Cu、Al协同强化含Nb铁素体不锈钢高温蠕变行为与机制

This project proposes a new idea to strengthen the Nb-bearing ferritic stainless steel (Nb-FSS) by the collaboration of Cu and Al with a view to improve the invalidation problem of high temperature creep strengthening caused by the currently conventional alloying method. In this investigation, the Fe-Cr-Nb-Cu-Al ferritic stainless steel is the research subject, and the Nb-FSS with different contents of Cu and Al will be prepared. The mechanism of synergistic effect of Cu and Al on creep behavior and creep fracture of Nb-FSS will be systematically studied. The dynamic microstructural evolution during high temperature creep deformation will be characterized, and the interaction mechanisms betweenε-Cu phase and Cottrell atoms atmosphere caused by Al solute atoms segregation and dislocations and grain boundary will be clarified. After the foregoing in-depth investigation, the intrinsic mechanism of high temperature creep deformation of the Nb-FSS by the collaboration strengthening of Cu and Al will be revealed, and the micro-model of high temperature creep will be developed by considering the collaboration effect of Cu and Al. Finally, the control methods on key factors of high temperature creep will be obtained, which will provide an important experimental reference and theoretical support for the optimal design of high performance and resource saving ferritic stainless steel used in automotive exhaust pipes.

本项目提出了Cu、Al协同强化含Nb铁素体不锈钢的新思路，以期改善目前常规合金化方法带来的高温蠕变强化失效问题。项目拟以Fe-Cr-Nb-Cu-Al系铁素体不锈钢为研究对象，制备出具有不同Cu、Al含量的含Nb铁素体不锈钢，系统研究Cu、Al协同效应对含Nb铁素体不锈钢高温蠕变行为和断裂损伤的作用机理。表征蠕变过程中微观结构的动态演化，阐明ε-Cu相和Al偏聚所致Cottrell气团与位错、晶界的微观交互作用。通过这些深入研究，揭示Cu、Al协同强化含Nb铁素体不锈钢高温蠕变变形的本征机制，建立基于Cu、Al协同强化作用的高温蠕变模型，寻求高温蠕变关键因素的控制手段。该研究成果可为汽车排气管用资源节约型高性能铁素体不锈钢材料的优化设计提供实验数据和理论支撑。

为解决传统铁素体耐热不锈钢高温蠕变强化失效的问题，设计开发了Cu、Al协同合金化的含Nb铁素体不锈钢，以实现Nb析出导致强度降低的补充强化目的，进而改善其高温力学性能。本文围绕Fe-Cr-Nb-Cu-Al系铁素体不锈钢高温蠕变行为和断裂损伤机制开展了系统研究，主要结论有：随着温度和加载应力的升高，稳态蠕变速率逐渐增大，蠕变寿命逐渐减小。应力指数介于6.0-9.7之间，平均蠕变激活能大于α-Fe自扩散激活能，表明蠕变过程受幂律位错型破坏机制和析出强化机制的控制。建立了描述蠕变行为的Monkman–Grant关系模型和Larson–Miller参数模型，实现蠕变寿命的预测。蠕变过程中主要存在Fe2Nb型Laves相、Fe3Nb3C、NbC和富铜相等四种析出相，详细分析了蠕变条件对各种析出相的形状、尺寸、分布和与基体错配关系的影响。富铜相会率先弥散析出，并与Fe2Nb型Laves相存在伴生关系，这一定程度上促进了Laves相的形核，有助于细化Laves相。与Nb的碳化物相比，Laves相呈棒状生长且粗化速率最大。加载应力本身也促进了析出相的形核与长大。分析了析出相与位错之间的交互作用，Orowan缠绕机制是实验钢蠕变过程的主要强化方式。沿晶界形成的PFZs使位错更容易穿过PFZs，从而在晶界上形成塞积，加快微空洞的形成，从而导致材料最终断裂，断裂方式为韧性断裂。Al合金化在高温和（或）低应力下作用明显，可以显著降低稳态蠕变速率和提高蠕变寿命。Al对析出相的粗化和组织回复起到抑制作用，减缓析出相的熟化，增加了蠕变过程中析出相的稳定性，提高了铁素体耐热不锈钢的高温蠕变性能。通过本项目研究，为汽车排气管用资源节约型铁素体耐热不锈钢的设计开发和应用提供实验依据和理论支撑。