H. W. Sheng

Atomic packing and short-to-medium-range order in metallic glasses

Unlike the well-defined long-range order that characterizes crystalline metals, the atomic arrangements in amorphous alloys remain mysterious at present. Despite intense research activity on metallic glasses and relentless pursuit of their structural description, the details of how the atoms are packed in amorphous metals are generally far less understood than for the case of network-forming glasses. Here we use a combination of state-of-the-art experimental and computational techniques to resolve the atomic-level structure of amorphous alloys. By analysing a range of model binary systems that involve different chemistry and atomic size ratios, we elucidate the different types of short-range order as well as the nature of the medium-range order. Our findings provide a reality check for the atomic structural models proposed over the years, and have implications for understanding the nature, forming ability and properties of metallic glasses.

Microstructure evolution and thermal properties in nanocrystalline Fe during mechanical attrition

The microstructural evolution and thermal properties of nanocrystalline (nc) Fe during mechanical attrition were investigated by using quantitative X-ray diffraction and thermal analysis techniques. Upon milling of the Fe powders with coarse grains, grain refinement takes place gradually and a steady-state grain size in the nanometer regime is reached after a certain period of milling. With the further milling of the nc Fe within the stage of the steady-state grain size, we observed a grain boundary relaxation process that was manifested by evident decreases in the thermal expansion coefficient and the stored enthalpy, as well as slight decreases in the lattice strain and the Debye-Waller parameter. The grain boundary enthalpy of the nc Fe was estimated, showing a decreasing tendency with the milling time. The present work indicated with clear experimental evidence that the nc materials with the same grain size may exhibit very different properties that depend upon the microstructure of the numerous metastable grain boundaries

Superheating and melting-point depression of Pb nanoparticles embedded in Al matrices

Two kinds of Pb-Al granular sample (with nanometre-sized Pb particles embedded in an Al matrix) were prepared by using melt-spinning and ball-milling techniques. The Pb particles were synthesized with similar particle sizes, about 5-30 nm in diameter, but were observed to have different Pb-Al interfaces for samples prepared using different approaches. In the melt-spun Pb-Al samples, the Pb nanoparticles were seen to have an epitaxial relationship with the Al matrix, while the Pb particles in the ball-milled sample were seen to be randomly oriented with respect to the Al matrix. The melting behaviour of the Pb particles was investigated by differential scanning calorimetry. It was found that the faceted Pb nanoparticles in the melt-spun sample could be superheated by about 11-40 K, whereas the irregularly shaped Pb particles in the ball-milled sample melted about 13 K below its equilibrium point. We suggest that different interface structures, which cause variations in energy differences between solid-Pb-solid-Al and liquid-Pb-solid-Al interfaces, are responsible for the different melting behaviour of Pb nanoparticles.

Long-Range Topological Order in Metallic Glass

The pressure-induced formation of a single crystal reveals the metallic glass Ce75Al25 to have long-range ordering. Glass lacks the long-range periodic order that characterizes a crystal. In the Ce75Al25 metallic glass (MG), however, we discovered a long-range topological order corresponding to a single crystal of indefinite length. Structural examinations confirm that the MG is truly amorphous, isotropic, and unstrained, yet under 25 gigapascals hydrostatic pressures, every segment of a centimeter-length MG ribbon devitrifies independently into a face-centered cubic (fcc) crystal with the identical orientation. By using molecular dynamics simulations and synchrotron x-ray techniques, we elucidate that the mismatch between the large Ce and small Al atoms frustrates the crystallization and causes amorphization, but a long-range fcc topological order still exists. Pressure induces electronic transition in Ce, which eliminates the mismatch and manifests the topological order by the formation of a single crystal.

Melting and freezing behavior of embedded nanoparticles in ball-milled Al–10 wt% M (M=In, Sn, Bi, Cd, Pb) mixtures

Abstract Nanometer-sized In, Sn, Bi, Cd, and Pb particles were homogeneously embedded in an Al matrix through ball milling of powder mixtures of pure immiscible elements. The melting and freezing behavior of the embedded nanoparticles were systematically investigated using differential scanning calorimetry. It was observed that the melting temperature as well as the latent heat of fusion of the embedded particles decreased significantly relative to their bulk values. Both the melting point depression and heat of fusion reduction are inversely proportional to the size of the embedded particles. The melting behavior is interpreted using a thermodynamic model and found to depend not only on the particle size but also on the structure and excess enthalpy of the particle/matrix interface. Solidification of the nanoparticles, presumably via heterogeneous solid nucleation at the Al/particle interfaces, exhibited significant undercooling. The degree of undercooling increased with decreasing particle size. A thermodynamic analysis based on the classical nucleation theory indicates that the large undercoolings observed result from the energy barrier for solid nucleation. The effect of particle size on undercooling is attributed to the size dependence of the melting point.

