Crystallization Kinetics and Phase Transformation Mechanisms in Cu 56 Zr 44 Glassy Alloy
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SINCE the discovery of glass formation in metals, reported a half-century ago,[1] numerous glass-forming metallic systems have been identified and investigated, owing to the promise and potential benefit of unique and remarkable combinations of properties. With glassformation tendency as a metric, these alloys are often compared according to the quench rate necessary to produce an ‘‘amorphous’’ or, more correctly, a ‘‘noncrystalline’’ solid alloy. Thus, alloys are typically categorized as strong, weak, or marginal glass formers, with the best glass formers termed ‘‘bulk metallic glasses,’’ signifying that these alloys can be cast in large cross-section without forming crystalline phases. Beyond the potential applications of non-crystalline metals, the interest in these materials arises largely from the access to novel structures that may become available through the intermediate glassy state. Indeed, the nanoscale structural elements present in metallic glasses may serve as effective templates or stimuli for certain metastable phases or even specific types of multiphase structures that are otherwise unattainable. Realizing novel materials through such glassy-state pathways requires a detailed understanding of alloy ILKAY KALAY, Assistant Professor, is with the Department of Materials Science and Engineering, Cankaya University, 06790, Ankara, Turkey. Contact e-mail: [email protected] MATTHEW J. KRAMER, Division Director, and RALPH E. NAPOLITANO, Professor, are with the Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011, and also with Materials Science and Engineering, Ames Laboratory, U.S. Department of Energy, Ames, IA 50011. Manuscript submitted September 8, 2014. Article published online April 28, 2015 3356—VOLUME 46A, AUGUST 2015
thermodynamics and transformation kinetics under conditions that are far from equilibrium, where multiple structural/chemical transformations may be highly driven and subtle aspects of ordering in the glass can serve to tip the competitive ‘‘scale,’’ favoring one structure over another. In this respect, the Cu-Zr binary system is of interest, given the wide range of compositions exhibiting high glass-formation tendency[2–6] and the existence of multiple stable and metastable phases that compete during devitrification processing.[7–11] Examining these issues, we have previously reported on alloy thermodynamics,[12,13] glass structure,[14,15] and crystallization mechanisms[12,16] in Cu-Zr alloys. In addition, we have reported on several aspects of competitive crystallization in the Cu50Zr50 alloy, elucidating the phase sequences, interfacial mechanisms, and structural dynamics associated with devitrification.[16] In the present study, we focus on the Cu56Zr44 (eutectic composition) alloy and investigate experimentally the competitive crystallization kinetics and overall devitrification behavior during both continuous heating and isothermal annealing of the quenched (non-crystalline) alloy. Our investigation of devitrification in the Cu50Zr50 alloy
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