Acceleration mechanism of Intergranular Cracking of SUS316L under Creep-Fatigue Loading at Elevated Temperatures

2020 ◽  
Vol 2020.56 (0) ◽  
pp. B5
Author(s):  
Yukako TAKAHASHI ◽  
Ken SUZUKI ◽  
Hideo MIURA
Keyword(s):  
Elevated Temperatures ◽  
Fatigue Loading ◽  
Intergranular Cracking ◽  
Acceleration Mechanism ◽  
Creep Fatigue

Molecular Dynamics Analysis of the Acceleration of Intergranular Cracking of Ni-Base Superalloy Caused by Accumulation of Vacancies and Dislocations Around Grain Boundaries

2020 ◽  
Author(s):  
Ryo Kikuchi ◽  
Shujiro Suzuki ◽  
Ken Suzuki
Keyword(s):  
Molecular Dynamics ◽  
Plastic Deformation ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Fatigue Loading ◽  
Transgranular Fracture ◽  
Dynamics Analysis ◽  
Intergranular Cracking ◽  
Creep Fatigue ◽  
Schmid Factor

Abstract Ni-based superalloys with excellent high temperature strength have been used in advanced thermal power plants. It was found that grain boundary cracking is caused in the alloy under creep-fatigue loading due to the degradation of the crystallinity of grain boundaries and the grain boundary cracking degrades the lifetime of the alloy drastically. In order to clarify the mechanism of intergranular cracking, in this research, static and dynamic strains were applied to a bicrystal structure of the alloy perpendicularly to the grain boundary using molecular dynamics analysis. In addition, the effect of the accumulation of vacancies in the area with high-density of dislocations on the strength of the bicrystal structure was analysed. It was found that the fracture mode of the bicrystal structure changed from ductile transgranular fracture to brittle intergranular one as strong functions of the combination of Schmid factor of the two grains and the density of defects around the grain boundary. The local heavy plastic deformation occurred around the grain boundary with large difference in Schmid factor between nearby grains and the diffusion of the newly grown dislocations and vacancies was suppressed by the large strain field due to the large mismatch of the crystallographic orientation between the grains. The accumulation of vacancies accelerated the local plastic deformation around the grain boundary. Therefore, the mechanism of the acceleration of intergranular cracking under creep-fatigue loading was successfully clarified by MD analysis.

Acceleration of Grain Boundary Cracking in Ni-Base Alloy 617 Under Creep-Fatigue Loading at 800°C

2020 ◽  
Author(s):  
Kenta Ishihara ◽  
Yifan Luo ◽  
Hideo Miura
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Power Plants ◽  
Base Alloy ◽  
Thermal Power ◽  
Fatigue Loading ◽  
Intergranular Cracking ◽  
Alloy 617 ◽  
Effective Lifetime ◽  
Creep Fatigue

Abstract In recent years, in order to solve the global warming issue, the operating temperature of advanced thermal power plants has attempted to improve thermal efficiency and reduce CO2 emissions. Under the creep and creep-fatigue conditions at elevated temperature, however, the effective lifetime of heat-resistant alloys such as Ni-base Alloy 617, which has high strength and good corrosion resistance at about 750°C, was found to decrease drastically. Main reason for this short lifetime was attributed to the change in the crack initiation and propagation paths from transgranular one to intergranular one. Therefore, it is important to understand and express the criteria for grain boundary cracking. In this study, electron back-scatter diffraction (EBSD) analysis was applied to the visualization of the degradation process of the quality of grain boundaries in the alloy. The change in the crystallinity of grains and grain boundaries were continuously monitored during creep and creep-fatigue tests. It was found that accumulation of vacancies and dislocations degraded the crystallinity of grain boundaries and thus, their strength. The accumulation occurred around the specific grain boundaries which consisted of grains with large difference of Schmid factor during creep test. On the other hand, it occurred around all grain boundaries under the creep-fatigue loading. Thus, the accumulation of defects was clearly accelerated under the creep-fatigue loading. The critical image quality (IQ) value of intergranular cracking was almost the same regardless of the loading mode. Once the IQ value of the damaged grain boundaries decreased to a critical value, intergranular cracking started to occur at the grain boundaries.

Evaluating the Critical Temperature of Creep-Fatigue Interaction a Nickel-Based Powder Metallurgy Superalloy

2013 ◽  
Vol 577-578 ◽  
pp. 625-628
Author(s):  
Hao Liu ◽  
Rui Bao ◽  
Wei Ming Lei ◽  
Bin Jun Fei
Keyword(s):  
Fatigue Crack ◽  
Critical Temperature ◽  
Powder Metallurgy ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Elevated Temperatures ◽  
Fatigue Loading ◽  
Damage Mechanism ◽  
Creep Fatigue ◽  
Creep Fatigue Crack

For the components working in high temperature and enduring fatigue loading, the fatigue fracture properties will be reduced remarkably when the working temperature is higher than the critical temperature of creep-fatigue interactionTcof the material. In consequence, the damage mechanism from creep-fatigue interaction becomes more complex. A method is presented in this paper to determine theTcof a nickel-based powder metallurgy superalloy. Pure fatigue crack growth and creep-fatigue crack growth tests were conducted in several different elevated temperatures. The fracture mechanism was investigated via observing the fractographic characteristics using scanning electron microscope (SEM). The test results show that theTcof this superalloy is a little bit lower than a half of the melting temperatureTm.

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