DOI: 10.1002/adem.201600066 Bulk Metallic Glasses as Structural Materials: A Review By Jamie J. Kruzic* Bulk metallic glasses (BMGs) can exhibit excellent combinations of strength and fracture toughness that cannot be achieved by traditional metals, making them attractive for load bearing, mechanical engineering applications. Furthermore, recent research on BMGs has shown that many early perceived shortcomings, such as apparent brittleness or poor fatigue resistance, are not as big of a problem as once thought. The purpose of this paper is to review some of the unique mechanical characteristics of BMGs along with some of the strategies that may be used to improve or exploit their characteristics so that BMGs may be used as structural materials in engineering applications. 1. Introduction While metallic glasses have been studied since 1960, [1] initially only thin (typically <100 mm) ribbons of material could be fabricated because of the fast quenching rates (e.g., 10 4 –10 6 Ks 1 ) that were required to avoid crystallization of the early binary compositions. Research in the 1970s and 1980s lead to the discovery of ternary compositions that could be cooled slower into bulk metallic glass (BMG) form, [2–6] where bulk is defined as >1 mm in the smallest dimension. However, at that time such compositions were very limited in number and were based on expensive elements such as Pd, Pt, or Au. Overall, study of metallic glasses was relegated to a scientific curiosity for many decades due to the limitations of thin ribbon dimensions with only very few compositions that could be made larger. This changed with the discovery of the bulk metallic glass Zr 41.2 Ti 13.8 Cu 12.5 Ni 10.0 Be 22.5 1 in the 1990s, commonly called Vitreloy 1. [7] This composition could be produced in large sizes (>1 cm) and was widely studied. [8–17] More importantly, this discovery sparked a large interest in BMGs and was followed by the development of numerous quaternary and quinary BMG compositions based on various non-precious metals (e.g., Zr, [18,19] Fe, [20,21] Ni, [22,23] Co, [24] Cu, [25,26] Ti, [27] Mg [28,29] ) with excellent glass-forming ability and a wide variety of physical and mechanical properties. Overall, it has been found that bulk metallic glasses (BMGs) can exhibit excellent combinations of mechanical properties that make them attractive for load bearing, mechanical engineering applications. For example, they have high yield strengths (1–5 GPa is common) and high elastic strain limits (up to 2% is common) compared to typical crystalline metals. [8,21,24,26,30] While fracture toughness values can range from highly brittle to very tough, the best BMG compositions can achieve excellent combinations of strength and toughness that cannot be achieved by traditional metals (Figure 1). [31,32] However, BMGs also have some unique mechanical characteristics relative to crystalline metals that can make their use for engineering applications appear challenging. For example, BMGs exhibit localized strain softening behavior which can lead to unstable shear failures. Also, when compared to crystalline metals, BMGs have demonstrated distinctly different effects of cold work, annealing, notches, sample dimensions, and loading configu- ration on the deformation and fracture responses. Relative to materials used in some applications (e.g., consumer electronic devices), the timeline for implementing newly discovered structural materials is invariably slow and often takes decades due to safety and reliability concerns. For example, the commercial aircraft industry is highly motivated to reduce the weight of aircraft to save fuel costs. Nonetheless, it took decades for carbon fiber-reinforced composites to move beyond niche sporting goods and military applications to make a significant impact in the lightweighting of commercial airframes. Similarly, it also took decades for titanium aluminide intermetallics to be implemented in 1 All compositions in this paper are given in terms of atomic percent. [*] Dr. J. J. Kruzic Materials Science, School of Mechanical, Industrial, and Manufacturing Engineering, Oregon State University, Corvallis, Oregon 97330, USA E-mail: jamie.kruzic@oregonstate.edu 1308 wileyonlinelibrary.com © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ADVANCED ENGINEERING MATERIALS 2016, 18, No. 8 REVIEW