基于两步等温淬火Cu合金化ADI的疲劳性能及机理

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.

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。

现代工业对结构材料提出了严格的轻量化、强韧化要求。两步法ADI能够大幅提高塑韧性，而不降低强度，为满足轻量化提供可能。但这种新型ADI的强化机制尚未明确，尤其是疲劳性能及机理更是未见报道。为此，本课题通过调控球化剂加入量、热处理工艺参数、Cu元素含量制备多种组织特征的两步法ADI试样，并对其力学性能及疲劳性能进行了研究。利用透射电镜观察了其形貌特征，并绘制了C3.6Si2.5Mn0.4Cu1.4球墨铸铁的TTT曲线及CCT曲线。.通过调控球化剂加入量，获得了不同石墨形貌及分布的两步法ADI试样。随球化效果的提升，两步法ADI各项性能显著提升。球化水平较高时，石墨球直径分布及其精细结构会决定两步法ADI性能。疲劳裂纹容易在聚集的石墨球团及大尺寸石墨球处萌生，大尺寸石墨球会促进疲劳裂纹扩展。.通过调控两步等温淬火工艺参数，观察分析了基体组织形貌及分布的变化规律。并研究了两步等温淬火相变程度对ADI组织性能的影响。随一步相变程度增加，力学性能显著提高，当一步相变量超过25%后，力学性能反而降低。随二步相变程度增加，ADI强度升高，塑韧性降低。疲劳性能受基体组织影响明显，疲劳极限随奥氏体含量增加变化不明显，但随奥氏体稳定性提高而增加。高周疲劳行为随奥氏体含量增加而提高，表明残余奥氏体能够抑制两步法ADI疲劳裂纹扩展，对疲劳裂纹萌生影响不大。疲劳裂纹萌生能力决定于组织粗细程度。.通过调控Cu加入量，改变了基体组织形貌及分布。随Cu含量提升，两步法ADI残余奥氏体含量增加，奥氏体稳定性提高，组织略有粗化。力学性能随Cu含量增加先升高后降低。Cu含量低于0.8%，随Cu含量增加，疲劳性能显著提升。Cu含量超过0.8%，随Cu含量提升疲劳极限略有降低，但高周疲劳行为得到增强。结果表明，Cu通过固溶强化及影响碳原子扩散，改变了组织形貌及分布，进而影响了力学性能。.总之，任何细化石墨球与基体组织、优化石墨球直径分布、增加残余奥氏体含量及其稳定性的手段都有利于提高两步法ADI力学性能及疲劳性能，这为相似研究提供了理论支持。