Simulation of Cu-Mg metallic glass: Thermodynamics and Structure

We have obtained effective medium theory (EMT) interatomic potential parameters suitable for studying Cu-Mg metallic glasses. We present thermodynamic and structural results from simulations of such glasses over a range of compositions. We have produced low-temperature configurations by cooling from the melt at as slow a rate as practical, using constant temperature and pressure molecular dynamics. During the cooling process we have carried out thermodynamic analyses based on the temperature dependence of the enthalpy and its derivative, the specific heat, from which the glass transition temperature may be determined. We have also carried out structural analyses using the radial distribution function (RDF) and common neighbor analysis (CNA). Our analysis suggests that the splitting of the second peak, commonly associated with metallic glasses, in fact has little to do with the glass transition itself, but is simply a consequence of the narrowing of peaks associated with structural features present in the liquid state. In fact the splitting temperature for the Cu-Cu RDF is well above $T_g$. The CNA also highlights a strong similarity between the structure of the intermetallic alloys and the amorphous alloys of similar composition. We have also investigated the diffusivity in the supercooled regime. Its temperature dependence indicates fragile-liquid behavior, typical of binary metallic glasses. On the other hand, the relatively low specific heat jump of around $1.5 k_B/\mathrm{at.}$ indicates apparent strong-liquid behavior, but this can be explained by the width of the transition due to the high cooling rates.Comment: 12 pages (revtex, two-column), 12 figures, submitted to Phys. Rev.

High glass forming ability correlated with microstructure and hydrogen storage properties of a Mg-Cu-Ag-Y glass

Thermal characterization of an as-cast Mg54Cu28Ag7Y11 bulk metallic glass revealed that this alloy exhibits excellent glass forming ability. High-resolution X-ray diffraction study and transmission electron microscopy show that heating and isothermal annealing treatment results in the nucleation of nanocrystals of several phases. The average size of these nanocrystals (~15-20 nm) only slightly varies with prolonged annealing, only their volume fraction increases. High-pressure calorimetry experiments indicate that the as-cast fully amorphous alloy exhibits the largest enthalpy of hydrogen desorption, compared to partially and fully crystallized states. Since the fully crystallized alloy does not desorb hydrogen, it is assumed that hydrogen storage capacity correlates only with the crystalline volume fraction of the partially crystallized Mg54Cu28Ag7Y11 BMG and additional parameters (crystalline phase selection, crystallite size, average matrix concentration) do not play a significant role

Storing hydrogen in the form of light alloy hydrides

Different hydrides are investigated to find a system with a sufficiently high storage density (at least 3%). The formation of hydrides with light alloys is examined. Reaction kinetics for hydride formation were defined and applied to the systems Mg-Al-H, Mg-Al-Cu-H, Ti-Al-H, Ti-Al-Cu-H, and Ti-Al-Ni-H. Results indicate that the addition of Al destabilizes MgH2 and TiH2 hydrides while having only a limited effect on the storage density

On the structure of defects in the Fe7Mo6 μ\mu-Phase

Topologically close packed phases, among them the $\mu$-phase studied here, are commonly considered as being hard and brittle due to their close packed and complex structure. Nanoindentation enables plastic deformation and therefore investigation of the structure of mobile defects in the $\mu$-phase, which, in contrast to grown-in defects, has not been examined yet. High resolution transmission electron microscopy (HR-TEM) performed on samples deformed by nanoindentation revealed stacking faults which are likely induced by plastic deformation. These defects were compared to theoretically possible stacking faults within the $\mu$-phase building blocks, and in particular Laves phase layers. The experimentally observed stacking faults were found resulting from synchroshear assumed to be associated with deformation in the Laves-phase building blocks