FGHxx系列合金疲劳损伤行为表征、评价与展望

Characterization, Evaluation and Perspective for Fatigue Damage Behaviors of FGHxx-series Nickel-based P/M Superalloys

FGHxx系列合金是我国近年来成功开发的损伤容限型粉末高温合金,由FGH95到FGH99系列组成.通过合金化及优化后续的热处理工艺参数,以较低的成本有效地提高FGHxx高温合金服役温度下的强度、断裂韧性、疲劳裂纹扩展阻力.论文通过对比现有文献的研究结果,对FGHxx系列合金的微结构特征,SEM原位测试方法,疲劳小裂纹的萌生和扩展行为,疲劳裂纹扩展速率和门槛值、高温疲劳寿命分散性,疲劳寿命预测模型,损伤容限设计准则进行了分析与讨论.在此基础上,重点关注了典型的FGH96合金.利用原位扫描电镜观察技术,观察了其疲劳小裂纹萌生与扩展行为,结果显示:在室温和400℃下,FGH96合金的低周疲劳小裂纹初期萌生和扩展优先发生在晶界上,当小裂纹扩展长度大于2-3倍平均晶粒尺寸后发生混合断裂模式,强化相的化学元素组成和弥散位置会影响晶界强度进而影响断裂机制;分析了断口形貌,发现微结构差异尤其是孔洞和夹杂是FGH96合金寿命分散性强的主要原因;利用DFR(细节疲劳寿命)方法预测了FGH96合金的具有Weibull分布特征的寿命特征值;建立了考虑温度和缺口系数的FGH96合金的3维疲劳损伤容限(3D K-T)图.最后探讨了该领域急需解决的问题和未来可能的研究方向.

The FGHxx-series nickel-based powder metallurgy(P/M) superalloys recently developed in China are one type of high-temperature structural materials with high damage tolerance, which consist of FGH95, FGH96, FGH97, FGH98 and FGH99. Through optimization of parameters for alloying and heat treatment, such as powder size, hot isostatic pressing(HIP) and rate of cooling, the strength, fracture toughness and resistance to fatigue crack initiation and propagation of FGHxx-series superalloys serving at high temperature can be effectively enhanced at a relatively low cost. In this study, the characteristics of microstructure, the observation of small fatigue crack initiation and propagation, the estimation of scatter in fatigue-life data from the S-N curves, and the models for fatigue life predication were analyzed by comparing various results reported in literature. For example, the 3 D K-T diagrams considering the effects of notch factor Kand elevated temperature under different experimental conditions were established as the design criteria based on the damage tolerance of FGHxx-series superalloys. The initiation and propagation behaviors of fatigue small cracks in a typical FGH96 superalloy were investigated using the in-situ SEM observation technology. These results indicated that the early-stage cracks in a typical FGH96 superalloy preferred to occur along the crystal boundary at both room temperature and 400 ℃. When the fatigue small crack length was over 2-3 times of the average grain size, the mixed model of fatigue crack propagation occurred mainly along and through the crystal. In addition, the effects of position, distribution, density, physical dimension, and composition of strengthening phases of(γ,γ’) such as the configuration ratio between Ti and Al elements, on the fatigue small crack propagation of FGH96 superalloy were also discussed. Furthermore, the effect of Co element on the spalling or step-like fracture between layers within the grain was analyzed. Finally, the urgent problems and promising directions for future research were highlighted.