Stress-Induced Acceleration of the Change of Microstructure of Ni-Base Superalloy CM247LC

2017 ◽  
Author(s):  
Ken Suzuki ◽  
Hideo Miura
Keyword(s):  
High Temperature ◽  
Creep Damage ◽  
Degradation Process ◽  
Atomic Diffusion ◽  
High Temperature Strength ◽  
Atomic Concentration ◽  
Superlattice Structure ◽  
Creep Loading ◽  
Change Of Microstructure ◽  
Base Superalloy

The degradation process of Ni-base superalloy CM247LC was investigated experimentally under the creep loading at 900°C. The initial excellent high-temperature strength of this alloy is attributed to its micro texture, the fine binary phase such as cuboidal γ’ (Ni3Al) precipitates orderly dispersed in the γ matrix (Ni-rich matrix). However, it was observed that γ’ precipitates started to coarse perpendicular to the applied uniaxial load direction during high temperature creep loading. The disappearance of the strengthened micro texture caused the acceleration of the crack growth along the phase boundaries of the layered texture and seriously degrades the strength of this material. Therefore, not only the outlook of micro texture but also the changes of the atomic configuration and atomic concentration which were based on the atomic diffusion behavior was investigated for the further explication of rafting mechanism more in detail. It was found that the distribution of Image Quality (IQ) value which was obtained from EBSD analysis monotonically shifted to lower values and the full width of half maximum became wider as the creep loading time increased. This degradation of the order of atomic alignment indicated that lattice defects density increased and ordered superlattice structure (Ll2 structure) became disordered. In addition, the initial periodic distributions of component elements which corresponded to the fine periodic alignment of the γ and γ’ phases also disappeared and the concentration of each element became uniform even though both the γ and γ’ phases still remained even after rafting. The observed creep damage of CM247LC was, therefore, dominated by the degradation of the order of atomic arrangement, and this degradation was attributed to the strain-induced atomic diffusion of component elements. It is very important, therefore, to suppress this strain-induced acceleration of atomic diffusion in this alloy by modifying the microstructure of this alloy.

Atomic Diffusion Induced Damage of Ni-Base Super Alloy at Elevated Temperature

2016 ◽  
Author(s):  
Motoki Takahashi ◽  
Ken Suzuki ◽  
Hideo Miura
Keyword(s):  
Power Plants ◽  
Creep Damage ◽  
Rotor Blades ◽  
Atomic Scale ◽  
Atomic Diffusion ◽  
Kernel Average Misorientation ◽  
Damage Process ◽  
Creep Loading ◽  
Ebsd Method ◽  
Base Superalloy

Ni-base superalloys consisting of binary phases such as cuboidal γ’ (Ni3Al) precipitates orderly dispersed in the γ matrix (Ni-rich matrix) have been generally used for rotor blades in energy power plants. However, fine dispersed γ’ precipitates are coarsened perpendicularly to the applied load direction during high temperature creep loading. As this phenomenon called “Rafting” proceeds, the strengthened micro texture disappears and then, cracks starts to grow rapidly along the boundaries of the layered texture. Thus, it is very important to evaluate the change of the crystallinity of the alloy in detail for explicating the atomic scale damage process. In this study, the change of the micro-texture of the Ni-base superalloy (CM247LC) was observed by using EBSD method. The change in the crystallinity was evaluated using both Kernel Average Misorientation (KAM) and image quality (IQ) values. The KAM value indicates the dislocation density and the IQ value shows the order of atom arrangement in the observed area. As a result, KAM value showed no significant change with increasing the creep damage. On the other hand, the IQ value monotonically shifted to lower values and the average IQ value gradually decreased as the creep loading time increased. Decreasing IQ value without change in KAM value implies that the density of point defects such as vacancies mainly increased under creep loading and ordered Ll2 structure became disordered. Therefore, the creep damage of this alloy is mainly dominated by not the accumulation of dislocations, but the increase in the disorder of atom arrangement in the micro texture caused by the diffusion of component elements.

