Bulk Metallic Glasses and Their Composites: A Brief History of Diverging Fields

Bulk Metallic Glasses and Their Composites: A Brief History of Diverging Fields Douglas C. Hofmann Jet Propulsion Laboratory, California Institute of Technology, MS 18-105, 4800 Oak Grove Drive, Pasadena, CA 91109, USA Received 12 November 2012; Accepted 13 December 2012 Academic Editor: Ram Gupta Copyright © 2013 Douglas C. Hofmann. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Bulk metallic glasses (BMGs) and their derivative metal matrix composites (BMGMCs) are emerging high-performance engineering materials that are on the precipice of widespread commercialization. This review article discusses the origin of these materials and how their applications and research focus have divided into two distinct fields, one primarily focused on the plastic-like processability of BMGs and the other on the enhanced fracture mechanics of BMGMCs. Although the materials are of similar composition and origin, it is argued that their implementation will be widely varying due to their different processing requirements and intended uses. BMGs will likely find use as plastic-replacement components in cosmetic applications (e.g., watches, cell phones, biomedical implants) while BMGMCs will be used in structural applications (e.g., golf clubs, hardware for defense, energy absorbing structures). Since 1960, with the first publication of a metallic glass system in AuSi [1], amorphous metals and their composites have produced widespread scientific and commercial interest [2–4]. Initially, the focus of research in this area was centered on rapidly quenched thin foils and ribbons of metallic glass and, in particular, the scientific curiosity of how deeply undercooled liquids could avoid nucleation and growth of crystals when cooled below their glass transition temperature [5, 6]. Commercial applications soon followed with the identification of unique mechanical and magnetic properties obtained in thin layers of the materials. These included transformer coils produced by winding ribbons of Ni-, Co-, and Fe-based metallic glasses and corrosion resistant spray coatings of glass-forming alloys on drilling pipes, for example [7]. Two decades of further research led to the development of multicomponent alloy compositions with deep eutectics, large atomic size mismatch between constituents and sluggish crystallization kinetics such that critical cooling rates to prevent crystallization could be reduced by orders of magnitude (from 106 in quenched ribbons to 0.7 K/s in Pd-Cu-Ni-P). These new alloys, identified as bulk metallic glasses (BMGs), exhibited robust glass-forming ability and could be fabricated at thicknesses greater than 1 mm, thus creating a potential for structural hardware [8, 9]. The development of BMGs in practical (low cost) alloy compositions, such as Cu-Zr-Ni-Al and Zr-Ti-Cu-Ni-Be, opened the door for widespread commercialization into applications such as cell phone cases and golf clubs. The surge in commercial interest, starting in the mid-1990s, identified a wide range of applications for BMGs but also identified a number of flaws in the material when used as structural hardware, particularly the low fracture toughness, low fatigue limit and complete lack of ductility [10, 11]. Some research shifted towards toughening strategies for BMGs and in the early 2000s, progress was made by recognizing that the brittle failure of BMGs could be mitigated with the addition of crystalline phases into a BMG matrix [12]. This started a parallel field of research in BMG matrix composites (BMGMCs), with the aim of producing centimeter or greater thickness amorphous alloys for structural (load bearing) applications. Vigorous funding and increased world-wide research interest on BMGs and BMGMCs led to a resurgence in scientific progress on the materials by the end of the decade. Further understanding of the intricacies of processing amorphous metals has now brought us to the brink of global commercialization for these novel alloys and to a place where future activities in BMGs and their composites, while equally bright, diverge. BMGs are poised to make an impact in low-thickness, high-precision cosmetic applications usually reserved for polymers, while the high-strength, toughness, and hardness of BMGMCs are being investigated for high-performance load-bearing applications, such as spacecraft shielding and panels for military vehicles. Metallic glass coatings, in contrast, have already broken into the commercial marketplace with successful ventures achieved primarily in the oil and gas industry (see, e.g., websites for Armacor, Scoperta and the NanoSteel Company). With access to infinite cooling rates, all metals and metal alloys can be undercooled into an amorphous state because crystal nucleation and growth are time dependent phenomena. 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