表层梯度结构广泛应用于工程材料，经表面强化技术处理后，零件表层结构呈梯度分布，可以改善零件的服役能力。综述了金属表层梯度微结构对力学性能和裂纹扩展的影响，从微观结构和力学性能的角度，结合非均匀材料中裂纹的扩展行为，考虑了微结构和力学性能之间的相互影响规律，分析了梯度结构中影响裂纹扩展的因素。经表面强化后，材料表面晶粒细化，晶粒尺寸沿纵深方向呈现梯度分布，改变裂纹萌生位置，同时对裂纹扩展起到阻碍，使零件疲劳寿命延长。梯度结构中金相组织由表层单一组织到芯部多相组织的转变，引起表层硬度和残余压应力向纵深方向逐渐减小，表面压应力可以减弱外力的作用，从而影响疲劳寿命。然而，梯度结构中微观组织和力学性能互相关联，像晶粒和位错能改变残余应力的分布，也会对裂纹扩展路径产生影响，如何定量表征他们的影响还有待进一步研究。

The surface gradient structure is widely obtained in engineering materials. After the surface strengthening technology treatment, the surface structure of the part shows a gradient distribution, which can improve the serviceability of the part. Reviews the influence of the gradient microstructure of the metal surface layer on the mechanical properties and crack growth. From the perspective of microstructure and mechanical properties, combined with the crack growth behavior in heterogeneous materials, the mutual influence law between microstructure and mechanical properties is considered and the factors affecting crack growth in the gradient structure are analyzed. After surface strengthening, the surface grain of the material is refined, and the grain size presents a gradient distribution along the depth direction, which changes the position of crack initiation, and at the same time hinders the crack propagation and prolongs the fatigue life of the part. The metallographic structure of the gradient structure is single from the surface. The transition from the structure to the multiphase structure of the core causes the surface hardness and residual compressive stress to gradually decrease in the depth direction. The surface compressive stress can weaken the action of external forces, thereby affecting the fatigue life. However, the microstructure and mechanical properties in the gradient structure are related to each other. For example, grains and dislocations can change the distribution of residual stress and also have an impact on the crack propagation path. How to quantitatively characterize their effects remains to be further studied.