Low cycle fatigue properties and fatigue mechanism of DD6 single crystal superalloy under asymmetrical cyclic loading
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摘要: 研究DD6单晶高温合金在700 ℃、R=0.05条件下的低周疲劳性能，采用扫描电镜观察断口形貌和断裂组织，分析疲劳裂纹萌生、扩展及断裂机制。结果表明：随着应变幅增加，合金的低周疲劳寿命降低，合金在非对称循环载荷条件下具有良好的低周疲劳性能，不存在过渡寿命，低周疲劳过程中弹性应变起主要作用，塑性变形量极小。随着总应变幅的增加，塑性变形损伤增加；疲劳断口由疲劳源区、裂纹扩展区和瞬断区三部分组成，所有试样断裂机制均为类解理断裂。疲劳裂纹萌生于试样的表面、亚表面或远离表面的显微孔洞，远离表面起裂断口呈现“鱼眼”特征。裂纹先沿与主应力轴垂直方向扩展，然后沿{111}平面扩展，裂纹扩展区有典型的疲劳条带、解理台阶、河流状花样特征，瞬断区有解理平面、滑移带、撕裂棱特征；断裂组织分析表明远离断口处γ′相仍保持立方状形态，断口附近的γ′相发生了塑性变形，断口附近可见滑移带，二次裂纹沿滑移带形成。Abstract: The low cycle fatigue（LCF） properties of DD6 single crystal superalloy were investigated at 700 ℃ and R of 0.05. SEM was used to study the fracture surface and fracture microstructure. The results show that the LCF life of the alloy decreases with the increase of strain amplitude. LCF properties of the alloy are excellent under asymmetrical cyclic loading. The alloy has no transition fatigue life during LCF tests at all total strain amplitudes. LCF fatigue damage can be dominantly contributed to elastic damage and the plastic deformation is very minimal. The plastic damage increases with the increase of total strain amplitude. The crack initiation site, the fatigue crack propagation area and the final fracture zone can be observed in the fracture surface and all specimens is similar to quasi-cleavage fracture. The fatigue cracks are initiated from the micro-pores on the surface, sub-surface or far from the surface. Far from the surface crack fractures have fish-eye feature. The fatigue crack propagates perpendicularly to main stress at first and then along {111} plane. Typical fatigue striation, cleavage steps and river pattern characteristic are formed on fatigue crack propagation zone. The cleavage plane, slip band and tearing ridge are seen in the final fracture zone. Fracture microstructure analysis shows that the γ′ phase morphology far from the fracture surface still maintains cubic shape, and the slip bands are visible seen near the fracture surface, and secondary cracks are formed along slip bands.
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Key words:
- DD6 /
- single crystal superalloy /
- asymmetrical cyclic loading /
- low cycle fatigue /
- fracture mechanism
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图 3 700 ℃时DD6合金疲劳寿命与应变幅的关系 （a）总应变幅；（b）弹性与塑性应变幅
Figure 3. Relationship between strain amplitude and fatigue life of DD6 alloy at 700 ℃ （a）total strain amplitude；（b）elastic and plastic strain amplitude
图 4 合金不同条件下的滞后环 （a）Δεt/2=1.1%， Nf=10171， 半衰期循环；（b）Δεt/2=1.1%， Nf=10171， 最后一个循环；（c）Δεt/2=1.6%， Nf=924， 半衰期循环；（d）Δεt/2=1.6%， Nf=924， 最后一个循环；（e）Δεt/2=1.9%， Nf=349， 半衰期循环；（f）Δεt/2=1.9%， Nf=349， 最后一个循环
Figure 4. Typical hysteresis loops of the alloy at different conditions （a）Δεt/2=1.1%， Nf=10171， half-life cycle；（b）Δεt/2=1.1%， Nf=10171， last cycle；（c）Δεt/2=1.6%， Nf=924， half-life cycle；（d）Δεt/2=1.6%， Nf=924， last cycle；（e）Δεt/2=1.9%， Nf=349， half-life cycle；（f）Δεt/2=1.9%， Nf=349， last cycle
图 5 疲劳断口整体形貌和源区形貌 （a）、（b）Δεt/2=1.3%， Nf=4554；（c）、（d）、（e）Δεt/2=1.6%， Nf=832；（f）、（g）、（h）Δεt/2=1.1%， Nf=18133
Figure 5. Overall morphology of fatigue fracture surface and crack initiation site （a），（b）Δεt/2=1.3%， Nf=4554；（c），（d），（e）Δεt/2=1.6%， Nf=832；（f），（g），（h）Δεt/2=1.1%， Nf=18133
图 6 疲劳断口扩展区特征形貌 （a）Δεt/2=1.1%， Nf=9203；（b）Δεt/2=1.3%， Nf=4902；（c）Δεt/2=1.1%，Nf= 10171；（d）Δεt/2=1.6%，Nf=832
Figure 6. Characteristics of fatigue crack propagation zone （a）Δεt/2=1.1%， Nf=9203；（b）Δεt/2=1.3%， Nf=4902；（c）Δεt/2=1.1%， Nf= 10171；（d）Δεt/2=1.6%， Nf=832
图 7 疲劳断口瞬断区特征形貌 （a）、（b）Δεt/2=1.6%，Nf=924；（c）Δεt/2=1.1%，Nf= 10171；（d）Δεt/2=1.1%，Nf=18133
Figure 7. Characteristics of the final fracture zone （a）、（b）Δεt/2=1.6%，Nf=924；（c）Δεt/2=1.1%，Nf= 10171；（d）Δεt/2=1.1%，Nf=18133
图 8 疲劳断裂组织 （a）、（b）、（c）Δεt/2=1.1%， Nf=18133；（d）Δεt/2=1.6%， Nf=832;（e）、（f）Δεt/2=1.9%， Nf=288
Figure 8. Microstructue of specimen after fatigue fracture （a），（b），（c）Δεt/2=1.1%， Nf=18133;（d）Δεt/2=1.6%， Nf=832；（e），（f）Δεt/2=1.9%， Nf=288
表 1 DD6合金的名义成分（质量分数/%）
Table 1. Nominal chemical compositions of DD6 alloy（mass fraction/%）
Co Re Nb Mo Cr W Ta Al Hf C Ni 9 2 0.5 2 4.3 8 7.5 5.6 0.1 0.006 Bal 
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表 2 DD6合金非对称循环载荷下的低周疲劳参数
Table 2. Low cycle fatigue parameters of DD6 alloy under asymmetrical cyclic loading
E/GPa σf'E/% σf'/MPa b εf'/% c 109.4 2.224 2433.1 –0.143 0.164 –0.425
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