Graduate Thesis Or Dissertation
 

Molecular Dynamics Studies on The Glass Forming Ability and Icosahedral Clusters in Cu(100-x)Zrx (x=32, 34, 36, 38.2, 40 at%)

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  • Metallic glasses (MGs) have attracted tremendous attention because of their unique properties and potential applications in the future industry. However, it is still a mystery for people to understand the formation mechanism of the bulk MGs (BMGs). To produce BMGs, the specific composition of materials, sufficiently high cooling rate, etc., have been considered critical to the glass forming ability (GFA). A material system with multiple components (at least 3 elements) had been regarded as necessary to form BMGs until Xu et al. discovered 2 mm Cu-Zr BMGs in 2004. From then on, the Cu-Zr binary alloys have become a particularly important BMG system for both experimental and theoretical studies. In 2020, Xu et al. further discovered the inverted core-shell potential energy of the icosahedral cluster (ICO cluster) which is a widely existing short-ordering structure in MGs by studying glasses of BCC and FCC pure metals. This discovery stimulated our interest in studying the correlation between population of ICO cluster and potential energy in Cu-Zr alloys in order to seek the mechanism behind the better GFA, especially for Cu64Zr36. Following Introduction and Literature Review, the top-four dominant clusters in Cu64Zr36 are observed and analyzed first for better understanding of the structural evolution in Chapter 3. The most dominant cluster, the icosahedral cluster, is further discussed. The cooling rate effect, system size effect and tensile stress effect on the cluster population fraction and potential energy state are investigated. In chapter 4, the collective engaging effect induced by shells in entire system is found. We observe that with rising Zr-content, the core–shell energy difference of the icosahedral clusters increases which makes these clusters more strongly encaged and harder to break before or during crystallization. This effect counteracts the decrease in the population of the icosahedral clusters. The net result of the interplay between these two, quantified by the collective encaging strength, exhibits a maximum at 36% Zr where the highest GFA has been observed. These findings provide new insights into the origin and peaking behavior of the GFA in this important binary bulk MG system, which could facilitate the search for the best glass-forming compositions in other alloy systems. After that, the thermodynamics of ICO cluster formation, such as formation enthalpy and entropy, are analyzed in order to understand how a supercooled liquid generally determines the ICO population fraction. We find that the formation of MGs in monatomic and binary systems involves more coordination shells than the first shell only. This investigation advances our fundamental understanding of supercooled liquid behavior.
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