【作者】 杨春；

【导师】 过增元；

【作者基本信息】 清华大学 ， 工程热物理， 2000， 博士

【摘要】 深过冷液态金属的快速凝固是制备高效金属材料的一种有效手段。但是由于对深过冷条件下的凝固机制和深过冷液态金属的物理化学性质(比热,表面张力等)的认识不足,人们尚不能准确地控制这类制备过程。液态金属的物理化学性质的测量和计算因此成为研究者十分关注的问题。 本文采取分子动力学方法对有广泛应用背景的两种纯金属以及三种二元合金的比热进行了计算和分析,并以铜镍合金为对象进行了实验研究,将实验结果与模拟结果进行相互验证。 采用熔融玻璃净化法得到四种铜镍系合金的过冷度与凝固平台的关系并研究了其结构随过冷度的变化。通过传热分析得到了凝固平台时间长度与过冷度成线性关系。在此基础上求得这四种成分合金的超过冷临界温度及其熔点以下液态的平均比热。在实验中获得的四种组分合金的最大过冷度分别达到0.83△T_h,0.83△T_h,0.78△T_h,0.82△T_h,超过了经典形核理论预言的均质形核临界过冷度0.2T_l。 以简单流体为研究对象,对汽液界面的密度、压力以及温度分布进行了模拟。模拟得到了与前人研究一致的界面密度和压力的分布,但同时发现界面中的温度分布呈非单调变化。 模拟了两种纯金属及三种合金在800K-2000K范围内的比热,以及其随温度的变化趋势,最大过冷度可达到500K,这是目前实验方法尚难以达到的温度范围。 模拟得到的液态铜、银以及铜镍合金的比热的模拟结果与实验数据的差别均在20%以内。其中铜、铜银合金以及铁镍合金的比热在较大的温度范围内维持在一个比较固定的数值上,相反,银以及铜镍合金的比热随温度的变化是非单调的。在700-900K区域内,液态金属银的比热随着温度的升高而有所升高:而在1000K到2000K范围内,比热随着温度的升高而略有下降。在1400K到2000K之间Cu-25%Ni的比热随温度的升高而略有降低;而在800K-1400K的区域内该合金的比热则随着温度的升高而升高。更多还原

【Abstract】 As an advanced processing technique for metallic materials, rapid solidification of supercooled liquid metals offer a promising way for preparing bulk amorphous metal, which is not expected for traditional rapid quenching techniques. To control the solidification process, the specific heat capacity of supercooled liquid metal is of particular importance. However, the specific heat capacity of supercooled metallic liquids is difficult to be completely determined by experiment, for the supercooled liquid metals are metastable and inaccessible for direct-contact measurement.Molecular dynamics simulation provides an available way to study the thermal properties of supercooled liquid metals numerically. The main purpose of this thesis is to investigate the heat capacities of two pure metals and three series of binary alloys numerically and experimentally.The average heat capacities of the a series undercooled Cu-Ni melts were derived by using glass fluxing technique. The undercoolings of these alloys were 381, 380, 349 and 431K respectively, which exceed the critical undercooling of the classical nucleation theory. A detailed analysis of the heat transfer condition during the solidification process was carried out, which suggested a linear relationship between the time duration of thermal arrest t_a and the undercooling ΔT. The hypercooling points of the alloys, derived from the relationship between t_a and ΔT, were determined to be 457.7, 461.1,448.4 and 528.3K respectively.Molecular dynamics simulations based on embedded-atom method and an effective pair potential are carried out to predict the specific heat of liquid copper and silver. The relationship between the specific heat of liquid metal and the undercooling are investigated. The simulations predict the specific heat of liquid copper and silver quite well and show that the specific heat of copper decreases slightly with the temperature decreasing linearly above and below the melting point, while the specific heat of silver behaves non-monotonously.A discussion about the influence of interatomic potential on the calculation of specific heat is given. The simulation results show that the bulk module has little influence on the simulation of heat capacity, while the cohesive energy and share stress module influence the heat capacity obviously.The simulation of heat capacities of Cu-25%Ni, Cu-33%Ag and Fe-33%Ni are carried out. The comparison between the predicted heat capacity of Cu-25%Ni alloy and the experiment showed reasonable agreement. The heat capacity of Cu-25%Ni behaves non-monotony, while the heat capacities of Cu-33%Ag and Fe-33%Ni keep constant in a rather large region.更多还原