High Packing Density of Zr- and Pd-Based Bulk Amorphous Alloys

Predicted Optimum Composition for the Glass-Forming Ability of Bulk Amorphous Alloys: Application to Cu-Zr-Al

Metallic glasses have been established to have unique properties such as ductility, toughness, and soft magnetism with promising engineering applications. However, the glass-forming ability (GFA) has not been sufficient to synthesize the bulk metallic glasses (BMGs) required for many engineering applications. Attempts to develop the understanding of the GFA required to predict the optimum alloys have not yet been proven successful. We develop here a computational model based on molecular dynamics simulations that explains the dramatic change of GFA with alloying small amounts of Al into Cu-Zr. We find that the high GFA to form BMGs depends on a combination of three factors, (a) a low thermodynamic driving force for crystallization, (b) a high melt viscosity, and (c) large ratios of icosahedral structures in the liquid phase. These computational methods to predict these factors that suppress formation of crystal nuclei and slow the dynamic motions in the liquids are practical for in silico prediction of new alloys with optimal GFA.

Atomic-scale mechanisms of the glass-forming ability in metallic glasses

The issue, composition dependence of glass-forming ability (GFA) in metallic glasses (MG), has been investigated by systematic experimental measurements coupled with theoretical calculations in Cu-Zr and Ni-Nb alloy systems. It is found that the atomic-level packing efficiency strongly relates to their GFA. The best GFA is located at the largest difference in the packing efficiency of the solute-centered clusters between the glassy and crystal alloys in both MG systems. This work provides an understanding of GFA from atomic level and will shed light on the development of new MGs with larger critical sizes.

Volume Effects in Bulk Metallic Glass Formation

Atomic volume effects in amorphous alloys obtained by rapid solidification and in bulk glasses are briefly reviewed. It is recalled that at high undercoolings, the release at the growth fronts, of the volume corresponding to the reduction of the molar volume upon solidification can sharply accelerate crystal growth in the melt through viscosity reduction. A method is then proposed for estimating the volume of mixing ΔVmix which is negative for elemental additions to glass-forming liquid alloy. It is argued that negative ΔVmix reduces atomic mobility in easy glass-forming alloys thus allowing the suppression during cooling, of nucleation and growth of crystallites. The ZrCuNi system is used as an example for applying this reasoning. It is shown that Al or Ti addition to ZrNiCu alloys lead to strongly negative ΔVmix and expected sharp drops in diffusion-controlled crystal growth kinetics in the melt.

Structural Characteristics and Crystallization of Metallic Glass Sputtered Films by Using Zr System Target

Zr-Al-Ni-Cu thin films were deposited by the radio-frequency sputtering method at low substrate temperature using three kinds of targets:Zr55Al10Ni5Cu30bulk metallic glass target (α-BMG target), crystallized bulk metallic glass target (c-BMG target), and an elemental composite target composed of each Zr, Al, Ni chips, and Cu plate. XRD profiles of the films prepared when using these targets indicated that all of the films showed amorphous structures. While XRD profiles of the films usingα- and c-BMG targets revealed a broad peak of2θ=38degree in the same way as theα-BMG target indicating amorphous structures, that of the film using elemental composite targets showed a broad peak of2θ=42degree, which is higher compared to the latter material. As a result of annealing the films at various temperatures for 900 seconds, the film using theα-BMG target showed a crystallization temperature of 748 K, higher than that of BMG with 723 K, while the other films had lower crystallization temperatures below 723 K. XRD profiles also indicated that the crystallized compounds of the films were different from those of BMG target.

Unusual glass-forming ability of bulk amorphous alloys based on ordinary metal copper

We report the unusual glass-forming ability (GFA) of a family of Cu-based alloys, Cu46Zr47-xAl7Yx (0<x< or =10, in at. %), and investigate the origin of this unique property. By an injection mold casting method, these alloys can be readily solidified into amorphous structures with the smallest dimension ranging from 4 mm up to 1 cm without detectable crystallinity. Such superior GFA is found primarily due to the alloying effect of Y, which lowers the alloy liquidus temperature and brings the composition closer to a quaternary eutectic. Other beneficial factors including appropriate atomic-size mismatch and large negative heat of mixing among constituent elements are also discussed.