A brief overview of bulk metallic glasses

Abstract

Bulk metallic glasses, formed at very low critical cooling rates, are fundamentally diff erent from traditional amorphous alloys, which are usually formed at very high cooling rates in order to suppress the nucleation of crystalline phases. Th e high cooling rates required for the formation of traditional glassy alloys restrict the range of possible structures that can be prepared to powders, fi lms and ribbons. Of the extensive family of glasses, BMGs are probably the youngest, possessing a number of characteristics, such as amorphicity and high strength, that are shared by other glasses, including the most familiar window (oxide) glasses. However, the most important feature of BMGs that, along with other glasses, distinguish them from general amorphous materials is the glass transition that transforms supercooled liquids into a glassy state when cooled from high to low temperature and vice versa. Metallic glasses are therefore scientifi cally defi ned as amorphous alloys that exhibit a glass transition, from which derives their unique properties of extreme strength at low temperature and high fl exibility at high temperature, along with thermodynamic and physical properties that change abruptly at the glass transition temperature (T g ).Research on metallic glasses is closely related to that on metallic liquids. It is known that the atomic confi gurations of molten metals and alloys are disordered. Th e noncrystalline structure is expected to be retained if the liquids can be quenched at a suffi ciently high cooling rate to suppress the formation of equilibrium crystalline phases. Th e critical cooling rate required to freeze the liquid structure is estimated to be on the order of 10 5 -10 6 K s -1 for alloys. Th is assumption was proved experimentally in 1959 by a research group at Caltech, who reported the fi rst glassy alloy with a composition of Au 75 Si 25 produced by splat quenching [1]. In this pioneering work, the Au 75 Si 25 melt was quenched using a cold metal plate instead of the water or oil used traditionally. Th e good contact between liquid droplets and the cold metal avoids the formation of gaseous layers that limit the heat release during solidifi cation. Moreover, the droplets spread into a thin layer for rapid thermal diff usion when striking the solid, cold metal plate. X-ray diff raction analysis revealed the disappearance of all crystalline structure and the formation of a noncrystalline structure in the resultant micrometer-sized fl akes. Following this discovery, a large number of metallic glasses were found in various alloy systems, and rapid research progress was made in the search for new metallic glasses. Th is research was accelerated by the invention of a melt spinning method [2], a rapid cooling technique that can routinely quench liquid alloys as 10-50 μm thick ribbons at cooling rates of 10 3 -10 6 K s -1 by impinging a melt stream on a spinning copper wheel. Most glass-form alloys reported at that time were binary alloys, and their glass-forming ability was found to be strongl...

3D nano-architected metallic glass: Size effect suppresses catastrophic failure

“…Advances in small-scale technological devices such as MEMS (microelectromechanical systems), biomedical devices and implants, microelectronics, micromanipulators, and microrobotics have increased the demand for miniature parts fabricated from materials with suitable properties [12,13]. The combination of metallic glasses' enhanced plasticity at small scales [5][6][7][8][9][10][11] with their other desirable properties (including high elasticity, an M A N U S C R I P T…”

The Development of Structure Model in Metallic Glasses

“…Bernal's model has been widely accepted for metallic glasses, but it fails to describe the metal-metalloid-based alloys with pronounced chemical short-range order 86 . In light of this, Gaskell 87-89 put forward a model shown in Figure 5.…”

Effect of Hafnium and Zirconium to Glass Forming Ability, Thermal Stability, Plasticity Deformation and Crystallization of Ni-Free Pentabasic Ti-Based Bulk Metallic Glasses

“…From the point of view in effective usage extension of these materials, Ti-based amorphous alloys are one of the optimized materials because of their own extraordinary specific strength, superior corrosion resistance as well as flow-viscous plastic workability and consolidation at supercooled liquid region (SLR). The expression of SLR and bulk forming ability (BFA) in amorphous state contributed to increase scientific interest and has a high potential of industrial application [2]. Recently, Ti-based amorphous alloys and Ti-based BMGs have been attempted to apply as bio-medical application by Ni-free Ti-based metallic glasses [3][4][5][6][7][8].…”