【作者】 张爱龙；

【导师】 刘让苏；

【作者基本信息】 湖南大学 ， 理论物理， 2005， 硕士

【摘要】 关于液态过渡金属的快速凝固过程,由于其外层电子的复杂性,一直是计算机模拟研究的热点和难点。目前虽已采用多种多样的势函数形式,但以EAM势函数在这方面的应用最为广泛,所以我们对原有采用非局域模型赝势的分子动力学模拟计算的程序进行改进,使之适用于采用EAM势函数。由于EAM势函数中的参数一般是由直接拟合系统的固态物理参数而得,为了使液态金属凝固过程的模拟研究得到更准确的计算结果,必须从势函数形式上反映出液态与固态之间的物理图像上的差别,必须对EAM势函数的具体形式进行适当改进。大量实验研究表明:液态金属凝固过程中,冷却速率对结构转变的快慢程度和最后结果有很重要的影响,但对其微观机理的研究还不够深入,这方面的文献也不是很多。基于上述构想,本文主要开展以下几个方面的工作: 1 对以前所用源程序进行改进,采用改进后的EAM势函数来描述液态过渡金属系统中原子之间的相互作用,对液态过渡金属Ni快速凝固过程进行模拟计算,并和量子Sutton-Chen势进行比较,发现用改进后的EAM势函数能更准确地描述系统中原子间的相互作用。 2 对液态金属Ni系统进行模拟研究时,重点考虑冷却速率对凝固过程中微观结构转变的快慢和最后形成的微观结构的影响。发现冷却速率对凝固微观结构的形成起着决定性的作用。当冷却速率在1.0×10¹⁶K·s^-1到4.0×10¹³K·s^-1之间时,系统的结构变化不大,都形成非晶态结构;而在4.0×10¹³K·s^-1到3.5×10¹³K·s^-1这个比较窄的速率区间时,系统却发生了根本性的结构转变,从非晶态结构变为以fcc和hcp二者共存的结构为主的部分晶体结构;而在3.5×10¹³K·s^-1到1.0×10¹²K·s^-1这个速率区间时,系统却发生了从以fcc和hcp二者共存的结构为主的部分晶体结构转变为仅以fcc结构占绝对优势的晶体结构了。 3 发现系统微观结构随冷却速率的变化是非线性的,即在冷却速率很快的非晶形成区段,微观结构对冷却速率不是很敏感;而在很窄的过渡区段,微观结构的变化剧烈,从非晶结构变成晶体结构;在晶体形成区段,微观结构变化在各个冷却速率下的差别相对于非晶形成段还是很大,不过随着冷却速率的继续减慢,这种差别越来越小。这就给我们提供了一个非常重要的启示:只要我们适当调整液态金属Ni在凝固过程中的冷却速率,就完全有可能获得所需的具有某种优良性能的微观结构。更多还原

【Abstract】 The rapid solidification processes of transitional metals have been considered to be a hot and difficult point for computer simulation study due to the complexity of their outer electrons. At present, various kinds of potential have been adopted in simulations, of which only the EAM potential has been widely applied, so the original program used before now, adapting energy independent non-local model pseudo-potential, has been modified to meet the need of EAM potential. Owing to the EAM potential parameters, in general, obtained by directly fitting to the physical properties of solid metals, for obtaining more accurate results in the simulation of solidification processes, it is necessary to reflect the differences between the structures of liquid and solid metals in the potential and make a suitable modification to the concrete form of potential. A lot of experiments have demonstrated that during solidification processes the cooling rate has an important effect on the transition speed and the final results of microstructures, however, their microscopic mechanisms have not deeply investigated up to now. Based on this thought, the main works in this paper are completed as follows:（1） By improving the original program used before now and using the modified EAM potential to describe the inter-atomic interaction in the system of liquid transitional metal, the rapid solidification processes of liquid transitional metal Ni have been simulated. And comparing these results with those obtained by using Quantum Sutton-Chen potential, it is found that the modified EAM potential can more accurately describe the inter-atomic interaction in the system.（2） Performing the simulation study of liquid metal Ni system, the emphasis is to consider the effects of cooling rate on the transition speed of microstructures and the structures formed in the end. It is found that the cooling rate plays a critical role in the formation of solidifying microstructures. When the cooling rates are in the range from 1.0×10¹⁶ K . s^-1 to 4.0× 10¹³ K. s^-1 , the structures in the system have not obvious change and the amorphous structures formed for all the cooling rates; while when the cooling rates are in the narrow range of 4.0×10¹³ K.s^-1 to 3.5×10¹³ K.s^-1, the essential transitions of structures are happened from amorphous to partial crystal structures, in which the fcc and hep structures are the main parts and co-exist in the system; and as the cooling rates in the range from 3.5×10¹³ K.s^-1 to 1.0×10¹² K.s^-1 , the structure transitions suddenly happened from the fcc and hep structures as the main part and co-existed in the system to the crystal structures in which only the fcc structures更多还原