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.

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.

Grain Boundary Cracking of Nickel-Based Alloy 625 Under Creep Loadings at Elevated Temperatures

2019 ◽  
Author(s):  
Yan Liang ◽  
Yifan Luo ◽  
Ken Suzuki ◽  
Hideo Miura
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Power Plants ◽  
Elevated Temperatures ◽  
Thermal Power ◽  
Thermal Power Plants ◽  
Alloy 625 ◽  
Creep Fatigue ◽  
Creep Loading ◽  
Initial Cracks

Abstract Since the operating condition of thermal power plants has become harsher for minimizing the emission of CO2, Ni-based superalloys, such as Alloy 617 and 625, have been used in the plants to replace the conventional ferritic materials. Unfortunately, the increase of coefficient of thermal expansion compared with conventional steels is a concern. In addition, Ni-based superalloys have to suffer creep-fatigue random loading because thermal power plants have to compensate the random output of various renewable energies. It was found that the lifetime of Ni-based superalloys under creep-fatigue loading was much shorter than that under simple fatigue or creep loading. Thus, it has become very important to clarify the crack mechanism and establish the quantitative theory for estimating their lifetime under various loading conditions at elevated temperatures. Thus, the elucidation of the initial damage mechanism of Alloy 625 under various loading is indispensable. Hence, the initial cracking mechanism of Alloy 625 at grain boundaries under creep loading was investigated experimentally. The creep test was applied to small specimens in Argon atmosphere. The change of the micro texture during the creep test was observed by using SEM. It was confirmed that all the initial cracks appeared at certain grain boundaries. The change of the crystallinity was observed by EBSD (Electron Back-Scatter Diffraction) analysis quantitatively. It was found that the local accumulation of dislocations at the cracked grain boundaries caused the initial cracks at those grain boundaries. The initiation of cracks appeared clearly between two grains which had difference of KAM (Kernel Average Misorientation) values larger than 0.2. Therefore, dislocations were accumulated at one side of the grain boundary. By measuring the KAM values near grain boundaries, the appearance of initial cracks can be predicted approximately.

Change of the Effective Strength of Grain Boundaries in Alloy 617 Under Creep-Fatigue Loadings at 800°C

2019 ◽  
Author(s):  
Wataru Suzuki ◽  
Kenta Ishihara ◽  
Ryo Kikuchi ◽  
Ken Suzuki ◽  
Hideo Miura
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Evaluation Method ◽  
Accumulation Rate ◽  
Ion Beam ◽  
Fatigue Loading ◽  
Test System ◽  
Total Density ◽  
Alloy 617 ◽  
Creep Fatigue

Abstract In this study, both EBSD (Electron Back-Scatter Diffraction) analysis and a micro tensile test system with FIB (Focused Ion Beam) equipment were applied to bicrystal specimens of Alloy 617 in order to establish a quantitative theory of the lifetime evaluation method under creep-fatigue loadings. The IQ (Image Quality) value which is obtained from the diffraction pattern (Kikuchi pattern) indicates the total density of defects such as vacancies, dislocations, and local strain was used for quantitatively evaluating the crystallinity of the alloy. KAM (Kernel Average Misorientation) value was also used for density analysis of GN (Generally Necessary) dislocations. Continuous changes of microtexture of grains and grain boundaries were observed by applying an intermittent creep fatigue test. As a result, it was confirmed that the IQ value around specific grain boundaries in damaged specimen was drastically degraded compared to that of the initial specimen. Intergranular cracks always occurred when the IQ value decreased to a certain value. Therefore, there is a correlation between the IQ value and the strength of a grain boundary in this alloy. The decrease of the IQ value was attributed to the accumulation of both dislocations and vacancies around the specific grain boundaries under creep loading. The accumulation rate under the creep-fatigue loading was clearly higher than that under simple fatigue and creep loadings. Finally, it was clarified that the degradation of the crystallinity in the vicinity of a grain boundary was a criterion of an intergranular crack under creep-fatigue loadings.

Evaluation of Damage Evolution in Nickel-Base Heat-Resistant Alloy Under Creep-Fatigue Loading Conditions

2017 ◽  
Author(s):  
Ken Suzuki ◽  
Takuya Murakoshi ◽  
Hiroki Sasaki ◽  
Hideo Miura
Keyword(s):  
Grain Boundaries ◽  
Fatigue Test ◽  
Damage Evolution ◽  
Power Plants ◽  
Hold Time ◽  
Fatigue Loading ◽  
Fatigue Tests ◽  
Electron Back Scatter Diffraction ◽  
Heat Resistant Alloy ◽  
Creep Fatigue

