镍基单晶合金低周机械疲劳寿命模型评述

A Review of Low Cycle Mechanical Fatigue Life Models for Nickel-based Single Crystal Superalloy

航空发动机叶片等先进动力推进系统涡轮叶片长期服役于高温、高压、高离心力的工作环境,对叶片材料的性能有很高的要求。普通多晶合金材料存在晶界,晶界处较为脆弱,裂纹易滋生于晶界并沿晶界扩展。采用定向凝固工艺的镍基单晶合金消除了晶界组织,具有较高的高温强度、良好的蠕变与疲劳抗力、优异的热稳定性,长久以来一直作为涡轮叶片材料使用。镍基单晶材料的疲劳损伤是直接影响叶片服役时间的一个重要因素,疲劳损伤的评估依赖于合理有效的疲劳寿命模型。镍基单晶材料的疲劳模型涉及范围很广泛。一方面,材料的工作环境复杂,涉及的疲劳问题包括机械疲劳、热疲劳、热机械疲劳以及蠕变疲劳等。另一方面,单晶材料本身的各向异性带来了疲劳性能的各向异性,取向偏离这一铸造缺陷决定了单晶材料的实际使用取向并非材料性能的择优取向。目前,研究者们主要从复杂环境带来的复杂疲劳状态和单晶本身的各向异性方面进行疲劳寿命模型的探究。针对复杂的疲劳状态,目前的疲劳模型从基本机械疲劳出发,向各个侧重探究方向延伸,尚没有广泛适用且机理清晰的模型。机械疲劳模型的探究仍处于前列。在针对材料本身各向异性的研究方面,学者们提出了不同的各向异性疲劳性能的处理方式,如基于单晶体弹性模量各向异性的取向因子类模型,这类模型因操作简单而适合工程应用,但其预测能力缺乏评估。由于复杂疲劳状态涉及范围太广,本文立足于低周机械疲劳,分类整理了其疲劳模型,按照疲劳损伤参量的定义思路将模型分为宏观损伤参量模型和微观损伤参量模型两大类,论述了各类模型的建模原理。同时收集了五种镍基单晶材料的11组疲劳试验数据,对典型模型进行了评估,以期为进一步探究镍基单晶的疲劳寿命模型提供参考。

The turbine blades of advanced power propulsion systems such as aeroengines have long been serving in high temperature,high pressure and high centrifugal force working environment,which requires high performance of blade materials.Common polycrystalline alloys have grain boundaries,which are relatively fragile.Cracks tend to grow at and propagate along grain boundaries.The nickel-based single crystal alloys,developed with directional solidification process,have been used as turbine blade materials for a long time with their elevated temperature strength,good creep and fatigue resistance and excellent thermal stability resulted from the elimination of grain boundary.Fatigue damage of nic-kel-based single crystal material is an important factor that directly affects the service life of blades.The assessment of fatigue damage depends on reasonable and effective fatigue life models.The research of fatigue model of nickel-based single crystal materials concerns a wide range of studies.On the one hand,the working environment of nickel-based single crystal material is complex.The fatigue problems occurring during service include mechanical fatigue,thermal fatigue,thermomechanical fatigue and creep fatigue.On the other hand,the anisotropy of the single crystal material itself leads to the anisotropy of fatigue properties.The orientation deviation caused in casting determines that the actual orientation of the single crystal material is not the preferred orientation of material properties.At present,researchers mainly study the fatigue life models by focusing on the complex fatigue states caused by complex environment and the anisotropy of crystal itself.In view of the complex fatigue states,the current fatigue models extend basic mechanical fatigue to various fatigue research directions.There is no widely applicable model with clear fatigue mechanism.The research of mechanical fatigue model still plays an important role.In the research of material anisotropy,scholars have proposed different ways to deal with the anisotropic fatigue properties,such as the widely recognized orientation parameter models based on single crystal modulus anisotropy.This kind of model is suitable for engineering application because of its simplicity,meanwhile the evaluation of their predictive ability is still absent.Since the complex fatigue states cover a wide range,this paper focuses only on low cycle mechanical fatigue.The basic low-cycle mechanical fatigue models of nickel-based single crystal superalloys are sorted and classified into two categories:macroscopic damage parameter model and micro-damage parameter model according to the definition of fatigue damage parameters.The modeling mechanisms of various models are discussed.11 sets of fatigue test data of five kinds of nickel-based single crystal materials were collected to evaluate the typical models,looking forward to providing insights in further study of the fatigue life model of nickel-based single crystals.