Abstract
The nucleation mechanism of Ti<sub>3</sub>Al alloy is simulated by the molecular dynamics method in this work. The atomic clusters on different spatial scales are identified in the solidification process by the cluster-type index method (CTIM), and the formation process and the growth process of critical nucleus are studied in depth. It is found that the solidification system contains ten thousands of different types of atomic cluster structures, but only 22 types play a key role in the nucleation process. In the nucleation and growth process of nuclei, the ICO-like cluster, the BCC-like cluster, and the defective FCC cluster and the defective HCP cluster respectively reach their saturation points at the characteristic temperature <i>T</i><sub>1</sub> (1110 K), <i>T</i><sub>2</sub> (1085 K) and <i>T</i><sub>3</sub> (1010 K). And the competition processes of these clusters are revealed according to the changes of their number and spatial distribution with temperature. By tracing the nucleation and growth process of the grain with parallel twin, it is found that the critical crystal nucleus is composed of single-phase FCC structures, and the preferent nucleation of metastable bcc structure is not observed. The twinned structure is formed by the layer-by-layer growth along the close-packed plane. It is also found that the CTIM is more accurate than other methods in revealing the microstructural characteristics during the solidification.
Highlights
Topological structure of larger atomic clusters characterized by cluster-type index method (CTIM): (a) Cluster with 220 atoms consisting of one HCP basic atomic cluster (12, 6/1421, 6/1422) and one FCC basic atomic cluster (12, 12/1421); (b) nanocluster with 38 atoms consisting of six FCC basic atomic clusters (12, 12/1421)
临界晶核的形成经历了无序原子局域 堆积紧密程度增加后, 逐渐从 ICO 结构或无序结 构转变为单相 FCC 结构, 未观察到 BCC 亚稳相 在 ICO 聚集区优先形核, 随后向 HCP 亚稳相和 FCC 稳定相顺序转化的过程
Summary
随后 Li 等 [12] 进一步借助于公共近邻分析 (CNA)[15] 方法对结晶形核过程各种微观结构组 态的数目进行了统计分析, 并结合可视化方法, 发 现 TiAl 合金结晶形核过程经历了 BCC 亚稳相在 二十面体聚集区优先形核, 随后向 HCP 亚稳相和 FCC 稳定相顺序转化的过程. 典型的液态或非 晶态结构中大量出现的二十面体 (ICO) 以 1551 键 对的大量存在为特征; 对于各种晶体结构, FCC 结 构以 1421 键对为特征, HCP 结构以 1421 和 1422 键对为特征, BCC 结构则以 1441 和 1661 键对为 特征. CNA 方法只能对 FCC, HCP, BCC 和 ICO 等 局域原子结构的部分结构组态进行识别, 为了有效 识别不同局域原子结构组态, 我们在 CNA 方法的 基础上建立了一种新的微观结构表征方法, 即 CTIM [17−19].
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