A Simulation Study On The Heredity And Evolution Of Clusters In Liquid Cu-Zr Alloys During The Rapid Solidification

It is well known that glass-forming ability(GFA) is intimately correlated with the local atomic structures of alloy melts. Thus, an investigation on the heredity and evolution of clusters of liquid alloys in the rapid solidification can not only provide a deep insight into glass transition phenomena, but also is helpful for the understanding of GFA from the point view of microstructure. In this thesis, the molecular dynamics(MD) method is used to simulate the rapid solidification processes of liquid Cu-Zr alloys under different conditions. By means of pair distribution function, Honeycutt-Andersen(H-A) bond-type index method, and extended cluster-type index method(CTIM), the microstructures of the systems are analyzed. An inverse tracking of atom traces method is further adopted to examine the heredity and evolution of clusters in supercooled liquid alloys during the rapid solidification. And the possible relationship between the heredity and GFA is discussed. The following provides the main contents of this thesis.Firstly, a simulation study on the rapid solidification process of liquid Cu50Zr50 alloy at a cooling rate of 5 K/ns is performed using Finnis-Sinclair(F-S) many-body potential. The results show that both of liquid and rapidly solidified solids mostly consist of(12 12/1551) standard icosahedra and(12 8/1551 2/1541 2/1431) defective icosahedra, and the most of which are Cu-centered Cu6Zr7 clusters, followed by Cu7Zr6 and then Cu8Zr5 clusters. The size distribution of icosahedral medium-range order(IMRO) clusters linked by intercross-sharing(IS) atoms in the liquid and the glassy solid respectively presents a magic number sequences of 13, 19, 23, … and 13, 19, 23, 25, 29, 37, … The track of atoms reveals no icosahedral clusters in rapidly solidified solid can be detected in the liquid alloy. The initial heredity emerges in the super-cooled liquid region of Tm~Tg and a significant ascent in heredity fraction fi takes place at temperature nearby Tg. Among perfect, core and segmental modes the perfect heredity of icosahedra is demonstrated to be dominant below Tg. Relative to(12 2/1431 2/1541 8/1551) defective icosahedra, the standard(12 12/1551) icosahedra are of high structural stability and large hereditary ability, and most of which are passed down by means of the configuration rather than the chemical order. By the partial heredity, some large extended icosahedral clusters, even IMRO in the rapidly solidified solid is also able to be inherited from super-cooled liquids.Further, the heredity of icosahedral clusters of rapidly solidified Cu 56Zr44 alloys and its correlation with the GFA under various cooling rates and pressures are investigated. With increasing cooling rates γ, the glass transformation temperature Tg rises linearly with lgγ, but the clustering degree towards icosahedra descends in the rapidly solidified solid, especially in the range of 300K~Tg. In the case of high γ, not only the descendible fractions fi of icosahedral clusters above Tg are enlarged, but their initial descendible temperatures Tonset in the super-cooled liquid region are elevated as well. At the high cooling rate of γ5>105K/ns, the onset of configuration heredity of(12 12/1551) icosahedra can even emerge in the liquid region of Cu56Zr44 alloys. The pressure P can also remarkably affect the hereditary behavior and descendible ability of icosahedral clusters. An increasing P is demonstrated to be profitable for elevating the initial descendible temperatures Tonset of(12 12/1551) standard icosahedra and increasing descendible fractions fi, especially the fraction fp of perfect heredity. The P-dependent GFA of Cu56Zr44 could be approximately estimated by the relative onset hereditary temperature ?Tc and reduced hereditary super-cooling degree Ks.Finally, a systemic simulation investigation is performed to establish the relationship between the GFA and the heredity of icosahedral clusters of rapidly solidified Cux Zr100-x alloys. It is found that the icosahedral clusters, especially the IMROs are demonstrated to play a key role in the formation of Cu x Zr100-x metallic glasses. An inverse tracking of atom traces reveals the heredity of icosahedral clusters is an intrinsic feature of rapidly solidified alloys; their onset temperature merely emerges in the super-cooled liquid region of Tm ~ Tg. As the icosahedral clusters inherited from the super-cooled liquid being selected as a candidate, it is found a direct and continuous heredity of IMROs from super-cooled liquids to rapidly solidified solids is closely related to the formation of Cux Zr100-x glassy alloys. Their relative quantity fcore(T) or fatom(T) not only is a measure of glass transition, but also the magnitude fcore(300K) or fatom(300K) at 300 K can be utilized to roughly evaluate the x-dependent GFA of Cux Zr100-x alloy system in the rapid solidification. As the difference bettwenhe onset temperature of heredity Tonset and critical transition temperature Tc is selected as an assessment parameter, their local maximums are found to exactly match with the best glass forming compositions in experiments.