Variation of Hardness and Modulus across the Thickness of Zr-Cu-Al Metallic Glass Ribbons
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Variation of Hardness and Modulus across the Thickness of Zr-Cu-Al Metallic Glass Ribbons Z. Humberto Melgarejo1, J.E. Jakes2, J. Hwang1,*, Y.E. Kalay3,**, M.J. Kramer3, P.M. Voyles1,4,D.S. Stone 1,4 1
Materials Science Program, University of Wisconsin-Madison, Madison, WI 53706, U.S.A. Performance Enhanced Biopolymers, United States Forest Service, Forest Products Laboratory, Madison, WI 53726, U.S.A. 3 Ames Laboratory (DOE), Ames, Iowa 50011, USA and Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011, U.S.A. 4 Department of Materials Science and Engineering, and Materials Science Program, University of Wisconsin-Madison, Madison, WI 53706, U.S.A. 2
*current address: Materials Department, University of California, Santa Barbara, Santa Barbara, CA 93106-5050, U.S.A. **current address: Department of Metallurgical and Materials Engineering, Middle East Technical University, Ankara, 06800 Turkey. ABSTRACT We investigate through-thickness hardness and modulus of Zr50Cu45Al5 metallic glass melt-spun ribbon. Because of their thinness, the ribbons are challenging to measure, so we employ a novel nanoindentation based-method to remove artifacts caused by ribbon flexing and edge effects. Hardness and modulus vary approximately linearly across the thickness but, unlike bulk ingots, the side of the ribbon that cooled most quickly had the highest hardness and modulus. This “inverse” variation may be caused by the fast-moving solidification front, which might conceivably, for instance, push free volume in advance of it. Annealing near Tg causes both hardness and modulus to increase and become more uniform across the thickness. INTRODUCTION Bulk metallic glasses (BMGs) are a promising class of materials for structural applications, especially at very small dimensions [1]. Large BMG ingots have higher hardness and modulus in the center than near the surface [2, 3], which is attributed to differences in free volume caused by different cooling rates during solidification [3]. Plummer et al. [2] demonstrated that this effect depends on sample size and glass fragility index. Melt spun ribbons have even more extreme cooling rates than ingots. Ribbons have been reported to have lower hardness and modulus than ingots, favored by the difference in cooling rates and free volumes [4], but this result has been questioned as an artifact arising from the small ribbon thickness [5]. In the present work, we report measurements of hardness and elastic modulus across thicknesses of Zr50Cu45Al5 as-spun and structurally relaxed ribbons using nanoindentation, corrected for the effects of thin ribbon geometry using rigorous techniques we have developed [6, 7]. We observe that hardness and modulus vary approximately linearly across the thickness, but the slope is opposite of cast bulk ingots: as opposed to ingots the surface that solidified at highest cooling rate has higher hardness and modulus. During annealing both the hardness and
modulus rise, but their slopes across the specimens diminish. Careful nanoinden
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