金属材料的强度和塑性是材料力学中极为重要的两项性能指标，也是关系到材料实际应用的关键因素，良好的强度和塑性的匹配能使得材料在生产生活中获得更为广泛的使用。然而，一直以来材料的强度和塑性都存在着本征的矛盾，即强度和塑性的此消彼长（trade-off），当粗晶态的金属通过晶粒细化得到强度的提升时，通常都伴随着塑性的显著下降。所以如何缓解强度和塑性的此消彼长，提升材料的综合力学性能就成为了关键的问题。高熵合金（high-entropy alloys，HEAs）或多主元合金（multi-principal element alloys，MPEAs）是近年来受到研究者广泛关注的金属材料，这种合金以高构型熵的设计理念使材料获得稳定无序固溶体，避免了元素偏聚和金属化合物的产生，获得了良好的力学性能。但这种合金仍然存在强度和塑性的本征矛盾，且大多数为均匀的微结构，强度都处于比较低的范围。异构（hetero-structure）是一种极为有效的缓解强度和塑性的本征矛盾的微结构设计思路，通过将材料的均匀微结构进行一些处理，得到强度塑性有差异的非均匀微观结构，从而在塑性变形的过程中产生非均匀变形诱导硬化（Hetero deformation induced hardening，HDI hardening），在获得优异的强度和塑性的匹配的同时，缓解了材料的强度和塑性的本征矛盾。此外本文也针对高熵合金的化学短程有序结构进行了研究，尽管中高熵合金的设计理念是最大化材料的构型熵以获得无序的固溶体，但实际的报道表明了，中高熵合金中仍然存在着各组成元素的有序分布，本文通过透射电镜观察与模拟计算相结合的方式对中高熵合金中的化学短程有序结构进行了研究。
本文选择了非等原子比双相高熵合金Fe50Mn30Co10Cr10作为研究对象，首先通过对均匀的粗晶态样品进行轧制和非均匀退火处理得到异构样品，不同退火温度和时间所得到的的异构样品的微结构特征各不相同。异构样品均显示出了优异的强度和塑性匹配，回复态异构样品在获得1 GPa以上屈服强度的同时仍然保持了10%的塑性。其他异构样品也展现了相比于粗晶态样品强度和塑性的同时提升。由于异构样品的微观结构是不均匀且存在强度差异的，所以在变形中产生了应变不协调，同时在异构的界面处产生了用以协调变形的几何必须位错，几何必须位错的塞积导致了HDI应力和HDI硬化的产生，使异构材料在变形的过程中得到了持续的应变硬化能力，这也是异构材料获得良好强度和塑性匹配的原因。同时异构的应变再分配也使得马氏体相变的尺寸效应得到缓解，促进了材料的相变诱导塑性（transformation-induced plasticity，TRIP）效应。
此外，本文选择了完全再结晶和部分再结晶的异构非等原子比高熵合金Fe50Mn30Co10Cr10进行化学短程有序的表征。首先通过多次实验确定适宜进行观察的晶带轴，随后通过透射电子显微镜的两种途径对高熵合金化学短程有序的结构进行表征。一种是通过高角环形暗场的图像，对其进行快速傅里叶变换和快速傅里叶逆变换，得到了fcc基体以及化学短程有序结构的点阵图像。另一种则是通过纳米束电子衍射得到样品的衍射结果，并得到化学短程有序的超点阵衍射斑所对应的能量过滤暗场像，获得化学短程有序的分布情况。随后对两种方法所得到的化学短程有序结构进行尺寸的统计，结果显示两种尺寸统计结果的匹配性很好。此外本文还通过能量分布面扫描的手段直接获得了化学短程有序结构中各元素的占位信息，观察到了在("3"  ̅"11)" 面上存在着的富Fe面/贫Fe面/富Fe面的交替分布现象，这也进一步阐述了透射电镜所观察到的化学短程有序结构的化学信息。最后，通过对能量分布面扫描所得到的元素浓度曲线进行空间相关性分析，分析了高熵合金在亚纳米尺度下各元素富集的原子列的具体位置以及同种或不同种原子在近邻和次近邻位置的空间相关性。通过上述方法本文成功地在高熵合金中直接观察到化学短程有序结构并获得了其具体的元素分布情况。
最后，本文通过对不同厚度的热轧态Fe50Mn30Co10Cr10高熵合金进行表面机械研磨处理，得到了梯度结构的双相高熵合金。材料在晶粒尺寸、硬度和位错的密度均沿着深度方向呈梯度的变化，两侧的梯度层厚度约为300 μm。力学性能测试的结果显示，材料具有优异的强度和塑性，在具有接近1 GPa的屈服强度的同时仍然具有良好的塑性，此外，对比不同厚度的梯度结构样品性能可以发现，初始厚度的降低能有效地提升材料的性能域。通过变形前后的微结构表征可以发现，材料的梯度层和芯部以及两相在变形过程中产生了变形的不协调和较为明显的应变梯度。在芯部和梯度层的交界区域产生了大量的几何必须位错，使材料获得了持续的HDI硬化。此外，粗晶态的芯部和软硬相的交界区域分别在变形的不同阶段产生显著的TRIP效应，使材料的强度和塑性获得进一步提升。
 

Both strength and ductility are crucial properties for metallic materials, and also they are significant factors for application because excellent combination of strength and ductility could make the material more widely used in society. However, a paradox between strength and ductility constantly obsessing researchers, which calls the trade-off. When people refine the soft coarse-grain material to strengthening it, the ductility usually decreased obviously. So it is a pivotal task to alleviate the trade-off and manage to get excellent combination of strength and ductility. High-entropy alloys (HEAs), or multi-principal element alloys (MPEAs), has becoming a scientific hot button in recent years. Getting the maximum entropy of whole system to form ideal random solid solution is the basic concept of HEAs. This concept could restrain the formation of segregation , cluster and intermetallic compound and benefit the mechanical behavior of materials. However, HEAs still suffer from trade-off of strength and ductility. Most of HEAs are homogeneous structure with rather low yield strength. Hetero-structure (HS) is an effective method to alleviate the trade-off between strength and ductility by some deliberate process for homogeneous materials. During the procession, the homogeneous material get both soft and hard area within microstructure and get hetero-deformation-induced (HDI) hardening. Besides, a study on chemical short-range order (CSRO) for HEAs is also been conducted. Although HEAs have a structure of random solid solution nominally, researchers have find a structure of local chemical order in HEAs for some degree. In this article, the CSRO of HEAs is characterized by TEM, EDS-mapping and spatial correlation analysis.
