涡轮叶片喷射成形高温合金的环境疲劳行为及超长寿命预测方法

The fatigue property of turbine blade for vehicle turbocharger is greatly associated with the microstructure and defect of material as well as the service environment. However, the currently used material, failure assessment and strength design philosophy have no longer been able to meet the service demand of turbine blade for high-performance development and long-term safety. In this project, the idea that the advanced spray-formed material is going to be used to the manufacturing of turbine blade will be regarded as a major breakpoint. Simultaneously, based on the presented scientific problems of fatigue damage, failure assessment and life prediction in the very high cycle regime under complicated service environment, we will study the fatigue properties of spray-formed superalloy for turbine blade under surface strengthening, high temperature and ultra-long cycle loading. The main contents include four aspects: (1) the constitutive modelling of surface gradient material and the prediction of macro/micro residual stress, (2) the failure mechanisms under the combined action of surface strengthening, high temperature and ultra-long cycle loading, (3) the characterization of crack nucleation and multi-scale growth and the description of damage evolution process, (4) the prediction of multiple competing failure modes and the establishment of probabilistic ultra-long life fatigue assessment approach. The research significance is to reveal the ultra-long life fatigue regularities and mechanisms of the spray-formed superalloy under typical service environment of turbine blade, and to establish the relevant probabilistic fatigue assessment approach greatly associated with failure mechanisms and microstructure characteristics. The research results will provide a theoretical basis for the failure and safety service of turbine blade.

车用增压器涡轮叶片的疲劳性能密切关联于材料微结构/缺陷和服役环境，但现行叶片的材质、失效评价及强度设计理念已远不能满足其高性能研制和长期安全性的服役要求。本项目以先进喷射成形材料在涡轮叶片制造上的应用为切入点，面向复杂服役环境下超长寿命疲劳损伤、失效评价及寿命预测等方面的科学问题，开展涡轮叶片喷射成形高温合金在表面强化-高温环境-超长周次载荷作用下的疲劳行为研究，主要涉及：（1）表面梯度材料循环本构建模及宏/微观残余应力预测；（2）强化-高温-超长周次载荷作用下的疲劳失效机理；（3）裂纹萌生/多尺度扩展行为表征及损伤演化过程描述；（4）多元化竞争失效预测及概率超长寿命疲劳评估方法构建。本项目的意义在于揭示喷射成形高温合金在典型涡轮叶片服役环境下的超长寿命疲劳失效规律及机理，进而建立基于失效机制且与微结构属性强相关的概率超长寿命疲劳评估方法，为涡轮叶片失效及安全服役提供基础理论依据。

增材制造的镍基高温合金构件的力学性能与传统制造的构件有着明显的不同，尤其是其抗疲劳性能。同时，航空航天、汽车等行业所包含的机械结构在一定环境下往往承受高频、超长周期的疲劳载荷，低于传统的疲劳极限，但仍然存在着失效问题。因此，传统的高周疲劳设计规范已不能满足构件超长寿命使用的需要，环境-超长寿命疲劳已成为研究热点。首先，本项目从制备工艺和力学行为表征入手，探讨了微观结构特征和热处理条件对其力学性能的影响。该材料晶粒具有择优取向，即沿{100}方向的强织构。后热处理期间的相变过程包括Laves相的溶解、更多强化相的析出和δ相的尺寸减小。沿着最高剪应力面，真空环境中由位错堆积和硬δ相周围不协调变形形成的微空洞和微裂纹的内部开裂行为成为典型的延性破坏模式。其次，在室温和高温（25℃和650℃）下进行了两种应力比（R=-1和R=0）的恒幅轴向加载疲劳试验，揭示了高温-力等其它因素相互作用下的超长寿命疲劳失效模式和机理。在长寿命范围内，即使在完全反向加载下，650℃下的内部失效概率也大大提高，相关疲劳寿命更长。内部破坏主要由与晶粒结构相关的小平面引起。小平面的形成主要是由于大颗粒沿最大剪应力方向断裂所致。相对于较小的夹杂物，较大的小平面更有可能促进微裂纹的生长。内部宏观裂纹扩展的阈值对高温不敏感，这归因于细晶粒结构和真空环境。然后，基于超高周疲劳裂纹的微观结构、晶体学特征和应力-应变关系，建立了具有代表性的体积元模型，从细观层面讨论了超高周疲劳裂纹的萌生和扩展行为。基于分子动力学理论，建立了分子微观模型，阐明了不同温度下的变形机理，探讨了循环饱和阶段与疲劳性能的关系，并从原子理论的角度深入的理解了疲劳机理。最后，结合喷射成形镍基高温合金的组织/缺陷特征，建立了超高周疲劳损伤行为模型，提出了跨尺度疲劳寿命预测方法，为增材制造高温合金的高性能-长寿命设计提供了基础理论依据。