Twin Boundary Evolution Under Low-Cycle Fatigue of C-HRA-5 Austenitic Heat-Resistant Steel at High Temperature
ZHOU Hongwei1, GAO Jianbing2, SHEN Jiaming1, ZHAO Wei3, BAI Fengmei3(), HE Yizhu1
1.Key Laboratory of Green Preparation and Surface Technology of Advanced Metal Materials, Ministry of Education, School of Materials Science and Engineering, Anhui University of Technology, Ma'anshan 243032, China 2.State Key Laboratory of Advanced Stainless Steel Materials, Taiyuan Iron and Steel (Group) Co., Ltd., Taiyuan 030003, China 3.Anhui Key Laboratory of Metallurgical Engineering and Comprehensive Utilization of Resources, School of Metallergical Engineering, Anhui University of Technology, Ma'anshan 243032, China
Cite this article:
ZHOU Hongwei, GAO Jianbing, SHEN Jiaming, ZHAO Wei, BAI Fengmei, HE Yizhu. Twin Boundary Evolution Under Low-Cycle Fatigue of C-HRA-5 Austenitic Heat-Resistant Steel at High Temperature. Acta Metall Sin, 2022, 58(8): 1013-1023.
C-HRA-5 steel is a new type of austenitic heat-resistant steel with excellent oxidation resistance and corrosion resistance, and high endurance strength at high temperatures. This steel can be used in superheaters and reheaters applied in 630-700oC advanced ultrasupercritical fossil-fired power plants, which is of great strategic significance to realize national energy savings and emission reduction targets. Detwinning often occurs in austenitic stainless steel when high-temperature fatigue and creep set in. However, the detwinning mechanism in C-HRA-5 steel during high-temperature fatigue and its influence on fatigue crack initiation and propagation remain unclear. Therefore, in this work, the detwinning behavior of C-HRA-5 steel under strain-controlled low-cycle fatigue (LCF) was studied at 700oC. The evolution of twin boundaries (TBs), detwinning mechanism, and influence of residual TBs on fatigue cracks were analyzed using SEM, EBSD, and TEM. After solution treatment at high temperatures, the fraction of low coincidence site lattice boundaries reached 69% in C-HRA-5 steel with dominant Σ3-type TBs. There was a remarkable detwinning effect under LCF loading. The degree of detwinning increased with increasing strain amplitude ranging from 0.3% to 0.7%. The detwinning mechanism was mainly related to the interactions at the dislocation TBs and precipitation of M23C6 at TBs. The plastic deformation of C-HRA-5 steel was dominated by planar slip under LCF loading, which resulted in dislocation gliding on particular planes. Therefore, numerous dislocation slip bands were formed. The collision of dislocation slip bands with TBs changed the coherent orientation of TBs and eventually led to detwinning. Meanwhile, the dislocation-TBs interaction induced the precipitation of M23C6 carbide at some TBs during LCF at 700oC, which strengthened the pinning of TBs on dislocations, thus accelerating the detwinning process. The residual TBs had higher strength than random grain boundaries and could inhibit the initiation and propagation of fatigue cracks.
Fund: Anhui Natural Science Foundation(2008085ME127);Natural Science Foundation of Anhui Provincial Education Department(KJ2020A0252);Key Project of Science and Technology in Shanxi Province(20181101014)
About author: BAI Fengmei, associate professor, Tel: (0555)2311570, E-mail: baifengmei@ahut.edu.cn
Fig.1 OM (a) and TEM (b) images of C-5 steel after water quenching at 1250oC for 30 min, SAED pattern of the twin in Fig.1b (c), and EDS analysis of Z phase marked by a box in Fig.1b (d) (TBs─twin boundaries, CTBs─coherent TBs, ITBs─incoherent TBs)
Fig.2 Inverse pole figures (IPFs) (a1-d1), band contrast (BC) maps (Red lines show the TBs) (a2-d2), and kernel average misorientation (KAM) maps (a3-d3) of the solution-treated (ST) C-5 steel (a1-a3) and under low-cycle fatigue (LCF) loading at 700oC with total strain amplitude εt = 0.3% (b1-b3), 0.5% (c1-c3), and 0.7% (d1-d3)
Fig.3 Variation of the length fraction of TBs with εt in C-5 steel under LCF loading at 700oC
Fig.4 IPFs (a1, b1), BC maps (Red lines show the TBs) (a2, b2), and KAM maps (a3, b3) of TBs fragmentation in the C-5 steel under LCF loading at 700oC with εt = 0.3% (a1-a3) and 0.7% (b1-b3) (RB─random boundary, p-TBs─pseudo-TBs)
Fig. 5 IPF (a1), BC map (Red lines show the TBs) (a2), KAM map (a3), and the contour lines of low angle grain boundary (LAGB, red lines) and high angle grain boundary (HAGB, black lines) (a4) of the C-5 steel under LCF loading at 700oC with εt = 0.7%, enlarged IPF and pole figures (PFs) of p-TBs in the selection area in Fig.5a1 (b), IPF and PFs of Σ3-type TBs with εt = 0.7% (c) and in ST C-5 steel (d)
Fig.6 TEM images of interaction between dislocation and TBs under LCF loading at 700oC with εt = 0.3% (a, b) and 0.7% (c)
Fig.7 SEM images of the precipitates at TBs in C-5 steel with εt = 0.5% at 700oC (a) precipitates along the CTBs (b) precipitates along the ITBs
Fig.8 TEM images, SAED patterns, and EDS maps of the precipitates at TBs with εt = 0.5% at 700oC (a) bright field TEM image (b) dark field TEM image (c, f) SAED patterns of the precipitates in the circular area in Fig.8b (d, e) EDS element maps of C and Cr, respectively
Fig.9 Schematics of detwinning in C-5 steel under LCF at 700oC (a) dislocations slip to TBs under fatigue loading (b) dislocation increases to form slip bands that impact TBs, resulting in TBs fragmentation (c) annihilation of partial TBs (d) precipitation at TBs accelerates detwinning
Fig.10 IPF (a1-c1) and BC (Red lines show the TBs) (a2-c2) maps of the C-5 steel under LCF loading at 700oC with εt = 0.3% (a1, a2) and 0.7% (b1, b2), and enlarged images of the square area in Figs.10b1 and b2 (c1, c2)
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