Structure and Phase Transformation of Atom Cluster of Alloy

本文利用高分辨电子显微镜、电子衍射和计算机模拟等手段，研究了Cu3Au和CuAu合金纳米颗粒中的有序相变和马氏体相变，以阐明尺寸效应的来源。虽然真空沉积制备的Cu3Au和CuAu合金的原子团簇具有与块体相同的晶体结构和缺陷结构，但它们的L1_2和L1_0原子有序分别存在着一定的最小尺寸，并且它们的临界温度Tc在纳米颗粒中显著降低。为了弄清这些尺寸效应的来源，我们对Cu3AU纳米颗粒中的L1_2有序化进行了计算机模拟。利用改进的嵌入原子法计算了纳米粒子的基态能量，并用…进行了计算机模拟用蒙特卡罗方法计算了更多的有序-无序转变，并用MEAM方法得到了多体势。对直径分别为2 nm和3 nm的321和1061个原子的纳米粒子模型晶体和5×5×5晶胞的块体模型晶体进行了周期边界条件的计算。结果表明，纳米粒子中完全有序与无序状态之间的基态能量之差变小，且T_c随粒子尺寸的减小而显著减小。这些结果与前人的实验结果一致。Fe-22.1-28.6at.%Ni合金沉积时的原子团簇呈岛状，由细小的体心立方马氏体晶体和可能的NiFe_2O_4氧化物晶体组成。在773K加热后，它们整体上成为面心立方单相，由平均尺寸为几纳米的孤立奥氏体颗粒组成。这样形成的奥氏体粒子即使在冷却到105K时也是完全稳定的。即使在Fe-22.1at.%Ni合金的情况下也是如此。但在77K保温10d后，奥氏体相转变为马氏体相。这一事实表明，与以前的报道相反，即使在纳米级的颗粒中也确实发生了马氏体相变，尽管是等温的。较少

In the present study, order-disorder and martensitic transformations in nano-scale particles of Cu-Au and Fe-Ni alloys, respectively, have been investigated by means of high resolution electron microscopy, electron diffraction and computer simulation, in order to make clear the origins of size effects on these phase transformations.Although atom clusters of Cu_3Au and CuAu alloys prepared by vacuum deposition have the same crystal structures and defect structures as those in bulk, it is found that there seem certain minimum sizes for the L1_2 and L1_0 atomic orderings of them, respectively, and that their critical temperatures, T_c, are remarkably depressed in the nano-scale particles. In order to make clear the origins of these size effects, we have made a computer simulation of the L1_2 ordering in the nano particles of Cu_3Au. The ground state energy of the nano particles is evaluated by utilizing the modified embedded atom method (MEAM), and then a computer simulation is made of th … More e order-disorder transition by Monte Carlo method with the many-body potential obtained by MEAM.The calculations have been made for three kinds of model crystals of the nano particles with 2 and 3 nm in diameter, containing 321 and 1061 atoms, respectively, and of a bulk with a size of 5x5x5 unit cells, on which the periodic boundary codition is imposed. As a result, it is found that the difference in ground state energy between perfectly ordered and disordered states becomes less in the nano particles than in the bulk, and that T_c decreases markedly with decreasing particle size. These results appear to account for the previous experimental ones consistently.Atom clusters of Fe-22.1-28.6at.%Ni alloys in as-deposited state are island-like, and consist of fine bcc martensite crystals and possibly oxide crystals of NiFe_2O_4. Upon heating at 773 K,they become an fcc single phase as a whole, consisting of isolated austenitic particles with average sizes of several nm in diameter. The austenitic particles thus formed are found to be entirely stable even when cooled to 105 K.This is true even in the case of an Fe-22.1at.% Ni alloy. But, when kept at 77 K for 10 d, the austenitic particles are found to be transformed into the martensitic ones. This fact indicates that contrary to previous reports the martensitic transformation does occur even in the nano-scale particles, although isothermally. Less