The Phase Transition And Structural Heterogeneity Of The Two-dimentional Melt

Ultrathin metal films with a thickness of one or a few monolayers attract much attention, since their spectial physical and chemical properties which are different from the bulk one. Generally, the growth process of ultrathin metal film will have a direct effect on the atomic structure of surface and interface, which in turn results in their’s heat, electricity and optics properties. Therefore, the understanding on the phase transformation and structural evolution in two dimensional ultrathin metal film is significantly theoretical importance for the design of the film devices. Currently, numerous researches are devoted to build the relationship between the atomic arrangement in ultrathin metal film and their properties in nanoscale.In the present work, molecular dynamic simulations have been performed to investigate the structural evolution of quasi-two dimensional ultrathin metal film during cooling and to reveal the nature of phase transformation and structural evolution.These theories are important to guide the practical application of ultrathin metal film materials by controlling their cooling process. The main content of this thesis is as follows:(1) The structural and dynamical features of a liquid metallic nano-film during cooling are investigated by molecular dynamics simulations. Several unique structural transformations from liquid to nanocrystalline or glass are observed. Meanwhile, the crystalline fraction change and dynaical propensity are detailed discussed with different cooling rate. Results indicate a close relation between the local structural ordering and the dynamic signature. We use a new parameter P(a,τ,v) as an effective indicator for predicting both local structural order and dynamical propensity in liquid metallic film, and find the fraction of crystalline clusters follows a negative power-law scaling with the cooling rate increasing, which is the inverse of the P(a,τ,v). A quasi-two-dimensional inhomogeneous structural model, which contains both crystal-like and fully disordered regions, is proposed to interpret the origin of the splitting of the second peak in pair correlation functions of metallic glasses. Contrary to the perspective taken in previous studies, the splitting of the second peak is not the signature of the amorphous structure formation, but that of the local well-organized crystal-like structure formation. In fact, the second-peak splitting is a prototype of the crystal-like peak exhibiting distorted and vestigial features, which is the result of the statistical average of the crystal-like and disordered regions in the system.(2) The local atomic structure of the two dimensional liquid silicon during solidification is further investigated. Results show that the appearance of the left subpeak in pair correlation functions is the signature of the liquid-liquid phase transition (LLPT). The structural origin of the LLPT is the formation of a crystal-like ordered structure with a short-or medium-range scale, which in turn forms the local well-organized paracrystalline region. Unlike in the bulk liquid silicon, the stages of the LLPT and liquid-solid phase transition (LSPT) in the quasi-two-dimensional liquid silicon do not overlap. The crystal-like ordered structures formed in the LLPT are precursors which are prepared for the subsequent LSPT. Also observed was a strong interconnection between the appearance of the local well-organized paracrystalline region and the transition from the typical metal to the semimetal in the two-dimensional silicon. This study will aid in better understanding the essential phase change in two-dimensional liquid silicon. The above analysis shows that the liquid-amorphous transformation in quasi two-dimension is not just the simple freeze of liquid structure. The atomic arrangement in two-dimensional materials up cooling is not the pure results of the fully disordered clusters. The split of the second peak in pair correlation functions is not the signiture of the formation of the amorphous, but the appearance of ordered clusters with a certain percentange. It can be viewed as the result of the statistical average of the crystal-like and disordered regions in the system. The findings of this study provide physical and dynamic insights into the solidification feature of the quasi-two-dimensional metallic melt.(3) The solidification process and the glass-forming ability of Al-Fe alloy, as well as Au-Si alloy, are mainly studied under quasi-two-dimensional states. In the cooling process of two-dimensional Al-Fe liquid, it is more likely to form the short-range ordered structures which serve as the embryo. The short-range ordered structure will grow and transform into long-range crystalline structure under certain conditions. However, for the two-dimensional Au-Si, due to the different atomic arrangement of Au and Si atoms, it is different to form the crystal nuclei, so the two-dimensional Au-Si exhibits a strong glass-forming ability.(4) Dynamic elongation of the core/shell CuZr metallic nanowire (MNW) has been examined by molecular dynamics (MD) methods. Results indicate that the deformation region in the core/shell MNW represents an evident martensitic phase-transforming feature, which subsequently results in local amorphization. Meanwhile, the finite-size effect on the MNW with different crystalline-amorphous ratio is discussed. In addition, stress variation and Honeycutt and Anderson (HA) bond-type index during elongation provide reliable evidence to explain why these phenomena take place in the MNW. Our results illustrate that the corresponding martensitic phase transformation and local amorphization are closely related to the finite-size effect and crystalline-amorphous interfaces.