二十面体团簇的遗传:一个与快凝Cu56Zr44合金玻璃形成能力有关的动力学参数

采用分子动力学方法模拟研究了液态Cu₅₆Zr₄₄合金在不同冷速γ与压力P下的快速凝固过程, 并通过基于Honeycutt-Andersen键型指数的扩展团簇类型指数法对其微结构演变特性进行了分析. 结果表明: 快凝玻璃合金的局域原子组态主要是(12 12/1551)规则二十面体、以及 (12 8/1551 2/1541 2/1431)与(12 2/1441 8/1551 2/1661) 缺陷二十面体. 通过原子轨迹的逆向跟踪分析发现: 从过冷液体中遗传下来的二十面体对快凝合金的玻璃形成能力(GFA)具有重要影响, 不仅其可遗传分数F_i =N_{300 K←Tg}ⁱ/N_Tg 与GFA密切相关, 而且其遗传起始温度(T_onset)与合金约化玻璃转变温度T_rg = T_g/T_m也存在很好的对应关系.

Heredity of icosahedrons: a kinetic parameter related to glass-forming abilities of rapidly solidified Cu56Zr44 alloys

To explore the origin of glassy transition and glass-forming abilities (GFAs) of transition metal-transition metal (TM-TM) alloys from the microstructural point of view, a series of molecular dynamics (MD) simulation for the rapid solidification processes of liquid Cu₅₆Zr₄₄alloys at various cooling rates γ and pressures P are performed by using a LAMPS program. On the basis of Honeycutt-Andersen (H-A) bond-type index (ijkl), we propose an extended cluster-type index (Z, n/(ijkl)) method to characterize and analyze the microstructures of the alloy melts as well as their evolution in the rapid solidification. It is found that the majority of local atomic configurations in the rapidly solidified alloy are (12 12/1551) icosahedra, as well as (12 8/1551 2/1541 2/1431) and (12 2/1441 8/1551 2/1661) defective icosahedra, but no relationship can be seen between their number N(300 m K) and the glassy transition temperature T_g of rapidly solidified Cu₅₆Zr₄₄alloys. By an inverse tracking of atom trajectories from low temperatures to high temperatures the configuration heredity of icosahedral clusters in liquid is discovered to be an intrinsic feature of rapidly solidified alloys; the onset of heredity merely emerges in the super-cooled liquid rather than the initial alloy melt. Among these the (12 12/1551) standard icosahedra inherited from the super-cooled liquids at T_m-T_g is demonstrated to play a key role in the formation of Cu₅₆Zr₄₄ glassy alloys. Not only is their number N_{300 K←Tg}^P inherited from T_g to 300 K closely related to the GFA of rapidly solidified Cu₅₆Zr₄₄alloys, but a good correspondence of the onset temperatures of heredity (T_onset) with the reduced glass transition temperature (T_rg= T_g/T_m) can be also observed. As for the influence of γ and P on the glassy transition, a continuous tracking of descendible icosahedra reveals that the high GFA of rapidly solidified Cu₅₆Zr₄₄ alloys caused by big γ and P can be attributed to their elevated inheritable fraction (f_p and f_total) above T_g.