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Science Foundation in China

High entropy state of orientationally-disordered clusters in Zr-based bulk metallic glass
Akira TAKEUCHI1,Makoto OGATA2, Akihisa INOUE1
1. World Premier International Research Center, Tohoku University,2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan;2. Graduate School of Engineering, Tohoku University,Aramaki, Aoba-ku, Sendai 980-9482, Japan
Abstract:  A phase transformation of a metastable Zr2Ni intermetallic crystalline phase to vitrify was studied with molecular dynamics (MD) simulations based on a plastic crystal model (PCM), which utilizes the orientational disorder of molecules of plastic crystals in a class of soft matter. The simulation results showed that computational operations in MD-PCM for the random rotations of the clusters in the crystalline phase around their center of gravity and subsequent annealing lead to form a dense random packing structure from the metastable Zr2Ni phase. These randomly-rotated clusters provide high degrees of freedom in terms of atomic positions, which results in enhancing the glass-forming ability of the alloy. The critical fraction of the number of rotated clusters for forming the glassy phase (fc R) is evaluated to be 0.75?0.80. Thus, the critical fraction of the number of un-rotated clusters (fc U=1?fc R) is close to the critical concentration of site percolation for metallic materials. The mechanism of the metastable Zr2Ni phase to vitrify with increasing f R was analyzed with a concept of communal entropy in the free volume theory with solid- and liquid-like cells proposed by Cohen and Grest. The MD-PCM for the metastable Zr2Ni phase suggests that Zr-based bulk metallic glass (BMG) can be regarded as an alloy in a high rotation entropy state of clusters and that glass transition takes place by percolation of the nuclei of a liquid-like glassy phase.
Keywords:  metallic glass; molecular dynamics simulation; plastic crystal; percolation; communal entropy
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