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Glasses: At the Cutting Edge of Metals Research

A.L. Greer and E. Ma, Guest Editors Abstract Glassy alloys (metallic glasses) are currently the focus of intense research in the international metals community. Setting aside elevated-temperature applications, these amorphous metals have exciting potential for structural applications. When metallic glasses were first widely studied in the 1960s, the alloy compositions then known to be quenchable into the glassy state from the liquid required high cooling rates on the order of 106 K s−1 and were consequently restricted to thin sections. The current interest in metallic glasses has its origin mainly in the increasing range of compositions that can now be cast into glasses at much lower cooling rates, permitting minimum sections of 1 mm to 1 cm or even larger. These bulk metallic glasses (BMGs) are the focus of the articles in this issue of MRS Bulletin. Our goal is to illustrate the major materials issues for BMGs, from processing to structures to properties and from the fundamental science of glasses to viable industrial applications. We hope that the articles, in providing a snapshot of a rapidly moving field, show why BMGs are attracting such intense interest and serve to highlight some challenging issues awaiting resolution.

Introduction The glassy state is reached when a cooled liquid solidifies without crystallization, which is associated with the phenomena of the glass transition. The focus of this issue of MRS Bulletin is on glasses strictly defined in this way, although it should be noted that glasses are but one category of noncrystalline (or amorphous) solids, which in general can be prepared by a variety of methods.1 The best known natural glass is obsidian, formed by solidification after volcanic activity. The capability to work obsidian to create extremely sharp cutting edges for knives, spears, and arrowheads made this glass attractive to early humans. Glassmaking by humans is thought to have begun in Mesopotamia (modern Iraq) around 2500 BCE with the fashioning of beads in imitation of precious stones. In the context of this long history of usage, glasses with nondirectional metallic bonding are parvenus among their established cousins. The metallic glasses (also often

referred to as glassy alloys or amorphous alloys) emerged some 50 years ago,2 defying the expectation that solid metallic states would always be crystalline due to the nature of metallic bonding. Metallic glasses offer attractive benefits, combining some of the desirable properties of conventional crystalline metals and the formability of conventional oxide glasses. For example, in the absence of the well-defined dislocation defects ubiquitous in crystalline alloys, metallic glasses exhibit room-temperature strength much closer to the theoretical strength of the material than their crystalline counterparts. Meanwhile, near-net-shape processing can be realized by exploiting the viscous flow in the supercooled-liquid regime. Cast glassy alloys exhibit a shiny finish and maintain dimension