Phase Equilibria and Glass Forming Ability in the Zr-Cu-Ni-Al System
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Phase Equilibria and Glass Forming Ability in the Zr-Cu-Ni-Al System Douglas J. Swenson and Sreenivas Bhattiprolu Department of Materials Science and Engineering Michigan Technological University 1400 Townsend Drive, Houghton, MI 49931, U.S.A. ABSTRACT The constitution of the 66.67 at.% Zr isopleth of the Zr-Cu-Ni-Al quaternary system has been studied, utilizing X-ray diffraction and electron probe microanalysis to establish phase equilibria at 800 ºC, and differential scanning calorimetry to study the liquidus hypersurface. As would be expected in a four-component system, phase equilibria were found to be complex. However, it was determined that the isoplethal subsection Zr2Cu-Zr2Ni is pseudobinary, and subsequently a large portion of the quaternary isopleth is pseudoternary, comprising equilibria involving the phases Zr2Cu, Zr2Ni and Zr6NiAl2. Moreover, these three phases participate in a pseudoternary eutectic reaction: L → Zr2Cu + Zr2Ni + Zr6NiAl2 at 852 °C. Utilizing the thermal analysis data, the composition of the eutectic point was estimated to lie very near that of the well-known bulk metallic glass forming alloy Zr65Cu17.5Ni10Al7.5. The compositional proximity of a “deep” eutectic valley to this quaternary alloy thus provides a thermodynamic rationale for its high glass forming ability. INTRODUCTION Several Zr-base alloy systems have been shown to exhibit exceptional glass forming ability (GFA), as demonstrated by their ability to be cast as amorphous ingots of substantial cross-sectional dimensions (see, for example, [1,2]), or as inferred by measurement of certain parameters that have been correlated with high GFA, including reduced glass transition temperature, Trg (Tg/Tm), and ∆Tx (Tx-Tg) (see, for example, [1-7]). One of the main families of Zr-base bulk metallic glasses comprises the Zr-Cu-Ni-Al system. This system has been the subject of much study. In particular, ∆Tx has been measured as a function of composition for both the quaternary system[3] and its Zr-bearing constituent ternary alloy systems, namely ZrNi-Al[4], Zr-Cu-Al[5], and Zr-Cu-Ni[6]. It is noteworthy that the highest values of ∆Tx within each system are in the compositional vicinity of 65 at.% Zr; indeed, the alloy Zr65Cu17.5Ni10Al7.5[3], exhibits one of the largest values of ∆Tx ever observed for a quaternary alloy. It has been suggested previously that details of phase equilibria in an alloy system, such as the presence of “deep” eutectic reactions, are often correlated with GFA for a given alloy system[1,2,8]. Presently, however, no phase equilibrium data are available for the Zr-Cu-Ni-Al system. Owing to the apparent significance of alloys comprising about 65 at.% Zr in this system, the present study was conducted to investigate phase equilibria in the Zr-rich portion of the quaternary system. Specifically, the investigation focused on alloys consisting of 66.67 at.% Zr, and the determination of an 800 °C isotherm and the liquidus hypersurface. This isopleth was selected because it was noted by inspection of the constituent binary
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