Crystallinity Degradation Caused by Alloying Elements Diffusion During Creep of Ni-Base Superalloy

2015 ◽  
Author(s):  
Ken Suzuki ◽  
Takuya Murakoshi ◽  
Hideo Miura
Keyword(s):  
High Temperature ◽  
Creep Damage ◽  
Fatigue Loading ◽  
Dislocation Motion ◽  
Dominant Factor ◽  
High Temperature Strength ◽  
High Temperature Mechanical Properties ◽  
Directional Coarsening ◽  
Creep Loading

High temperature mechanical properties of Ni-base superalloys are improved by the fine cuboidal γ’ (Ni3Al) precipitates orderly-dispersed in the γ matrix (Ni-rich matrix) because the dispersed texture in a grain inhibits dislocation motion. However, it is well known that directional coarsening of the γ’ precipitates perpendicular to a principal stress occurs not only during creep loading but also during cyclic loading and, the formation of the raft causes the decreasing of high temperature strength drastically. Therefore, it is very important to evaluate the damage of the alloys caused by creep and fatigue loading based on the change of their micro texture. In this study, the change of crystallinity of the Ni-base superalloys (CM247LC) under creep loading was analyzed by applying Electron Back-Scattered Diffraction (EBSD) method. The image quality (IQ) value obtained from the EBSD analysis was used for the quantitative evaluation of the crystallinity in the area where an electron beam of 10 nm in diameter was irradiated. The quality of the atomic alignment of both γ’ and γ phases was found to degrade with increasing creep damage. The degradation of crystallinity suggests that the ordered L12 structure of Ni3Al became disordered and the density of dislocations and vacancies increased. However, KAM (Kernel Average Misorientation) value did not change significantly with increasing creep damage. Therefore, the dominant factor of the creep damage of this alloy is the strain-induced diffusion of elements under loading, and the decrease of the crystallinity.

High Temperature Damage of Ni-Base Superalloy Caused by the Change of Microtexture due to the Strain-Induced Anisotropic Diffusion of Component Elements

2011 ◽  
Author(s):  
Hideo Miura ◽  
Ken Suzuki ◽  
Yamato Sasaki ◽  
Tomohiro Sano ◽  
Naokazu Murata
Keyword(s):  
High Temperature ◽  
Anisotropic Diffusion ◽  
Axial Strain ◽  
Creep Damage ◽  
Face Centered Cubic ◽  
Directional Coarsening ◽  
Damage Process ◽  
Temperature Damage ◽  
Face Centered ◽  
Base Superalloy

In order to assure the reliability of advanced gas turbine systems, it is very important to evaluate the damage of high temperature materials such as Ni-base superalloys under creep and fatigue conditions quantitatively. Since the micro texture of the gamma-prime (γ′) phase was found to vary during the creep damage process, it is possible, therefore, to evaluate the creep damage of this material quantitatively by measuring the change of the micro texture. The mechanism of the directional coarsening of γ′ phasesof Ni-base superalloy under uni-axial strain at high temperatures, which is called rafting, was analyzed by using molecular dynamics (MD) analysis. The stress-induced anisotropic diffusion of Al atoms perpendicular to the finely dispersed γ/γ′ interface in the superalloy was observed clearly in a Ni(001)/Ni3Al(001) interface structure. The stress-induced anisotropic diffusion was validated by experiment using the stacked thin films structures which consisted of the (001) face-centered cubic (FCC) interface. The reduction of the diffusion of Al atoms perpendicular to the interface is thus, effective for improving the creep and fatigue resistance of the alloy. It was also found by MD analysis that the dopant elements in the superalloy also affected the strain-induced diffusion of Al atoms. Both palladium and tantalum were effective elements which restrain Al atoms from moving around the interface under the applied stress, while titanium and tungsten accelerated the strain-induced anisotropic diffusion, and thus, the rafting phenomenon.

scholarly journals Creep Damage Process of Ni-Base Superalloy Caused by Stress-Induced Anisotropic Atomic Diffusion

2009 ◽  
Vol 3 (3) ◽  
pp. 487-497 ◽  
Author(s):  
Ken SUZUKI ◽  
Hiroyuki ITO ◽  
Tatsuya INOUE ◽  
Hideo MIURA
Keyword(s):  
Creep Damage ◽  
Atomic Diffusion ◽  
Damage Process ◽  
Base Superalloy

High Temperature Resistant Intermetallic Nial-Based Alloys with Refractory Metals Cr, Mo, Re - Structures - Properties - Applications -

2002 ◽  
Vol 753 ◽  
Author(s):  
G. Frommeyer ◽  
R. Rablbauer
Keyword(s):  
High Temperature ◽  
Creep Resistance ◽  
Refractory Metals ◽  
Solid Solution Hardening ◽  
High Temperature Strength ◽  
Fibre Reinforcement ◽  
Matrix Composites ◽  
Superlattice Structure ◽  
High Temperature Mechanical Properties ