High Cycle Fatigue Behavior of Second Generation Single Crystal Superalloy
LI Jiarong(),XIE Hongji,HAN Mei,LIU Shizhong
Science and Technology on Advanced High Temperature Structural Materials Laboratory, Beijing Institute of Aeronautical Materials, Beijing 100095, China
Cite this article:
LI Jiarong,XIE Hongji,HAN Mei,LIU Shizhong. High Cycle Fatigue Behavior of Second Generation Single Crystal Superalloy. Acta Metall Sin, 2019, 55(9): 1195-1203.
Ni-based single crystal superalloys have excellent comprehensive properties and become the preferred material for advanced aeroengine turbine blades. DD6 alloy which has been widely used in China and DD5 alloy are the second generation single crystal superalloy, and their chemical compositions and mechanical properties are quite different. In the past few decades, high cycle fatigue failure has become one of the main causes of turbine blade failure. More and more attention has been paid to the high cycle fatigue properties of single crystal superalloys. Therefore, it is important to study the high cycle fatigue behavior of single crystal superalloys, especially the second generation single crystal superalloys. In order to compare high cycle fatigue performance, two typical second generation single crystal (SC) superalloys DD6 and DD5 with [001] orientation were subjected to high cycle fatigue (HCF) loading at temperatures of 760 and 980 ℃ in ambient atmosphere. The results demonstrate that the fatigue limit of DD6 alloy is 414 and 403 MPa at temperatures of 760 and 980 ℃, respectively. DD6 alloy exhibits an excellent HCF performance under a condition of stress ratio of -1 regardless of medium or high temperature. Analysis on fracture surfaces of DD6 and DD5 alloys at 760 and 980 ℃ demonstrate that quasi-cleavage mode is observed. In addition, different types of dislocation structures were developed during the cyclic deformation. When the stress amplitude is low, dislocation movement in the γ matrix by bowing and cross slip is the main deformation mechanism and shearing γ' particles by dislocation pairs occurs occasionally under high stress level. The analysis shows that the carbon content of DD5 alloy is eight times than that of DD6 alloy, which makes the carbide content much higher than DD6 alloy, and there are significant differences in carbide morphology. In the process of fatigue fracture, carbide plays two roles of secondary crack initiation position and crack propagation channel, which greatly accelerates the fatigue crack growth rate. In the end, the fatigue resistance of DD5 alloy is reduced.
Fig.4 Low (a, b) and high (c, d) magnified microscopic fracture surface SEM images of DD6 alloy at 760 ℃ (a, c) and 980 ℃ (b, d)
Fig.5 SEM images of longitudinal sections near the fracture surface of fatigue-ruptured DD6 and DD5 alloys at 760 ℃(a, b) center and edge of DD6 alloy, respectively, σa=600 MPa, Nf =1.19×105 cyc(c, d) center and edge of DD5 alloy, respectively, σa=500 MPa, Nf =1.73×105 cyc (Arrow in Fig.5c shows the stress direction)
Fig.6 TEM images of dislocation configuration near fatigue fractures of DD6 and DD5 alloys at 760 ℃(a) DD6, σa=500 MPa, Nf =8.45×105 cyc (b) DD6, σa=700 MPa, Nf =5.74×104 cyc(c) DD5, σa=400 MPa, Nf=1.41×106 cyc (d) DD5, σa=600 MPa, Nf =8.96×104 cyc
Fig.7 SEM images of carbides in DD6 (a) and DD5 (b) alloys
Fig.8 Fatigue crack initiation of DD6 (a, c) and DD5 (b, d) alloys at 760 ℃ (a, b) and 980 ℃ (c, d)
Fig.9 SEM images of longitudinal sections near the fatigue fracture surface of DD5 alloy specimens(a~c) 760 ℃, σa=400 MPa, Nf =1.41×106 cyc (d~f) 980 ℃, σa=400 MPa, Nf =8.48×106 cyc
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