The competing crystalline and amorphous solid solutions in the Ag–Cu system

Abstract Upon nonequilibrium processing using vapor quenching or mechanical alloying, the supersaturated fcc solid solution predominates over the amorphous solution in the Ag–Cu system. The thermodynamic and kinetic origins of this phase selection are explored. The enthalpy of formation of both solutions has been determined as a function of composition using calorimetry measurements and molecular dynamics (MD) simulations. The enthalpy of the fcc solution is found to be lower than that of the competing amorphous phase. The preference for the fcc crystalline state is enhanced by the low kinetic barrier to crystallization of the amorphous solution, which occurred during quenching even when high cooling rates were employed or during annealing at low temperatures. Consequently, the retention of an amorphous Ag–Cu solution required kinetic trapping using ultrahigh quenching rates achievable only under stringent vapor deposition conditions or in MD simulations. However, transmission electron microscopy revealed the presence of local regions of amorphous Ag–Cu after cold rolling of multilayers of elemental Ag and Cu foils at room temperature. This result of partial amorphization demonstrates the possibility of mechanically driven solid-state amorphization in a system with a positive heat of mixing.

Alloying strongly influences the structure, dynamics, and glass forming ability of metallic supercooled liquids

The addition of a relatively small amount of alloying element(s) can induce major changes in the viscosity, fragility, and glass forming ability of supercooled liquids. A microscopic understanding of this behavior from the structural perspective has been elusive. Through comparisons between Cu–Zr–Al and Cu–Zr supercooled liquids, here we demonstrate the strong effects of Al alloying on the atomic-scale structure, in particular, the evolution of icosahedral local motifs, as well as the resulting dramatic slowing down of relaxation dynamics. The composition-structure-dynamics relationship uncovered for realistic bulk metallic glass forming liquids is important for understanding the glass transition and glass forming ability.

Ab initio and atomistic study of generalized stacking fault energies in Mg and Mg-Y alloys

Magnesium-yttrium alloys show significantly improved room temperature ductility when compared with pure Mg. We study this interesting phenomenon theoretically at the atomic scale employing quantum-mechanical (so-called ab initio) and atomistic modeling methods. Specifically, we have calculated generalized stacking fault energies for five slip systems in both elemental magnesium (Mg) and Mg-Y alloys using (i) density functional theory and (ii) a set of embedded-atom-method (EAM) potentials. These calculations predict that the addition of yttrium results in a reduction in the unstable stacking fault energy of basal slip systems. Specifically in the case of an I2 stacking fault, the predicted reduction of the stacking fault energy due to Y atoms was verified by experimental measurements. Wefind a similar reduction for the stable stacking fault energy of the {11¯

Epitaxial dependence of the melting behavior of In nanoparticles embedded in Al matrices

Nanometer-sized In particles (5-45 nm) embedded in the Al matrix were prepared by using melt-spinning and ball-milling techniques. Different crystallographic orientationships between In nanoparticles and the Al matrix were constructed by these two approaches. Melting behavior of the In particles were investigated by means of differential scanning calorimetry (DSC). It was found that the epitaxially oriented In nanoparticles (with the Al matrix) in the melt-spun sample were superheated to about 0-38 degrees C, whereas the randomly oriented In particles in the ball-milled sample melted below its equilibrium melting point by about 0-22 degrees C. We suggest that the melting temperature of In nanoparticles can be either enhanced or depressed, depending on the epitaxy between In and the Al matrix.

Nanocrystalline grain structures developed in commercial purity Cu by low-temperature cold rolling

Nanocrystalline pure copper was obtained by cold rolling a commercial bulk Cu to very large extensions at subambient temperatures. The eventual formation of nanocrystalline grain structures is attributed to dynamic grain refinement (recrystallization) mechanisms activated by the low-temperature continuous plastic deformation that leads to ultrahigh densities of dislocations.