Structural origin of the pinpoint-composition effect on the glass-forming ability in the NiNb alloy system
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Great research efforts to investigate the glass-forming ability (GFA) in alloys have been made, leading to an observation that a pinpoint composition produces the best glass-forming characteristics. The reason for this observation is still unknown, limiting the development of bulk metallic glasses (MGs) with a relatively large size. In this work, systematic experimental measurements coupled with calculations were performed to address this issue using the NiNb binary alloy system. It is found that the atomic-level packing efficiency and the clusters-level regularity parameters strongly contribute to their GFA. In particular, the best glass former found in a pinpoint composition possesses the local maximum of the atomic-packing efficiency and the highest degree of the cluster regularity. This work provides an understanding of GFA from atomic and cluster levels and will shed light on the development of more MGs with relatively large critical casting sizes.
I. INTRODUCTION
Understanding the mechanisms of glass formation in alloys has been a long-standing issue since the discovery of the first metallic glass (MG) with the composition of Au75Si25 in 1960.1 So far, various rules, principles, and criteria have been presented to guide the development of amorphous alloys having high glass-forming ability (GFA)2–7 because high GFA is required for a broad application of MGs as engineering materials. MGs with high GFA usually can be fabricated in multicomponent alloy systems containing at least three elements. However, recently, MGs with relatively high GFA were successfully prepared in binary alloy systems, such as the CuZr8 and the NiNb9 systems. These simple alloys have attracted intense interest because they have unique properties10–12 and they are the representative models for studying glass-forming mechanisms.13–16 Further more, studying the binary MGs may enhance our understanding of the formation of their corresponding multicomponent MGs.17–21 An interesting phenomenon was found that although these binary systems have the broad composition regions forming MG, the best glass formers are located at some pinpoint compositions.8,9 The existing rules and criteria fail to explain this phenomenon, resulting in the absence of the guiding principle for developing more MG compositions with high GFA in binary alloy systems. The microstructures of MGs have been largely studied. The explicit or direct structural picture is far from being a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.318 3170
J. Mater. Res., Vol. 28, No. 22, Nov 28, 2013
http://journals.cambridge.org
Downloaded: 18 Nov 2014
established due to the structural complexity in this kind of glassy materials. Nevertheless, fortunately, many structural models have been proposed by studying the clusters theoretically or experimentally, including the hard-sphere random-packing model,22 the stereochemically designed model,23 the efficient cluster-packing model,24 and the quasi-equivalent cluster model.25 These structural model
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