Understanding the Glass-forming Ability of Cu 50 Zr 50 Alloys in Terms of a Metastable Eutectic
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J.J. Lewandowski Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106
A.L. Greer Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom (Received 17 October 2004; accepted 3 March 2005)
Interest in finding binary alloys that can form bulk metallic glasses has stimulated recent work on the Cu–Zr system, which is known to show glass formation over a wide composition range. This work focuses on copper mold casting of Cu50Zr50 (at.%), and it is shown that fully amorphous rods up to 2-mm diameter can be obtained. The primary intermetallic phase competing with glass formation on cooling is identified, and the glass-forming ability is interpreted in terms of a metastable eutectic involving this phase. Minor additions of aluminum increase the glass-forming ability: with addition of 4 at.% Al to Cu50Zr50, rods of at least 5-mm diameter can be cast fully amorphous. The improvement of glass-forming ability is related to suppression of the primary intermetallic phase.
I. INTRODUCTION
There has been much interest in finding a binary alloy composition capable of forming a bulk metallic glass (BMG), i.e., of forming a glass with a minimum dimension greater than 1 mm. If a BMG were found with a true binary or near-binary composition, it would facilitate fundamental analysis of glass formation; in particular, atomistic modeling would be much easier than it would be for the multicomponent systems usually considered necessary for BMG formation. Also, any binary BMG would be an excellent starting point for the development of multicomponent systems giving still better glassforming ability (GFA). The search for a binary BMG has recently focused on the Cu–Zr system. From early work, it is known that by melt-spinning Cu100−xZrx (at.%), metallic glasses can be obtained in an unusually wide composition range of x ⳱ 25 to 60 at.%.1 The origins of such comparatively good GFA have been extensively discussed.2–7 An atomic radius difference of more than 10% is important for glass formation,8,9 and Cu–Zr meets this criterion (Goldschmidt radii: Cu, 0.128 nm; Zr, 0.161 nm). The size difference leads to copper being an anomalous fast a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0302 J. Mater. Res., Vol. 20, No. 9, Sep 2005
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diffuser in zirconium10 but zirconium being a slow diffuser in copper. Good GFA is also associated with a negative heat of mixing, and Cu–Zr shows a particularly large value of –23 kJ mol–1.11 The diffusional asymmetry and the negative heat of mixing are favorable for forming amorphous Cu–Zr by mechanical alloying12 and by the solid-state amorphization reaction.13 Looking for BMG formation, Xu et al.14 studied Cu100–xZrx alloys in the composition range x ⳱ 34 to 40 at.%. They reported that at the best glass-forming composition in this range, Cu64Zr36, 2 mm rods can be cast completely amorphous. In related work, a simila
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