The density of the amorphous phase of metals is generally thought to be related to glass formation, but this correlation has not been demonstrated experimentally to date. In this work, systematic deflection measurements using microcantilevers and a combinatorial deposition method show a correlation between glass-forming ability and the density change upon crystallization over a broad compositional range in the copper-zirconium binary system. Distinct peaks in the density of the amorphous phase were found to correlate with specific maxima in the critical thickness for glass formation. Our findings provide quantitative data for the development of structural models of liquids that are readily quenched to the amorphous state. The experimental method developed in this work can facilitate the search for new glass-forming alloys. Metallic glasses are amorphous metals that do not have a structure with longrange atomic order like crystalline materials do, but have pronounced short-and medium-range order at the atomic scale. Because of their very different properties as compared to those of their crystalline counterparts, metallic glasses are very promising materials for future structural, chemical, and magnetic applications (1, 2). The packing density of the amorphous phase is a key consideration in studying the formation of metallic glasses (2-5). A liquid of high packing density (6-8) has a low free volume content and a correspondingly low atomic mobility (9-11). Upon quenching, such a liquid is expected to have a strong kinetic constraint on nucleation and the subsequent growth of crystals. This has been the basis for recent theoretical studies (12, 13) of structural models of metallic glasses, in which a correlation between compositions having especially dense packing and compositions that are known to quench to the glassy state at relatively low cooling rates was sought, but not obtained. Earlier studies of the density of glasses, based on the Archimedes method, have been mostly limited to relatively narrow compositional ranges of ternary and quaternary alloys with large critical sample sizes for glass formation (14-16), and no correlation between density and the ease of glass formation has been demonstrated.We have developed a method for the measurement of density changes, using microfabricated Si-rich silicon nitride (SiN) cantilevers (17). Owing to the small size and close spacing of the cantilevers, the deposition of alloy films with compositions that varied in a controlled way from cantilever to cantilever allowed a combinatorial approach to measurements of density changes for a broad range of compositions, with high compositional resolution. This process is schematically illustrated in fig. S1. Heatinginduced crystallization of the initially amorphous Cu-Zr films causes an increase in density (a decrease in volume) that causes an upward deflection of the cantilevers, as a result of the tensile elastic mismatch strain developed at the interface between the film and the cantilever. By measuring the magnitude of ...

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The deformation of metallic glasses involves two competing processes: a disordering process involving dilatation, free volume accumulation, and softening, and a relaxation process involving diffusional ordering and densification. For metallic glasses at room temperature and under uniaxial loading, disordering usually dominates, and the glass can fail catastrophically as the softening process runs away in a localized mode. Here we demonstrate conditions where the opposite, unexpected, situation occurs: the densifying process dominates, resulting in stable plastic deformation and work hardening at room temperature. We report densification and hardening during deformation in a Zr-based glass under multiaxial loading, in a notched tensile geometry. The effect is driven by stress-enhanced diffusional relaxation, and is attended by a reduction in exothermic heat and hardening signatures similar to those observed in the classical thermal relaxation of glasses. The result is significant, stable, plastic, extensional flow in metallic glasses, which suggest a possibility of designing tough glasses based on their flow properties.