Firstly, the coarse-grained quaternary equimolar Fe50Mn30Co10Cr10 HEA is chosen to rolling and anneal at various temperature and times to get HS samples. All of HS samples show excellent combination of strength and ductility. The recovery HS sample get more than 1 GPa yield stress with 10% uniform elongation. And the strength and ductility are simultaneously improved in other HS samples compared with coarse-grained sample. The strain gradient is observed during the deformation of HS samples because of the difference of strength in the heterogeneous microstructure. Thus, a large number of geometrically necessary dislocations (GNDs) pile up in the hetero-boundary so that HDI stress and HDI hardening are caused. This is the mechanism in the deformation of HS samples. Meanwhile, the reassignment of strain caused by HS alleviate the size effect of martensite transformation and promote the transformation-induced plasticity (TRIP) effects.
Secondly, the fully recrystallized and partial recrystallized HS samples are chosen to characterize the CSRO in HEAs. Initially, the suitable zone axis is explored by several TEM observations. Then the CSRO is demonstrated by two methods. High-angle annular dark-field (HAADF) observations were carried out to get whole lattice structure. Then the lattice structure of CSRO and face-centered cubic (fcc) matrix is obtained by fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) of HADDF image. Another way is the energy-filtered dark-field image of the extra diffuse in the nanobeam electron diffraction. Then, the size distribution of CSRO can also obtained by these two ways. The two results match up nicely. Chemical information is important to unravel what kind of CSRO is present by probing into the detailed arrangements of the four chemical species (Fe, Mn, Co, Cr) in the CSRO regions. To this end, energy dispersive X-ray spectroscopy (EDS) mapping was carried out based on HAADF imaging. The CSRO structure consists of two Fe-enriched planes sandwich one Fe-depleted plane is observed and explain the reason for extra diffuse in diffraction pattern. Besides, the spatial correlation analysis of EDS-mapping results is carried to quantitatively evaluate the extent of correlation between each element pair. The presence of CSROs is firstly evidenced and the spatial extent of the CSRO regions is clearly identified.
    Finally, the gradient structured (GS) Fe50Mn30Co10Cr10 HEA is prepared by surface mechanical attrition treatment (SMAT) to hot rolled plates. The grain size, hardness, and dislocation density show gradient distribution. The gradient layer have a thickness of 300 μm. The results of tensile tests for GS samples show excellent combination of strength and ductility with a yield stress around 1 GPa accompany with considerable ductility. It is notable that a reduction of thickness for initial hot rolled plate is effective to further increase the mechanical properties. The incoordination deformation is observed during the deformation of GS samples due to the hardness difference between the gradient layer and core. So that, high-density GNDs are produced to accommodate the strain gradient and cause HDI stress and HDI hardening. Moreover, the core area and the junction between core and gradient layer contribute to TRIP effect in different period of deformation and benefit the mechanical property of GS material.