面心立方金属凝固过程中孪生枝晶的形核机理研究

Twinned dendrites of primary phase often occur in solidification of face-centered cubic (fcc) metals with a high stacking fault energy, but their formation mechanism is unclear. One of the possible mechanisms is quasicrystal-induced nucleation of twins. In this proposal, samples of pure Al and Al-Mn alloys with twin boundaries or twinned dendrites are prepared by vacuum suction casting and rapid solidification. The effects of microalloying elements and solidification conditions on formation of icosahedra and twins or twinned dendrties are investigated experimentally. The crystallographic orientation relation between fcc and quasicrystal phases and that between fcc matrices in the microstructures of the samples are characterized by means of high resolution transmission electron microscopy and electron back-scattering diffraction technique of the scanning electron microscopy for the purpose of providing evidence for quasicrystal-induced nucleation of twins. Based on experimental observations, the thermodynamic conditions of epitaxial growth of fcc crystals on quasicrystals are established by calculating and analyzing the changes of Gibbs free energy crossing liquid-solid transformations aiming at an understanding of the physical mechanism of quasicrystal-induced nucleation of twins of fcc crystalline phases. Finally, mechanical properties of commercial Al alloys containing a higher volume fraction of twin boundaries are measured to reveal and understand the influence of twin boundaries or twinned dendrites. The proposed studies may extend the classical theories of heterogeneous nucleation in liquid metals to twin formation. The studies may also bring about ideas of developing new technologies and theories for tailoring of solidification microstructure of engineering materials with a fcc structure as advanced Al alloys.

高层错能面心立方（fcc）金属凝固过程中初生相常会呈现孪生枝晶，但是其形成机理尚不清楚。一种可能的机理是准晶诱发了孪晶的生成。为了检验这一假设，本项目以工业纯铝和Al-Mn合金为研究对象，采用真空吸铸和急冷快速凝固法制备样品，系统研究微合金化与凝固条件对二十面体准晶和孪晶形成的影响规律，通过电子背散射衍射技术和高分辨透射电镜分别对样品凝固组织中面心立方相晶粒之间、及其与准晶相间的晶体学取向关系进行表征，获得准晶诱发孪晶形核的实验证据。在实验基础上，通过相变自由能计算分析准晶外延形成面心立方相的热力学条件，揭示准晶诱发孪晶形核的物理机制。最后，项目对凝固组织中含有较高体积分数孪晶界的工业铝合金试样的力学性能进行测试，分析孪晶对力学性能的影响规律和微观机制。项目研究结果有望为丰富液态金属的异质形核理论提供新的实验素材，同时为调控先进铝合金等这类面心立方工程材料的凝固组织提供新技术和新原理。

项目研究了微量元素Ti、Sc、Zr及熔体过热处理对铝合金凝固过程中晶粒细化的影响，利用SEM-EBSD、TEM技术表征了二十面体准晶相以及孪晶界的特征，分析了具有孪晶界的面心立方相间、以及准晶相与面心立方相间的多重孪生取向关系，探讨了准晶相的优先形核与依附于准晶外延形成面心立方相的晶体学条件，阐明了铝合金面心立方合金凝固过程中准晶诱发孪晶的形核机理；利用FIB、HRTEM技术表征了形核颗粒（Al3Ti、Al3Sc、Al3Zr）与铝基体间的晶体学结构关系，结合E2E模型计算，分析了形核颗粒与面心立方相的晶体学条件，提出了基于晶体学取向关系的晶粒细化机制；研究了铝合金定向凝固组织随溶质含量与凝固条件的影响，利用SEM-EBSD技术表征了合金组织取向的演变规律及孪生枝晶的结构特征，揭示了铝合金凝固组织中孪生枝晶的形成条件，阐明了孪生枝晶凝固组织的形成机理。在此研究基础上，对凝固组织中含有较高体积分数孪晶的工业铝合金试样的力学性能进行了测试，分析了孪晶或孪生枝晶对力学性能的影响，实现铝合金中孪晶界凝固组织及其力学性能的调控。.项目获得的面心立方金属合金凝固过程中准晶诱发孪晶的形核机理、基于晶体学取向关系的铝合金晶粒细化机制，以及铝合金孪生枝晶凝固组织的形成机理，不仅丰富了面心立方金属中的晶体形核理论，为晶体形核理论提供新的实验支撑，而且对于先进高性能铝合金工程材料的晶粒细化及凝固组织缺陷控制具有实践指导意义，为先进铝合金等这类面心立方金属合金工程材料的凝固组织调控提供新技术和新原理。