| In this thesis, the local structure and dynamical properties of elemental zirconium and silicon alloys by using ab initio molecular dynamics simulations and experiments. The work can be divided into three parts:local structure of Zr liquid and undercooled liquid; local structure and dynamical properties of Cu80Si20; the effect of Sr addition to the structure and glass forming ability of Al88Si12.Zr is one of the elements that can easily form glasses. The Zr-Cu system is a good glass former. Though classical theory proposed that glass can be formed at rapid enough quenching rate for almost all materials, the Zr glass is still not found in experiments. In this study, atomic cluster alignment method was adopted to analysis the local structure. Among the five temperatures studied in this work, no clear order is found at2500,2400,2290and2135K, respectively, after alignments, indicating complex short-range orders in the system. As temperature decreases, the alignment result turns out to be bcc like at1830K, which means that bcc becomes the dominant phase in the undercooled liquid while the existence of other short-range orders is still possible. After that, we divided the molecular dynamics steps into10short steps at2135and1830K, and do collective alignment. At2135K, different steps turn out to be icosahedral or bcc like. The icosahedral ones are more than the bcc ones. At1830K, however, the bcc order comes from behind. The above results suggest that competition and fluctuation between icosahedral and bcc short-range orders exist in the liquid and undercooled liquid Zr. The existence of bcc order stops the effect of icosahedral orders’ restraining the crystalline process, hindering the glass transition.The concentration of20%Si (atomic) is close to the eutectie point of Cu-Si alloys. There exist several complex crystalline phases about the melting temperature. Thus, this study can deepen our understanding on the relation between liquid structure and metallic glasses. In this work, the local structure and dynamical properties of Cu80Si20liquid and glass is studied by using molecular dynamics simulation and X-ray diffraction experiment. In the liquid, as temperature becomes lower, icosahedral short-range order develops while other orders decline. The coordination number derived from the Voronoi tessellation method of Si centered clusters is smaller. Besides, most fee clusters in the liquid are Cu centered. This can be interpreted like this:the Cu atoms tend to form crystals while the Si atoms with larger atomic radius can fill the interstice in the space, holding up the non-crystalline characteristic. Using the same quenching rate, the sample at300K is obtained. With respect to liquid state, the pair correlation function shows the glass behavior with sharper peaks and a shoulder peak at the larger r side of the second peak. The155index becomes dominant while142and other crystalline like indexes decrease to below5%. The Voronoi tessellation gives the same result. The perfect icosahedron index (0,0,12,0) exceeds10%, which is a big portion for Voronoi (Voronoi is a strict analysis method). By using the atomic cluster alignment, we find that pure Cu crystalizes quickly after the temperature drops below the melting point. CugoSi2o remains to be icosahedral short-range order dominated, suggesting that Si is positive for Cu to form glass.The concentration of12%Si (atomic) is the eutectie point of Al-Si alloys. The Si atoms tends to form coarse flake-like dendrites in the alloy, degrading the mechanical property. It is reported that addition of subtle elements can modify the micro structure, refine the grain size, and decrease the gaps. Sr was proved to be able to modify the AlSi alloys. Nevertheless, the underlying mechanism has not been clarified yet. In this work, different amount of Al atoms was replaced by Sr in Al88Si12liquid to study the modification effect, Al88Si12, Al87.5Si12Sr0.5and Al84Si12Sr4. The results turn out that0.5%Sr atom has only negligible affection on the structure of Al88Si12. The calculated coordination numbers’becoming smaller. Besides, the diffusion of Sr is much slower than that of Al and Si, and thus lowers the total diffusion coefficient of the system. By replacing4%Al atoms, the correlation between Al and Al increases slightly; Si and Si tend to separate; the correlation between Si and Sr turns out to be stronge. These results suggest that the addition of Sr greatly modified the local structure of Al88Si12liquid by disaggregating the Si atoms. By further quenching the sample to room temperature, the effect of Sr to Al-Si alloys under melting point is studied. The potential glass formatting mechanism is discussed. |
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