镍基单晶高温合金在高周疲劳断裂机制与疲劳应力预测模型

The fracture mechanism and fatigue stress prediction model of nickel-based single-crystal high-temperature alloys under high-cycle fatigue conditions

摘要:

开展了不同温度下镍基单晶高温合金的高温高周疲劳试验,运用光学显微镜和扫描电子显微镜对高周疲劳断口进行宏/微观观察,总结了镍基单晶高温合金高周疲劳疲劳断裂特征,开展了对应条件下的疲劳裂纹扩展试验,获取了稳定扩展特性,最终建立了新的疲劳断口定量分析参量和疲劳应力预测模型,并对试样承受的高周疲劳应力进行预测,对比计算了不同模型误差。结果表明：760℃、860℃,应力比R=0.05、-1条件下的高周疲劳裂纹萌生位置、扩展方向、断面特征相似,呈现典型的阶段性特征；裂纹均起源于材料内部疏松空洞处,裂纹扩展区前期为光滑平整的晶体学平面特征,裂纹扩展区后期可见“脆性疲劳条带”特征；断面与载荷施加方向呈一定角度,为复合型裂纹。镍基单晶合金的“脆性疲劳条带”特征与钢、铝多晶合金在形貌上、间距尺寸数量级存在明显不同,根据获取的疲劳裂纹稳定扩展阶段特性,计算得到疲劳应力预测模型所需的参数；选用体现镍基单晶高温合金高周疲劳裂纹萌生、扩展走向的裂纹形状因子“圆柱半椭圆表面裂纹”,计算体现表征合金斜裂纹特点的复合型裂纹形状因子修正系数；分别选用传统Paris模型和考虑循环特性应力比R、断裂韧性K_IC的Forman模型的疲劳应力预测模型进行应力预测,将预测结果与名义应力进行比对计算误差；Paris模型预测误差倍数在1.0~1.9,Forman模型预测误差在1.05~1.30；表明疲劳应力预测需选择与疲劳断口特征匹配的疲劳应力预测参量和应力预测模型。

Abstract:

High-temperature and high-cycle fatigue tests have been conducted on nickel-based single crystal superalloys at 760℃and 850℃. The macroscopic and microscopic features of the high-cycle fatigue fracture surfaces were assessed by optical and scanning electron microscopy. Fatigue crack propagation tests were conducted to establish the stable propagation characteristics. In addition, a new quantitative analysis parameter for fatigue fracture and a fatigue stress prediction modelling were proposed and applied in the prediction of high-cycle fatigue stress effects. The errors associated with different models are calculated and compared. The results demonstrate that, at 760℃ and 860℃ and a stress ratio (R) of 0.05 and -1, the location of high-cycle fatigue crack initiation, the propagation direction, and the fracture surface characteristics are similar, presenting typical staged characteristics. The cracks all originate from internal porosity and material voids, where the early stage of the crack propagation zone exhibits smooth and flat crystallographic plane characteristics. At the later stage, the crack propagation zone shows "brittle fatigue band" characteristics; the fracture surface is inclined at a certain angle to the loading direction, generating a composite crack. The brittle fatigue band features of the nickel-based single crystal alloys are significantly different from those associated with steel and aluminum polycrystalline alloys in terms of morphology and spacing size order of magnitude. Based on the stable propagation characteristics of the fatigue cracks, the parameters required for the fatigue stress prediction model were calculated. The crack shape factor, "cylindrical semi-elliptical surface crack", which reflects the initiation and propagation direction of the fatigue cracks, was selected. The correction coefficient for the composite crack shape factor that represents the characteristics of the alloy inclined cracks was calculated. Moreover, conventional Paris and Forman models that incorporate the stress ratio and fracture toughness (Kc) were selected as the fatigue stress prediction models. The predicted results were compared with the nominal stress, with a prediction error for the Paris model between 1.0 and 1.9, and between 1.05 and 1.30 in the case of the Forman model. The results indicate that accurate fatigue stress prediction requires the selection of prediction parameters and models which match the fatigue fracture characteristics.