Effects of Thermal Exposure and Test Temperature on Structure Evolution and Hardness/Viscosity of an Iron-Based Metallic
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BY their very nature, amorphous alloys exist in a metastable condition. Considerable research has been conducted over the last number of decades in order to understand their physical behavior, such as structural relaxation and crystallization, upon annealing within the supercooled liquid region. On the atomic scale, the rearrangement of atoms is often described by free volume model.[1,2] Based on this model, the amount and change of excess free volume can strongly affect the physical and mechanical properties (i.e., viscosity, flow, and fracture). In some cases, the thermal annealing that produces devitrification, especially nanocrystallization, can be useful for improving the mechanical, magnetic, and some other properties.[3,4] Characterization of the mechanical response that accompanies the loss of free volume or devitrification can help to better understand the behavior of these glasses and how such properties are related to atomic scale structural rearrangements. A. SHAMIMI NOURI, Graduate Assistant, and J.J. LEWANDOWSKI, Leonard Case Professor of Engineering, are with Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106. Contact e-mail: JJL3@case. edu Y. LIU, formerly Research Associate, Department of Materials Science and Engineering, Case Western Reserve University, is TEM Lab Manager, with The Department of Chemistry, Wayne State University, Detroit, MI 48202. This article is based on a presentation given in the symposium entitled ‘‘Iron-Based Amorphous Metals: An Important Family of High-Performance Corrosion-Resistant Materials,’’ which occurred during the MSandT meeting, September 16–20, 2007, in Detroit, Michigan, under the auspices of The American Ceramics Society (ACerS), The Association for Iron and Steel Technology (AIST), ASM International, and TMS. Article published online August 7, 2008 1314—VOLUME 40A, JUNE 2009
From an application standpoint, the thermal stability of bulk metallic glasses (BMGs) is one of the most important properties influencing their potential adaptation in various structural applications or as coatings. For example, the Fe-based BMGs are of great interest because of their combination of exceptional strength (4 GPa) and very high hardness (i.e., 11 to 13 GPa) with excellent corrosion resistance.[5] Recently, various coating techniques (e.g., high velocity oxygen fuel (HVOF), plasma spray, cold spray, etc.) have been used to create amorphous coatings in which the BMG particles/powders undergo significant deformation as well as thermal hysteresis.[6] Therefore, the thermal stability and effects of changes in test temperature on the flow behavior are vitally important to both the processing and subsequent properties of these coatings. A convenient way to quickly assess the effects of changes in test temperature (and time at temperature) on the hardness/strength is to conduct microhardness tests over a range of temperatures, as has been reported elsewhere for various BMGs.[7–9] In the present study, the effect of changes in test tempe
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