Increased Toughness of Zirconium-Based Bulk Metallic Glasses Tested under Mixed Mode Conditions
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INTRODUCTION
DURING the last decade, bulk metallic glasses (BMGs) have attracted widespread attention and interest because of their potential engineering applications. Several multicomponent BMGs that can be formed at relatively low critical cooling rates (below 102 K/s) have been developed that exhibit exceptional glass formability. Much work has been done to investigate the strength[1–4] and fatigue behavior[5–9] of BMGs. The fracture toughness of different BMG systems has also been measured using different specimen configurations, notch conditions, and experimental conditions.[6,10–12] Whenever a defect exists in the form of a crack in a material, the applied stresses acting on the crack can produce fracture under different loading modes. Mode I is generally considered the most severe case of loading and is the most widely studied mode of fracture. However, loading under multiaxial conditions will introduce non-mode I loading, and fracture is often not under pure mode I conditions. Thus, it is important to study failure under non-mode I and mixed mode conditions. In crystalline materials such as polycrystalline tungsten, which fail by brittle fracture, it has been shown that the addition of mode II and III loading lowers the mode I toughness contribution.[13] Tungsten, which fails by transgranular cleavage under mode I condi-
RAVIKUMAR VARADARAJAN, Research Associate, ALEX K. THURSTON, Graduate Student, and JOHN J. LEWANDOWSKI, Professor, are with the Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106. Contact e-mail: [email protected] Manuscript submitted April 1, 2009. Article published online October 27, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
tions, exhibits an increasing amount of intergranular fracture with mixed mode I/II conditions. It has also been shown that the addition of mode II and III loading lowers the mode I toughness contribution in materials such as polymethylmethacrylate (PMMA),[14] which fails by a shear yielding or crazing brittle fracture mechanism. The response of different materials to mixed mode loading appears to be dependent on the operative fracture mechanisms. Metallic materials that fail via tensile-stress-controlled cleavage fracture often exhibit low values for mode I fracture toughness, KIC. In these materials, if sufficient mixed mode loading is applied, crack propagation may occur via a different (e.g., ductile fracture) mechanism, thereby increasing the fracture toughness. In materials that fail via ductile fracture (i.e., HY100[15] and HY130[16] steels and 2034 Al alloy[17]) that exhibit high mode I fracture toughness under mixed mode loading, void nucleation and coalescence continues to occur under different loading modes, resulting in a decrease in mode I fracture toughness. In these materials, localized flow is present on the trajectory of the crack plane ahead of the crack tip, limiting the mode I plastic flow, causing less plastic work dissipation and a decrease in the mode I fracture toughness.[18] Stable crack growth is
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