Effects of Test Temperature and Loading Conditions on the Tensile Properties of a Zr-Based Bulk Metallic Glass
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sses have been produced in bulk sizes (i.e., >1 mm) for more than a decade. The size of these bulk metallic glasses (BMGs), combined with their high strengths and high elastic limits, make them appealing candidates for structural applications.[1] LiquidMetal 1 (LM1), formerly Vitreloy 1, of the composition Zr41.2Ti13.8Cu12.5Ni10Be22.5, is one of the most commercially-available BMGs, due to relatively sluggish recrystallization kinetics and, thus, its low critical cooling rate.[2] It is also the most heavily studied BMG. The thermodynamic, kinetic, toughness, and compressive viscoelastic properties have been well characterized.[3–6] However, tension testing has primarily been limited to room-temperature conditions for A.H. VORMELKER, formerly Graduate Student, Department of Materials Science and Engineering, Case Western Reserve University, is Plant Metallurgist, Marine Mechanical Corporation, 24703 Euclid Avenue, Cleveland, OH 44117, USA. O.L. VATAMANU, formerly, Post Doctorate Researcher with Case Western Reserve University, is Senior Research Scientist, Powdermet, Inc., Euclid, OH 44117, USA. L. KECSKES, Research Physical Scientist, is with the U.S. Army Research Laboratory, Aberdeen Proving Ground, MD 21005-5069, USA. J.J. LEWANDOWSKI, Leonard Case Jr. Professor of Engineering, Department of Materials Science and Engineering, Case Western Research University, Cleveland, OH 44106, USA. Contact e-mail: [email protected] This article is based on a presentation given in the symposium entitled ‘‘Bulk Metallic Glasses IV,’’ which occurred February 25– March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee. Article published online December 21, 2007 1922—VOLUME 39A, AUGUST 2008
LM1.[5,6] Relatively few high-temperature tensile studies have been conducted on BMGs (e.g., Reference 7). The flow and fracture behavior of BMGs is quite different than that of ordinary crystalline metals. Spaepen classified the plastic flow of metallic glass to be either homogenous or inhomogeneous, and adapted a free volume model[8] originally developed for polymers[9] to describe the flow. Inhomogeneous catastrophic shear failure at low macroscopic strains was expected at all strain rates applied at temperatures well below the Tg. In the Spaepen free volume model, flow occurs when a hydrostatic tensile stress causes a critical amount of hydrostatic dilatation (expansion). A rapid increase in free volume is proposed to occur in a thin layer of a plane, accompanied by a rapid loss of viscosity in that plane, resulting in local viscous flow at significantly reduced stresses. At temperatures near the Tg, models predict homogeneous viscous flow at sufficiently low strain rates.[8,10] Spaepen defined homogeneous flow as uniform deformation or, in other words, a constant cross-sectional area along the gage length during deformation to very high strains. Free volume is now assumed to remain constant, as all atoms present in the gage length rearrange themselves to the applied shear s
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