【作者】 秦博；

【导师】 赖文生；

【作者基本信息】 清华大学 ， 材料科学与工程， 2010， 硕士

【摘要】 自从1960年在Au-Si系统中首次通过液态熔融淬火得到金属玻璃(或非晶态合金)以来,人们对该领域的一个重要科学问题,即玻璃形成能力,开展了很多实验和理论研究。对一个特定的系统,玻璃形成能力可以用很多经验参数来表征,如约化温度Trg,稳定因子S和Tl-Tg。实际上玻璃形成能力的定量表征方法是非晶形成成分范围。在理论研究中,柳百新课题组提出了从原子作用势出发,通过分子动力学模拟方法来计算二元系统的非晶形成范围。本论文中,我们采用同样的方法研究了Cu-Ti二元系统和Cu-Zr-Ti三元系统非晶形成范围。首先构建了Cu-Ti和Cu-Zr-Ti系统的紧束缚(TB-SAM)多体势,然后用分子动力学模拟研究了玻璃转变、非晶形成范围及Cu-Ti系统中的hcp-to-fco马氏体相变。基于固溶体模型,分子动力学模拟研究表明:Cu-Ti系统的非晶形成范围为22at.%~71at.%Cu,与实验结果相符。配位数和公共近邻分析揭示了固溶体和非晶相能量差异的物理机制:与固溶体相比,随着溶质原子增加,非晶相的配位数和异类键数目变化更快,致使非晶相的能量下降更快,使其与固溶体能量曲线相交。这些结果表明固溶体和同成分的非晶相的相对稳定性与它们的微观结构有密切的联系。Cu-Zr-Ti系统的非晶形成范围是一个变形的四边形,四个顶点的成分分别为:Cu22Zr78Ti0,Cu24Zr0Ti76,Cu56Zr0Ti44和Cu72Zr28Ti0。位于该四边形内的成分点,非晶相比固溶体更稳定。模拟结果也与Egima玻璃形成经验规则和柳百新的结构差异规则相符。在Cu-Ti系统的富Ti一端不断地溶入Cu原子,观察到了hcp-to-fco马氏体相变。公共近邻分析表明fco相是一个类bcc的结构。通过对fco新相的不同相貌的分析,我们提出了一个简单合理的相变机制,即hcp晶体沿<100>密排方向伸长,(001)密排面之间有轻微的收缩,且相邻密排面之间沿<120>晶向的相对滑移,伴随点阵的调整,从而形成了新的fco相。更多还原

【Abstract】 Metallic glass (or amorphous alloy) was first obtained in the Au-Si system by liquid melt quenching (LMQ) in 1960 and since then a number of experiments and theoretical studies have been carried out to investigate one of the basic issues in the field, i.e. glass-forming ability (GFA). For a specific alloy, the GFA is commonly estimated by a number of empirical parameters, such as the reduced temperature Trg, stability parameter S and Tl-Tg. Practically, a quantitative measure of the GFA is glass-forming composition range (GFR), within which amorphous alloy can be obtained via some glass-producing technique. In theoretical studies, Liu et al. have proposed an atomistic method to determine the GFR of a binary metal system directly from the interatomic potential of the system through molecular dynamics (MD) simulations. In this thesis, we first study the GFRs of the Cu-Ti binary system with Liu’s method, and then extend the same idea to the Cu-Zr-Ti ternary system. The tight binding (TB-SMA) many-body potentials for the Cu-Ti and Cu-Zr-Ti systems were constructed and applied in MD simulations to study crystalline-to-amorphous transition and GFR in both binary and ternary systems, as well as an hcp-to-fco martensitic transformation in the Cu-Ti system.Based on solid solution models, the MD simulations using the constructed potentials show:The GFR of the Cu-Ti system is predicted to be 22at.%~71at.%Cu, in good agreement with experimental results. Coordination number and common-neighbor analyses clarified the physical mechanism responsible for the energy difference between solid solution and amorphous phase as follows: with increasing the solute atom content, the coordination number and unlike bond of amorphous phases increase greatly than those of solid solutions, leading to a cross-over point between their energy curves because of a fast energy drop of amorphous phases than the solid solutions. These results show that the relative stability between solid solution and amorphous phase is correlated to their microstructure.The GFR of Cu-Zr-Ti system is located in an approximate distorted quadrilateral region, and the compositions of the four vertexes of the quadrilateral are Cu22Zr78Ti0, Cu24Zr0Ti76, Cu56Zr0Ti44 and Cu72Zr28Ti0, respectively. In addition, the simulation results are in accordance with Egami’s glass-forming and Liu’s structural difference empirical rules.An hcp-to-fco martensitic transformation was observed in Ti lattice, upon dissolution of Cu atoms. Common-neighbor analysis showed that the new formed fco phase features bcc-like structure. Based on the detailed analysis of different manifestations observed in simulations, the hcp-to-fco phase transformation mechanism is proposed as follows: the hcp lattice elongates along the <100> close-packed directions and slightly shrinks along [001] direction, accompanying with a relative movement (or slide) along <120> directions between the adjacent close-packed planes and the lattice constants adjustment to form the new fco phase.更多还原