crystalline lattice and the formation of a glass, i.e. a material with a “domain size” of 1.5 to 2 nm. The potential of these processes in preparing materials with novel microstructures and properties, the evolution of the microstructure and the thermal stability of the structure against coarsening and glass formation are discussed. In addition, Mosbauer-spectroscopy data characterizing the magnetic properties of iron-based alloys with a cluster size below and above 2 nm are presented.

We have studied the sequence of phase transformations induced in the Zr-Al binary system by mechanical alloying of mixed Zr and Al powders. The structure of these materials has been studied by transmission electron microscopy and by x-ray diffraction measurements. Three different metastable phases have been found experimentally with variation of the initial composition xAl: (1) a nanocrystalline supersaturated solid solution of alpha-Zr for xAl

For the thermodynamic description of solid-state amorphization processes in binary alloy systems the T 0 concept has been combined with the isentropic condition for glass formation from the undercooled liquid and extended to the pressure variable. A polymorphous Clapeyron diagram for melting and glass transition is developed which displays a triple point between supersaturated crystal, undercooled liquid and glass. As an example, the polymorphous phase diagram has been calculated for the Ni-Zr alloy system based on available thermodynamic data. A triple point is found at 11·5at, % Ni and 638 K at which the crystal-to-glass transition becomes continuous in entropy.

We report on high-energy ball milling of Ru and AlRu. The deformation results in a drastic decrease of the crystal size to a nanometer scale and in an increase of atomic-level strain. This is accompanied by a disordering of the crystal lattice as is shown by means of the long-range-order parameter in AlRu. The specific heat increases by more than 15%–20%, indicating large changes in the vibrational and configurational part of the entropy. The stored energy of cold work is up to 6 kJ/mol for AlRu and 10 kJ/mol for Ru. This is almost 40% of the heat of fusion of Ru and exceeds by far the energies stored by other deformation processes.

Several metastable phase boundaries in the Pb-Sn alloy system have been determined in undercooled droplets containing different phase mixtures by means of X-ray diffraction lattice parameter and thermal analysis measurements. With droplets equilibrated as (α-Pb+L) mixtures, the metastable α-Pb solidus has been determined to reach 46 at. pct Sn with a corresponding liquidus at 87 at. pct Sn for a temperature 73°C below that for the stable eutectic. An extension of the metastable liquidus and solidus boundaries implies a melting of fcc Sn at about 5°C which agrees well with thermodynamic model calculations. With some solidification treatments a metastable, intermediate, body-centered tetragonal α1 phase can form in alloys with compositions between 35 and 73 at. pct Sn. Upon heating, the α1-phase decomposes to a supersaturated α-Pb phase and liquid by a peritectic reaction at 127°C. Calorimetric measurements yield an enthalpy of ΔHα1 = 3.16 ± 0.3 kJ/g atom for the α1/(α-Pb + L) peritectic reaction. The combination of the measured stable equilibria, the partial metastable equilibria determination, and phase equilibria modeling has allowed for a more complete description of metastable equilibrium behavior and for the analysis of some of the solidification pathways that are available for the synthesis of metastable product structures.

The lattice stability of Sn is reassessed in view of additional pressure-temperature data since the earlier assessment by Kaufman. The metastable phase boundary of the α-Pb phase reported in Part I in connection with available thermochemical data is used to define more accurately the model paraaeters for the stable L, α, β, and metastable α1-phases. Using these models, the stable phase diagram of Pb−Sn, the metastable extension of the stable phase diagram, and several possible metastable phase diagrams are calculated. Thermodynamic and kinetic criteria are used to discuss the formation of various phases during rapid solidification processes.

We have utilized differential scanning calorimetry to study decomposition of a large number (≊105) of independent, metastable, highly supersaturated Sn63Bi37 single crystals. Three different growth regimes are observed: first, t1/2 diffusion‐controlled growth, followed by a t1/3 regime attributed to Lifshitz–Slyozov coarsening [J. Phys. Chem. Solids 19, 35 (1961)] extended to the case of finite volume fraction, and eventually ‘‘freezing’’ of the growth possibly because of defects. The activation energy of the t1/3 growth is determined to be 0.54±0.05 eV.

The transition from a crystalline material with long-range order to a glass-like disordered structure has been observed in several metal alloys and minerals using experimental techniques such as solid-state reaction, mechanical alloying, pressure application, ion-beam mixing and hydriding1–3. In all these examples the vitrification occurs below the glass transition temperature, and because the glass can be thought of as a highly undercooled liquid, one may draw an analogy between the vitrification process and melting. The melting process is often viewed as a catastrophic instability of the crystal lattice4–6, although none of these theories predicts the melting temperature correctly. The transition from crystal to glass could also be triggered by some type of instability7; indeed, Kauzmann8 has argued that an undercooled liquid whose entropy falls below that of the crystalline phase must undergo massive freezing to a glass. Applying an 'inverse' Kauzmann argument to the problem of melting, an entropy catastrophe is predicted when the entropy of a superheated crystal exceeds that of the liquid phase. We propose that this temperature represents an ultimate stability limit for superheated or supersaturated crystals.