过热温度和冷却速率对过冷Ti熔体凝固过程的影响

Effect of Superheated Temperature and Cooling Rate on the Solidification of Undercooled Ti Melt

采用分子动力学方法研究了金属Ti熔体的凝固过程,通过径向分布函数、H-A键型结构以及最大原子团簇方法分析了Ti的凝固组织。结果表明,金属Ti熔体的凝固过冷度随过热温度的升高而增大,且过冷度与过热温度的变化曲线上出现2次转折:T_1=2100 K和T_2=2490 K,分别对应于形核团簇的原子键破坏起始温度和破坏终了温度。在此温度区间,过热熔体中微观晶核团簇随温度升高而减少。当过热温度增大到一定程度(大于T_2),其过冷度将维持定值;同时,金属Ti熔体的过冷度也随冷速的增大而增大,直到非晶结构形成;金属Ti形成非晶的临界冷速为1.0×10^(13)K/s。

Undercooling is an important parameter to characterize the process of solidification and the physical properties of the melt. However, the traditional experimental conditions do not provide mature technical conditions and experimental platforms for the study of this subject. Molecular dynamics simulation method can not only study the experimental process and the organization structure, but also break through the limited conditions of the laboratory, and provide advanced prediction for scientific research. In order to study the influences of superheated temperature and cooling rate on the undercooling of the homogeneous nucleation and the solidified structure, the solidification of undercooled Ti melt was studied by molecular dynamics simulation in this work; and the solidified structure was then analyzed by the radial analysis, the H-A key type analysis and the largest groups of cluster analysis. The results show that, the nucleation undercooling of Ti melt increases with the rise of superheated temperature. In the undercooling vs temperature curve there are two inflection points at 2100 K（T1） and 2490 K（T2）, which correspond to the breaking-start temperature and breaking-end temperature for bond pair of nucleation cluster. In this temperature range, the number of nucleation clusters decreases with rise of temperature.When the superheated temperature is higher than T2, the nucleation undercooling approaches a constant. On the other hand, the nucleation undercooling of Ti melt increases with the accelerate of cooling rate until an anomalous structure is formed, and in the numbers of the bonds of the structure vs different cooling rate curves, the number of 1541, 1551 and 1431 bond types gradually adds with cooling rate go-ing up. In addition, when the cooling rate is less than 1.0×10^（11） K/s, the hcp and bcc inlaid crystalline structures are obtained after the solidification of Ti melt. When the cooling rate is greater than or equal to 1.0×1013 K/s, two kinds of crystalline structure are reduced, and the microstructures are mainly amorphous.When the cooling rate ranges between 1.0×10^（11） K/s and 1.0×1013 K/s, its structure is a mixture of crystalline and amorphous. From the results of radial distribution, H-A bond type and atomic cluster analysis, it was found that the critical cooling rate for amorphous structure is determined as 1.0×1013 K/s.