镁合金中二十面体准晶固态形核与生长机制的原子尺度研究

Mg alloys with icosahedral quasicrystals (IQCs) show superior mechanical properties, due to the unique physical and mechanical properties of IQCs, and excellent resistance to cracking at IQC/Mg interfaces as a result of compatible interfacial connection between IQCs and Mg matrix. Precipitation of nanometersized IQC particles can be an effective way to strengthen Mg alloys. In spite of great achievements on quasicrystal studies in the past decades, the fundamental questions concerning nucleation and growth of quasicrystals are still not well understood. We will determine atomic structures of icosahedral clusters that formed during heat treatments of Mg-Zn-RE alloys, based on sub-angstrom resolution imaging by aberration-corrected electron microscopy, in combination with theoretical calculations and modeling. The formation of core icosahedral clusters consisting of 12 atoms and their growth into Bergman clusters consisting of 136 atoms are the key information to understand the mechanism of IQC nucleation. Further, we will determine the atomic configuration at the surface layer of tiny IQC precipitates and defects may exist herein, in order to understand the growth mechanism of IQC. Besides homogeneous nucleation of IQCs in hexagonal matrix of Mg solid solution, the heterogeneous formation of IQCs along dislocations, at grain boundaries and interfaces between Mg matrix and other crystalline phases will be addressed in the present study. Our research is devoted to the understanding of solid state origin of quasicrystals in crystalline materials, which may advance the theory of phase transformations of quasicrystals, and shed light on optimization of mechanical properties of Mg alloys through precipitation of IQC nanoparticles.

二十面体准晶（IQC）独特的物理和力学性质，及IQC与Mg基体间界面良好的抗裂能力，使得IQC强化的Mg合金具有优良的力学性能。IQC在固态下的弥散析出可能作为强化镁合金的一种新的途径。但准晶的原子尺度形成机理一直是个难题。本项目拟通过亚埃分辨率像差校正电子显微学实验研究结合理论计算模拟，首先确定固态相变过程中，Mg-Zn-RE合金内形成二十面体团簇及其长大成Bergman团簇的原子尺度机理，然后获得细小IQC析出颗粒表层（生长前沿）结构和缺陷的原子构型，并借助透射电镜观察获得IQC颗粒尺寸随时效时间的变化规律，揭示其生长过程和影响因素，从而阐明Mg合金中IQC的固态形核生长机制。除了均匀形核析出，本项目还研究IQC在位错、晶界及相界上固态形核析出。研究结果将阐明固态下准晶起源和析出机制，并为通过准晶沉淀优化Mg合金的力学性能提供理论指导。

本项目利用先进的亚埃分辨率电子显微学方法，在原子尺度上获得了关于在镁合金内部形成二十面体结构单元的过程中关键合金元素聚集和重新占位等方面的结构信息，为揭示准晶的形核与生长机理提供了重要数据。利用原子分辨率Z衬度成像结合高维超空间投影的方法解析了Mg合金中Mg3Zn6Y二十面体准晶的三维原子结构。Mg3Zn6Y准晶的Bergman团簇最外层是由20个Y原子和12个Mg原子构成的菱边三十面体，Y原子构成一个五角十二面体。我们阐明了位错作为异质形核点诱发准晶形核的原子尺度机理。此外，除了广泛报道的三维二十面体准晶，我们还在镁合金中发现了多种二维十次准晶，并制备了多种纳米尺寸准晶颗粒强化的镁合金。我们的研究结果不仅在准晶的基础研究方面取得了很有意义的进展，而且为开发准晶强化的镁合金提供了重要科学基础。在项目的资助下，培养毕业3名博士研究生和1名硕士研究生，已有16篇研究论文发表在国内外学术期刊上，还有部分工作有待发表。