In this study, interrupted creep and creep-fatigue tests of Alloy 617, which is a candidate alloy for boiler tubes and pipes of A-USC (advanced ultra-supercritical) power plants of the 700°C-class, were conducted to investigate damage evolution process. Also, the change of the micro texture of the alloy was continuously observed at a fixed area to elucidate the mechanism of damage evolution under creep and creep-fatigue loading from the viewpoint of the change of the order of atom arrangement using EBSD (Electron Back-Scatter Diffraction) analysis. The conditions of the creep test were a temperature of 800°C and the stress of 150 MPa in inert gas (99.9999% Ar). The stress-controlled creep-fatigue tests were carried out at 800°C in Ar using stress ratio R = −1 and hold time of 10 minutes at peak tension. IQ (Image Quality) values, which are the average sharpness of the obtained diffraction pattern, were used for evaluating the change of the micro texture during the tests. In both creep and creep-fatigue test, intergranular cracks appeared. The IQ value decreased monotonically in the vicinity of grain boundaries with the decrease of fracture life, indicating that the crystallinity of grain boundaries degraded faster than that of grains. This localized damage around grain boundaries was attributed to the intergranular crack propagation in the creep and creep-fatigue test. In addition, all the grain boundaries with IQ value lower than 85% of IQ value in as-received specimen were found to be cracked during both creep and creep-fatigue test. Therefore, there was the critical IQ value around grain boundaries at which intergranular cracks occurred under creep or creep-fatigue loading condition.

Degradation of the Strength of a Grain Boundary of Ni-Base Superalloys Under Creep-Fatigue Loading at Elevated Temperature

2020 ◽  
Author(s):  
Yukako Takahashi ◽  
Yifan Luo ◽  
Kenta Ishihara ◽  
Shujiro Suzuki ◽  
Hideo Miura
Keyword(s):  
Grain Boundary ◽  
Tensile Test ◽  
Grain Boundaries ◽  
Elevated Temperatures ◽  
Base Alloy ◽  
Tensile Load ◽  
Test System ◽  
Uniaxial Tensile ◽  
Total Density ◽  
Creep Fatigue

Abstract The degradation of the strength of a grain boundary was measured by using a micro tensile test in a scanning electron microscope. The change of the crystallinity of grain boundaries during creep-fatigue tests of Ni-base alloy such as Alloy 617 and 625 at elevated temperatures was monitored by electron back-scatter diffraction analysis. The image quality (IQ) value obtained from the analysis, which indicates the total density of defects, was applied to the quantitative evaluation of the crystallinity. It was clearly observed that the accumulation of defects occurred at grain boundaries which were perpendicular to the loading direction and consisted of grains with large difference of Schmid factor. Bicrystal specimens with different crystallinity were cut from the tested samples and the strength of the bicrystal specimens were measured by using the micro tensile test system. It was confirmed that the strength of a grain boundary decreased monotonically by about 50% with the decrease of IQ value, in other words, the increase in the total density of various defects such as vacancies and dislocations. On the other hand, the effective yielding stress of grains increased monotonically with the decrease of the IQ value. This is because the increase in the total density of these defects suppresses the movement of dislocations, in other words, plastic deformation. Therefore, there were three independent strengths, the strength of two grains and that of a grain boundary which consisted of the bicrystal specimen. Since the strength of grains increased, at the same time, that of a grain boundary decreased monotonically with the decrease of the IQ value, it was confirmed that there was critical IQ value at which the fracture mode of a bicrystal specimen changed from conventional transgranular cracking to intergranular cracking under the application of uniaxial tensile load.

Degradation of the Strength of Grains and Grain Boundaries of Ni-Base Superalloy under Creep and Creep-Fatigue Loadings

2017 ◽  
Vol 754 ◽  
pp. 31-34
Author(s):  
Takuya Murakoshi ◽  
Hayato Sakamoto ◽  
Taichi Shinozaki ◽  
Ken Suzuki ◽  
Hideo Miura
Keyword(s):  
Image Quality ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Ion Beam ◽  
Fatigue Loading ◽  
Small Sample ◽  
Loading Conditions ◽  
Bulk Specimen ◽  
Creep Fatigue ◽  
Quality Value

The degradation process of the micro texture of Ni-base superalloys was observed under fatigue and creep-fatigue loading conditions at elevated temperatures higher than 700oC by applying electron back-scatter diffraction (EBSD) analyses. The local distribution of the crystallinity of a grain and a grain boundary was defined quantitatively by analyzing the Kikuchi pattern obtained from each electron-beam-irradiated area with a diameter of 50 nm. The calculated image quality value was used for the analysis. It was found that the crystallinity of grain boundaries degraded seriously under creep-fatigue loading conditions due to the acceleration of anisotropic strain-induced diffusion of component elements. Since the initial finely-controlled strengthened micro texture disappeared due to the anisotropic diffusion of component elements, this degradation was found to cause the drastic decrease of the strength of grains and grain boundaries and thus, lifetime of the material. The decrease of the strength of both grains and grain boundaries was measured by micro tensile test system in a scanning electron microscope by making a small sample from bulk specimen using focused ion beam. The strength of a grain and a grain boundary varied drastically depending on their crystallinity, in other words, the image quality value.

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