包含三维复杂形状枝晶的多相组织变形行为研究

Microstructures of most engineering materials are typically characterized by three-dimensional, heterogeneous, polycrystalline, multiphase, and interconnected network features. Real 3D microstructure-based or direct finite element modeling to predict mechanical behavior and time-resolved in situ four-dimensional characterization under loading are the cutting-edge research fields. Magnesium and aluminum alloys are important lightweight materials, and they, generally, undergo the solidification process and form complex dendritic structures before the final product. Therefore the study of such topologically and morphologically complex 3D microstructures and their deformation behavior remains long-standing interest from both fundamental and practical point of view. The present work includes three main parts: 1) As-solidified alloy specimens are processed using two solidification methods and their microstructures are characterized by synchrotron-based X-ray computed microtomography (μCT); 2) The mechanical parameters and constitutive equation, of single microconstituents, will be obtained as a function of orientations using a combination of focused ion beam (FIB) and electron backscatter diffraction (EBSD), and micro/nanopillars nanoindentation; 3) Mechanical response of microstructures containing complex dendrites geometrically will be modelled utilizing real 3D microstructures and mechanical information of microconstituents as a direct input, and both in situ μCT and small-angle X-ray scattering and wide-angle X-ray diffraction (SAXS/WAXD) tensile experiments will be carried out to verify microstructures–deformation correlation model. This research highlights the potential to apply experimental and modelling approaches to understand the deformation behavior of, complex geometrically, dendrites and inhomogeneous duplex or multiphase alloys and composites, and its methodology and universality also can be benefited by other fields.

多数工程材料的微观组织为典型的三维异质多相多晶网状互联结构，基于真实三维组织的力学性能模拟，以及加载条件下考虑时间的四维原位实时研究有着重要意义。镁铝合金是重要的轻量化合金，在成形过程中一般都会经历凝固行为并形成三维复杂形状树枝晶，研究复杂形状三维枝晶凝固多相组织的变形行为有着重要的科学内涵。具体内容为：1）首先用两种凝固方法制备镁铝合金样品并用同步辐射X射线μCT技术获取凝固合金的三维微观组织；2）用聚焦离子束技术结合EBSD取向标定对合金样品定位制备微柱试样，并用平头微柱单压技术获取各个组成相的力学性能参数和本构关系；3）将真实三维组织和组成相的力学参数作为输入进行有限元建模，研究复杂形状多相组成的力学行为响应，提出模型并用原位的μCT及小角/宽角散射实验验证模拟结果及修正力学模型。该研究及方法论对认识包含复杂结构的异质两相及多相合金以及复合材料的三维组织/力学行为有着普适性。

一、背景.已有的研究多集中在简单晶粒形状和单相组成，对复杂形状枝晶组织组成的三维凝固组织力学行为研究甚少。同时在基于真实组织的三维模拟过程中，也没有使用微力学测试实验去获取单个微观组成的本构关系并将其作为有限元建模的初始输入。.二、主要研究内容.1）对Mg–Ca、Mg–Zn和Mg–Sn开展了同步辐射X射线μCT的静态表征和原位观察实验。选取镁合金体系，因为Mg为h.c.p.结构、Ca为f.c.c.结构、Zn为h.c.p.结构、Sn为b.c.t.结构，从而研究不同晶格类型固溶元素对α-Mg(X)先析出相本身晶格畸变的影响进而对形貌及性能的影响等现象。.2）对Mg–Ca、Nb–Si和Nb–Si–Ti合金系开展了FIB微柱加工和微力学测试实验，先采用EBSD在不同组成相上进行晶体学取向信息的收集，然后用FIB对EBSD取向标定后的区域进行离子束加工制备200nm–10μm范围内不同直径、长度约为直径二倍的圆柱形试样，获得单个组成相依赖于取向的力学性能参数和应力应变关系。.三、重要结果及关键数据.1）结果证明Mg–9 wt.%Ca合金中的α-Mg树枝晶形貌主干周围发生了3、4、5次分枝样式，通过精细的晶体学分析得出Mg–9 wt.%Ca合金中α-Mg(Ca)树枝晶的优先生长方向为<1120>和<2245>。发现随着Zn含量的添加，α-Mg(Zn)树枝晶倾向于分裂并向基面连续偏转的形貌转变现象，进而表明了α-Mg树枝晶的演化机制。Mg–15 wt.%Sn合金的凝固过程出现了6模对称的18次分枝树枝晶组织，论证了α-Mg(X)树枝晶生产过程演变的复杂性及形貌形成的多样性。.2）获取了Nb–Si体系中各个组成相的力学性能参数。实验表明合金化元素Ga的添加在增强Nb5Si3等金属间化合物相的强度和断裂韧性的同时，对Nbss固溶体相却起到了相反作用。.四、科学意义.用同步辐射X射线μCT原位技术和微力学测试手段分析镁合金、铌硅合金复杂形状枝晶组织及异质多相组成力学行为体现了原创性的科学思维和方法论，本项工作也对诸如压铸工艺、半固态加工及缺陷形成的认识以及微尺度树枝晶韧化大块金属玻璃等科学思想的理解有着重要的启发性。