熔融TiAl合金纳米粒子在TiAl(001)基底表面凝结过程中微观结构演变的原子尺度模拟

采用基于嵌入原子方法的分子动力学方法模拟了附着于TiAl合金(001)面的TiAl合金纳米粒子在不同温度下的原子堆积结构演变. 在模拟中, 熔融态(1500 K)的纳米粒子先被放置在温度分别为1100, 1000, 900, …, 200和100 K的基体(001)面, 随后急冷降温至基体温度. 通过逐层分析粒子内和基体表面的原子排列情况, 发现温度主要影响粒子内的原子堆积结构. 当基体温度很高时, 粒子内除了靠近基体的几个原子层外, 其他区域内均未形成有序的原子堆积结构. 随基体温度降低, 粒子内大部分原子逐渐形成了有序的原子堆积结构, 且粒子内出现了一个以基体(001)晶面为底面、以基体101], 101], 011], 011]晶向为轴的近四棱锥形内区域, 此区域内外的原子均呈有序排列, 但原子面的取向不同, 因而形成了明显的界面. 随基体温度进一步降低, 这个内区域仍然存在但其体积不断减小, 同时在纳米粒子顶部有越来越多的原子再次呈现无序排列, 使此内区域愈加难以辨别.

Atomical simulations of structural changes of a melted TiAl alloy particle on TiAl (001) substrate

Atomic packing structures of a melted TiAl alloy nanoparticle on TiAl(001) substrate at different temperatures are investigated by molecular dynamic simulation within the framework of embedded atom method. In order to obtain a melted TiAl alloy nanoparticle, a larger TiAl alloy bulk in nano-size is initially constructed, subsequently it is heated up to 1500 K and finally melted. A smaller sphere is extracted from the center of the melted bulk to serve as the melted nanoparticle. Periodic boundary conditions are employed in the x and y directions when constructing the sheet-like TiAl alloy substrate. In this simulation, the melted nanoparticle at 1500 K is laid on a TiAl(001) substrate, separately, at 1100, 1000, 900, …, 200 and 100 K as integral systems, and then they experience rapid solidification process. With the analysis of atomic arrangements of the nanoparticle and substrate surface layer by layer, it is found that temperature greatly affects the atomic packing structure of the nanoparticle. When the temperature of the substrate is 1100 K, most atoms in the nanoparticle disorderly pack, indicating that the nanoparticle is still melted at this temperature. At 1000 K, nearly all the atoms in the nanoparticle occupy TiAl lattice points, indicating that the nanoparticle is already solidified at this temperature. With the substrate temperature decreasing, most atoms in the nanoparticle are still of orderly pack. Meanwhile, a pyramid-like inner region, which takes TiAl(001) crystallographic plane as undersurface and TiAl 101], 101], 011], and 01 1] crystallographic axis as edges, abruptly emerges in the nanoparticle. Different atomic packing structures are observed inside and outside this region. Atomic layers composed of atoms inside this region are parallel to the (001) crystallographic plane of TiAl alloy substrate while atomic layers composed of atoms outside this region arranges along other different directions, which therefore leads to four interfaces separating the inner region from other parts of the nanoparticle. At low temperatures, this inner region still exists but its volume decreases with temperature decreasing. Besides, more and more atoms in the upper part of the nanoparticle gradually pack disorderly, which makes it more difficult to distinguish the inner region. In addition, the melted nanoparticle has very limited influences on the central and bottom parts of the substrate. However, thermal motion of atoms of substrate surface which touches the nanoparticle is intensified, thus leading to more obvious lattice distortion.