强度和塑性是金属材料的两个重要的力学性能参数。研究表明异构金属材料因其结构特点可以提供额外的加工硬化（Hetero-deformation induced hardening, HDI hardening），从而缓解了强度和塑性间的此消彼长，最终获得优异的强塑性匹配。异构金属优异的强塑性匹配能否带来好的疲劳行为是值得研究的重要课题。异构的形貌特征、尺寸、软硬区域间力学性能差异、HDI应变硬化对疲劳裂纹扩展行为会产生什么影响仍不够明确。本文针对以上问题开展研究，以揭示表征异构的特征参数、参数与疲劳裂纹扩展行为间的关联，并探讨异构疲劳裂纹扩展的微观机理。
本文选用单相面心立方 (Face centered cubic, FCC) CrCoNi 中熵合金 (Medium entropy alloy, MEA) 作为实验材料，通过冷轧（压下量88 %）和600 ℃下不同时长的再结晶退火处理，获得四种不同异构（HSx）。此外通过800 ℃ 60 min 处理获得完全再结晶的粗晶组织(Coarse grain, CG) 与上述四种异构进行对照试验。对以上五种微结构在室温进行了疲劳裂纹扩展试验。设计中途停机实验，通过电子背散射衍射(Electron Backscattered Diffraction, EBSD) 和背散射电子图像(Back scattered electron, BSE)技术对比裂纹扩展前后的微结构变化与裂纹路径特征；利用纳米压痕和EBSD 对不同异构进行定量表征，提炼异构表征参量并与力学性能进行关联。主要结果如下：
1）根据异构的定义，定义了两类表征异构度的特征参数：相对硬度比HR、硬度差HD和硬度变异系数CoVH（力学差异）和再结晶百分数θ（组织形态）。将这两类参数与力学性能关联并分析，结果表明上述参数在表征异构程度时存在不同的结果，导致力学性能随异构程度参数的变化规律不同，甚至矛盾。这表明唯象方法在表征异构时存在局限性。
2）疲劳裂纹扩展结果表明，再结晶程度低的HS1和HS2具有相近的疲劳裂纹扩展门槛值为8.5 MPa·m^1/2。再结晶程度高的HS3和HS4的门槛值有所增加，分别为9.9 MPa·m^1/2和12.9 MPa·m^1/2。完全再结晶的CG具有最高门槛值为17.9 MPa·m^1/2。综合试样的拉伸试验结果表明疲劳裂纹门槛值、均匀延伸率、强塑积均随着再结晶百分数的增大而增加。
3）综合BSE、EBSD 对裂纹扩展路径观测与断口分析表明：对于不同异构度的CrCoNi，裂纹扩展在变形回复区以剪切变形与开裂为主，而当裂纹尖端遇到再结晶晶粒时，会引发塑性区内滑移变形或孪生，裂纹沿滑移面扩展或在相邻区域孪晶界引发小裂纹。损伤主要发生在裂纹尖端的循环塑性区内，以外没有观察到明显的损伤。

4）由于再结晶晶粒取向不同会导致裂纹偏折，使得裂纹扩展模式变成混合型，引发粗糙度诱导的裂纹闭合效应，降低裂纹尖端的应力强度因子。因此，增大再结晶百分数，可以提高疲劳裂纹扩展门槛值并降低裂纹扩展速率。

Strength and plasticity are two important mechanical properties of metal materials. The study showed that the heterogeneous metal materials can provide additional hardening (Hetero-deformation induced hardening) due to its structural characteristics, which alleviates the trade-off between strength and plasticity and finally makes the material have excellent strength and plastic matching. Whether the excellent strength and plastic matching of heterogeneous metals can bring good fatigue behavior is an important topic worth studying. It is still unclear how the heterogeneous structures' morphology, size, differences in mechanical properties between soft and hard domains, and the influence of HDI strain hardening on fatigue crack propagation behavior are affected. In this paper, the above problems were studied to reveal the characteristic parameters of heterogeneous structures, the correlation between the parameters and the fatigue crack growth behavior, and to explore the microscopic mechanism of fatigue crack growth in heterogeneous structures
In this paper, single-phase FCC medium entropy alloy CrCoNi was selected as the experimental material, four kinds of heterogeneous microstructures (HSx) were obtained by cold rolling (reduction of 88 %) and recrystallization annealing at 600 ℃ with different annealing times. In addition, a contrast test was conducted with Coarse grain (CG) fully recrystallized at 800 ℃ for 60 min. The fatigue crack propagation tests of the five microstructures were carried out at room temperature. The shutdown experiment was designed. The microstructure changes and crack path characteristics before and after fatigue crack propagation were compared by Electron Backscattered Diffraction (EBSD) and Backscattered electron (BSE) techniques. The heterogeneous structures were quantitatively characterized by nanoindentation and EBSD, and the heterogeneous characterization parameters were extracted and correlated with mechanical properties. The main results are as follows:
1) According to the definition of heterogeneous structure, two kinds of characteristic parameters were defined: relative hardness ratio HR, hardness difference HD, hardness variation coefficient CoVH (mechanical difference) and percentage of recrystallization θ (microstructure morphology). The correlation and analysis of these two types parameters with mechanical properties show that the above characteristic parameters have different results in characterizing the degree of heterogeneous, which leads to different and even contradictory changes in mechanical properties with the characteristic parameters. This indicates that phenomenological methods have limitations in characterizing heterogeneity.
2) The fatigue crack propagation results show that HS1 and HS2 with low recrystallization degree have similar fatigue crack propagation threshold values of 8.5 MPa·m^1/2. The fatigue crack propagation threshold values of HS3 and HS4 samples with higher degree of recrystallization increased to 9.9 MPa·m^1/2and 12.9 MPa·m^1/2, respectively. The completely recrystallized CG has the highest fatigue crack propagation threshold value of 17.9 MPa·m^1/2. Combined tensile test results，it shows that the fatigue crack threshold, uniform elongation and the product of strength and elongation increases with the increase of the percentage of recrystallization.
3) The observation and fracture analysis of crack growth path based on BSE and EBSD showed that: for the CrCoNi with different heterogeneity, the crack propagation is mainly shear deformation and cracking in the deformation recovery zone. When the crack tip reaches the recrystallized grain, the slip deformation or twinning in the plastic zone will be induced, and the crack propagation along the slip plane or the twin boundary in the adjacent area will cause small cracks. The damage mainly occurs in the cyclic plastic zone at the crack tip. Not obvious damage and tissue changes were observed outside this zone.
4) The different orientation of recrystallized grain orientation will lead to crack deflection, resulting in a mixed crack growth mode, which leads to crack closure effect induced by roughness and reduces the stress intensity factor at the crack tip. Therefore, increasing the percentage of recrystallization can increase the fatigue crack growth threshold and reduce the crack growth rate