Rejuvenated metallic glass strips produced via twin-roll casting

Abstract

The energy state and atomic level structure of metallic glasses (MGs) are very sensitive to their cooling rates, and a lower cooling rate generally causes a lower energy and more relaxed state of MGs. In this work, the Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit. 1) ribbons with a thickness of 40 μm and 110 μm and the strips with a thickness of 320 μm and 490 μm were produced by single-roll melt spinning and twin-roll casting, respectively. The increase in thickness of either ribbons or strips results in a lower energy state with a smaller relaxation enthalpy, a lower content of free volume, and a higher hardness. Although the cooling rate of the twin-roll produced 320 μm-thick strip is almost one magnitude lower than that of the single-roll produced 110 μm-thick ribbon, the former, however, possesses a rejuvenated energy state as compared to the latter. Molecular dynamics simulations reveal that the squeezing force during twin-roll casting affects the evolution of connection types of clusters, and the 2-atom and 4-atom connections are prone to be retained, which results in a higher energy state of MGs. Such a rejuvenation process during twin-roll casting can overwhelm the relaxation process caused by the lower cooling rate. Therefore, twin-roll casting is not only a method being capable for producing strips with a large thickness, but also prone to obtain a high energy state of the MG strip.