Effect of cooling rates on clustering towards icosahedra in rapidly solidified Cu_(56)Zr_(44) alloy

冷却速度对快凝Cu_(56)Zr_(44)合金二十面体团簇化的影响（英文）

The rapid solidification processes of liquid Cu56Zr44 alloys at different cooling rates （γ） were simulated by a molecular dynamics （MD） method. In order to assess the influence of cooling rate on the clustering tendency and degree towards icosahedrons, a ten-indices＇ cluster-type index method was suggested to characterize the local atomic structures in the super-cooled liquid and the rapidly solidified solid. And their clustering and ordering degrees as well as the packing density of ieosahedral clusters were also evaluated by an icosahedral clustering degree （fI）, the chemical order parameter （ηαβ） and densification coefficients （D0, DI and DIS）, respectively. Results show that the main local atomic configurations in Cu56Zr44 alloy system are Z12 clusters centered by Cu, and most of which are （12 0 12 0 0 0 0 0 0 0） standard icosahedra and （12 0 8 0 0 0 2 2 0 0） as well as （12 2 8 2 0 0 0 0 0 0） defective icosahedra. Below glass transition temperature （Tg）, these icosahedral clusters will be coalesced to various icosahedral medium-range orders （IMROs） by IS linkages, namely, icosahedral bond, and their number N, size n, order parameter ηαβ as well as spatial distributions vary with y. As the cooling rate exceeds the critical value （γc） at which a glassy transition can take place, a lower cooling rate, e.g., γ1=10^1K/ns, is demonstrated to be favorable to uplift the number of icosahedra and enlarge the size of IMROs compared with the higher cooling rates, e.g., γ5=10^5 K/ns, and their packing density and clustering degree towards icosahedra in the rapidly solidified solid can also benefit from the slow cooling process.

采用分子动力学方法模拟研究液态Cu56Zr44合金不同冷速(γ)下的快速凝固过程。为了评估冷速对二十面体团簇化趋势与致密化程度的影响,采用十指数的团簇类型指数方法表征过冷液体和快凝固体的局域原子结构。利用二十面体团聚度(fI)和化学序参数(ηαβ)以及致密化系数(D0,DI和DIS)分别对其团簇化和有序度以及二十面体团簇密堆积度进行评估。结果表明,Cu56Zr44合金系统中主要的局域原子结构为Cu心的Z12团簇,其中大多数是(12 0 12 0 0 0 0 0 0 0)规则二十面体和(12 0 8 0 0 0 2 2 0 0)及(12 2 8 2 0 0 0 0 0 0)缺陷二十面体。在Tg以下,二十面体基本团簇以IS铰链(二十面体键)结合成二十面体中程序(IMRO),其数目N、尺寸n、序参数ηαβ和空间分布随γ而变化。在玻璃转变的临界冷速γc以上,较低的冷速有利于二十面体数目的增加和IMRO尺寸的增大。同时,较慢的冷却过程有助于快凝固体中IMRO堆积密度的增加和二十面体团簇化程度的提高。