【摘要】 本文以模型合金SCN-Ace.的强制性晶体生长实验为基础,系统分析了低熔化熵材料的生长端部、固/液界面和形态的稳定性。发现稳定的生长形态是在体系中诸因素影响下,通过初始非稳态的过渡过程形成的。本文报告的规律对于优化传统材料和开发新型材料具有广泛意义。更多还原

【Abstract】 The stability of constrained crystal growth is foundation of fabricating structural or functional materials during directional solidification, rapid solidification, crystal growth and solidification in space. The widely quoted theories, such as "constituational supercooling" and M-S stability analysis [1,2], have laid a basis for the understanding of the planar interface of melt/solid only. Although there has been a little progress on constrained dendritic growth recently [5,8,10], the physical nature of the growth stability and the pattern selection for the whole constrained growth process are not well understood. Because of this, it has limited the optimization of traditional materials and the development of advanced materials. In this paper, new experimental phenomena have been presented for steadystate crystal growth as well as planar-cell or planar-dendrite and cell-dendrite transitions in a typical single-phase-alloy succinonitrile-acetone system. A general framework has been proposed for the stability of growing fronts, interface, and pattern based on the experimental results. It has been found for the first time that the steady-state growth patterns are always formed during the nonsteady-state transient process at the early stage of destabilized planar interface under the influence of various factors in the solidification system, e.g., heat and solute transport, crystalline anistropy, noise and internal dissipation. The final microstructure is also dependent on the initial nonsteady-state transient process. This paper’s discussions on the problems addressed can be used to guide the fabrication of high quality single crystals and the cellular or dendritic functional and structural materials. The major conclusions are as follows: 1. There are different stability mechanisms for steady-state cellular and dendritic growth. The dendritic tip pattern agrees with the principle of marginal stability and the growth kinetics of cellular pattern is more complex. 2. There is a relaxation time when cellular or dendritic pattern is selected from the destabilized planar interface. The early state and random fluctuation have strong influence not only on the spacing but also on the relaxation time for cell, but only on the relaxation time for dendrites. 3. The planar-cell or planar-dendrite is a mutation transition because the destabilizing of planar interface is of mutability, but the cell-dendrite may not be so. The cell-dendrite transition can be described as follows: there is a perturbation wave propagating along the sides of solidified cell; the cell will become dendrite if the imaginary part of the increasing rate of oscillating propagation of perturbation wave is not zero and the real part is larger than zero.更多还原