两步等温淬火球铁具有优异的综合力学性能，是一种应用前景广阔的工程材料。研究两步等温淬火球铁(ADI)的疲劳性能及机理是促进和保障其安全应用的关键。本项目通过调整两步等温淬火工艺参数，制备不同微观组织的ADI；研究基体组织、石墨形态和分布对疲劳极限和疲劳裂纹扩展率的影响，阐明基体组织、石墨形态和分布与疲劳断裂的内在联系，揭示两步等温淬火球铁的疲劳断裂机理。 在此基础之上，本项目提出添加Cu元素提高两步等温淬火球铁疲劳性能的新思路。研究Cu元素对ADI基体组织及疲劳性能的影响，研究疲劳断口形貌和断裂方式，阐明Cu合金化ADI疲劳裂纹萌生及扩展的内在原因，建立Cu合金化、基体组织、石墨形态及分布与疲劳断裂的内在联系，揭示两步等温淬火Cu合金化ADI的疲劳断裂机理，为发展兼具优异常规力学性能及良好疲劳性能的新型ADI奠定基础。

Austempered ductile iron prcessed by two-step austempering process is a promising engineering material due to its excellent combination of mechanical properties. Investigation on fatigue properties and mechanism of the two-step austempering ductile iron (ADI) is the key to provide its security applications. In this project, the ADI with different microstructure is prepared by adjusting the two-step austempering process parameters. The effect of matrix microstructure, morphology and distribution of graphite on fatigue limit and fatigue crack growth rate is studied. Moreover, the internal relationships between the matrix microstructure, morphology and distribution of graphite and fatigue fracture are clarified. The mechanism of fatigue fracture of two-step austempering ductile iron is revealed. Furthermore, the new idea of adding Cu element to improve the fatigue performance of two-step austempering ductile iron is proposed. The effect of Cu element on the matrix microstructure and fatigue properties is studied. In addition, the fatigue fracture morphology and fracture mode are investigated. The underlying reasons of fatigue crack initiation and propagation of Cu alloyed two-step austempering ductile iron are clarified. The internal relationships between Cu alloying, matrix microstructure, morphology and distribution of graphite and fatigue fracture are founded. The mechanism of fatigue fracture of Cu alloyed two-step austempering ductile iron is revealed. The researches mentioned above provide the guidance for developing novel ADI exhibiting both the excellent conventional mechanical performance and good fatigue performance.