The effect of Ti, Nb, and Ta additions to ZrAlCu metallic glass on the crystallization and formation of the icosahedral
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The addition of Ti, Nb, or Ta altered the crystallization behavior of the Zr65Al7.5Cu27.5 glass, and a metastable nanoscaled I-phase was obtained on primary crystallization of these quaternary glasses. The complete substitution of Al by Nb also led to the precipitation of I-phase upon crystallization. Negative heats of mixing between the addition metals and the transition metals of the mother alloy were demonstrated to be not a necessary condition for I-phase formation in the present alloy system.
I. INTRODUCTION
Some Zr-based Zr–Al–Ni–Cu metallic glasses are very stable against crystallization to show large undercooled liquid spans,1,2 ⌬Tx ⳱ Tx − Tg, where Tx is the onset temperature of crystallization and Tg is the calorimetric glass-transition temperature. The alloys have large glassforming ability and can be made into bulk metallic glasses (BMGs).1–3 Following Frank’s argument, the icosahedral atomic clusters in the undercooled liquids are energetically favored and could therefore prevent the transition of the liquid to a periodic crystal, thus leading to a large undercooling.4–6 In the devitrification process of these BMGs, Köster et al. first reported the formation of Zr-based icosahedral quasi-crystals (I-phase), which have intrinsic icosahedral point symmetry.7 This throws some light on understanding the local atomic structures of BMGs and their formation mechanism. In BMGforming systems, the formation of I-phase has been drawing increased attention in recent studies. After the discovery of the Zr-based I-phase, ample experimental evidence appears that I-phase formation in Zr–Al–Ni–Cu glassy alloys is favored by the addition of oxygen8 and noble metals,9–12 as well as early transition metals (ETMs) such as Ti, Nb, and Ta.13–15 Murty et al. presented experimental evidence in a family of Zr-based alloys that Al and Ni are not essential for I-phase formation in the presence of Pd. In these alloys Pd has an exceptionally strong negative heat of mixing with Zr, but interacts only weakly with Ni or Cu.9 In particular,
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Address all correspondence to this author. e-mail: [email protected] On leave from Dalian University of Technology, People’s Republic of China. DOI: 10.1557/JMR.2007.0130 J. Mater. Res., Vol. 22, No. 4, Apr 2007
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I-phase formation was observed by the substitution of Cu with Pd in the Zr65Al7.5Cu27.5 alloy. With regard to the addition of ETMs in the Zr–Al–Ni–Cu glasses, Ni–ETM interatomic interactions are strong while Cu–(or Zr)– ETM show zero or positive heats of mixing. Kelton et al. suggested that the ETMs stabilize the Ni-centered icosahedral atomic structure and catalyze the nucleation of I-phase.16 In the present work, we show that the addition of ETMs such as Ti, Ta, and Nb in the Ni-free Zr65Al7.5Cu27.5 alloy also leads to the precipitation of I-phase during the crystallization processes of these metallic glasses, though these elements have zero or positive heats of mixing with Cu and Zr. II. EXPERIMENT
Alloy ingots of compo
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