Glass Formation Dependence on Casting Atmosphere in a Zr-Al-Ni-Cu-Pd Alloy System
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Glass Formation Dependence on Casting Atmosphere in a Zr-Al-Ni-Cu-Pd Alloy System Albertus D. Setyawan1, Hidemi Kato2, Junji Saida1, and Akihisa Inoue2 1 Center for Interdisciplinary Research, Tohoku University, Aramaki-aza Aoba 6-3, Aoba-ku, Sendai, 980-8578, Japan 2 Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai, 9808577, Japan ABSTRACT A Zr65Al7.5Ni10Cu17.5-xPdx (x=0-17.5 at. %) alloy system is found to exhibit a unique dependence of glass-forming ability (GFA) on casting atmosphere during mold-casting process. High-Pd containing alloys (x=7.5-17.5) showed a remarkable increase of critical size (dc) for glass formation, in contrast with the low-Pd containing alloys (x=0-5) in which no significant change in dc was observed, as the pressure of Ar atmosphere varied from vacuum to ambient. The origin of the phenomenon has been successively unveiled by correlating the cooling characteristic during casting process and phase transformation behavior during continuous cooling of the alloy system. The consequence in the structure and property control is investigated and discussed in detail.
INTRODUCTION Assessing glass-forming ability (GFA) of an alloy can be performed by evaluating either the kinetics of transformation, such as the nucleation or crystal growth rate, or more practical criteria, such as the large of supercooled-liquid region ∆Tx (=Tx - Tg, where Tg and Tx are the glass transition and the crystallization temperatures, respectively) and the well-known reduced glass transition temperature Trg (=Tg /Tl, where Tl is the liquidus temperature) [1,2]. However, a more direct and straight-forward method sometimes is necessary, for example by comparing the critical size for glass formation derived from as-prepared structure of samples of various sizes. Mold-casting is a simple and one of the most utilized techniques in fabrication of bulk metallic glassy specimens. This work reports a direct observation of apparent GFA in the Zr65Al7.5Ni10Cu17.5-xPdx (x=0-17.5) alloy system performed by mapping extensively all the structures of alloys fabricated by mold casting. The work is successful in unveiling the origin of different dependence of apparent GFA on casting-atmosphere among the alloys in the system following a thorough investigation on the cooling characteristic and the phase transformation during continuous cooling. The result provides a guide in determining the appropriate strategy in structure and property control for the alloy system.
EXPERIMENTAL DETAILS Master ingots of Zr65Al7.5Ni10Cu17.5-xPdx (x=0-17.5 at.%) alloys were prepared by arcmelting mixtures of pure elements (99.9 mass % Zr, 99.99 mass % Al, 99.9 % Ni, 99.99 mass % Cu and 99.9 mass % Pd) in an Ar atmosphere. Rod samples with various diameters and 5 mm in length were produced by a copper mold casting technique in which the atmosphere pressure in
the casting chamber was varied ranging from vacuum (p~2x10-3 Pa) to ambient (p~105 Pa). The atmosphere gas used was Ar or He. The melts were injected int
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