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Acta Metall Sin  2020, Vol. 56 Issue (9): 1185-1194    DOI: 10.11900/0412.1961.2020.00026
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Microstructure Evolution of K4169 Alloy During Cyclic Loading
WU Yun1, LIU Yahui1, KANG Maodong1,2(), GAO Haiyan1,2, WANG Jun1,2, SUN Baode1,2
1 School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
2 Shanghai Key Laboratory of Advanced High-Temperature Materials and Precision Forming, Shanghai Jiao Tong University, Shanghai 200240, China
Cite this article: 

WU Yun, LIU Yahui, KANG Maodong, GAO Haiyan, WANG Jun, SUN Baode. Microstructure Evolution of K4169 Alloy During Cyclic Loading. Acta Metall Sin, 2020, 56(9): 1185-1194.

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Abstract  

K4169 nickel-based superalloy has been widely used to fabricate high-strength components in aircraft engine. When in service, especially affected by vibration and start-stop process, this alloy is inevitably affected by the external cyclic stress. Therefore, it is of great significance for researchers to understand the microstructure evolution in K4169 while cyclic loading. In the present study, the microstructure evolution of K4169 during cyclic loading has been examined and discussed in detail by using investment casting, cyclic loading and microstructure characterization methods. The cyclic loading test with stress amplitude of 380 MPa was carried out on a pull-push type fatigue machine at room temperature. The dependence of cycle times or fatigue life of specimens with different casting conditions on microporosity content has been discussed. Special emphases have been put on investigating the deformation and fracture characteristics of Laves and δ-Ni3Nb phases under the influence of microporosity. The results show that the cyclic life was mainly dominated by the content of microporosity. The crack initiation occurred mainly near the microporosity of the specimen surface. The specimen with high microporosity content exhibits the characteristic of complete brittle fracture, while the specimen with low microporosity content exhibits obvious transgranular fracture characteristics. In addition, the fracture of Laves phase was not apparently affected by cycle number. At the beginning of cyclic loading, the long-striped Laves phase near the microporosity was easy to crack, which became the sensitive area of crack growth, and extending in the manner of parallel secondary cracks. The δ-Ni3Nb plates near microporosity exhibited two obvious cyclic deformation and fracture characteristics depending on their arrangement (or growth orientation) relative to external loading axis: cracking along length direction (or denoted as branch cracking); and exhibiting slip lines and cracks on the surface of δ-Ni3Nb plates. At the initial stage of cyclic loading, δ-Ni3Nb plates were prone to crack along the length direction, while the surfaces of the δ-Ni3Nb plates far from microporosity appear the characteristics of slipping, bending and fracture in turn with the decrease of microporosity content or increase of cyclic cycles. Edge dislocations have been found within δ-Ni3Nb plates, indicating the transition from screw dislocations to edge dislocations under cyclic loading. Additionally, the twinning deformation of γ-Ni matrix during cyclic loading has been scrutinized through TEM and TKD analyses. The results have been linked to the evolutions of Laves and δ-Ni3Nb phases, i.e., the evolutions were influenced by the increase of strain localization around Laves and δ-Ni3Nb phases.

Key words:  K4169 superalloy      cyclic stress      microstructure evolution      Laves phase      δ-Ni3Nb phase     
Received:  17 January 2020     
ZTFLH:  TG132.3  
Fund: National Natural Science Foundation of China(51971142);National Science and Technology Major Project of China(2017-Ⅵ-0013-0085);Aeronautical Science Foundation of China(2018ZE57012);Startup Fund for Youngman Research at SJTU(18X100040027)

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2020.00026     OR     https://www.ams.org.cn/EN/Y2020/V56/I9/1185

Fig.1  Dimension of the cyclic loading specimen (unit: mm)
Fig.2  Typical OM images containing microporosity of casted K4169 bars with hot spot diameters of 41.8 mm (a), 35.8 mm (b), 21.7 mm (c), 19.6 mm (d), 18.1 mm (e), 16.5 mm (f) and 11.8 mm (g) named specimens 1#~7#, respectively
Fig.3  Microstructure characteristics of K4169 alloy in standard heat treatment state
Fig.4  Distributions of Feret diameter of Laves phase in different specimens
Fig.5  Relationship between fatigue cycle (Nf) and volume fraction of microporosity (fv)
Fig.6  SEM images showing the fracture surfaces of low-cycle (394 cyc) (a~c) and high-cycle (5457 cyc) (d~f) specimens (PSBs—persistent slip bands)
Fig.7  SEM images of cyclic fractured Laves particle near the fracture surface in longitudinal section (a) and cyclic fractured Laves particle in fracture surface (b), and EDS results of the spot 1 in Fig.7a (c) and spot 2 in Fig.7b (d) (Inset in Fig.7a shows the secondary cracks)
Fig.8  SEM images of δ-Ni3Nb plates characterized by branch cracking and bending (a) and fractured δ-Ni3Nb plates induced by slip (b)
Fig.9  EBSD analyses of band contrast image of cyclic fractured Laves particle (a), inverse pole figure (IPF) image and 3D crystal orientation (insets) of the Laves particle (b) and local misorientation angle distribution within Ni-matrix nearby the Laves particle (c)
Fig.10  Deformation and fracture analyses of δ-Ni3Nb phase by SEM and TEM
Fig.11  TEM image (a) and TKD band contrast image (inset shows Kikuchi pattern of γ-Ni matrix) (b) showing twin related deformation characteristic of γ-Ni matrix after cyclic loading
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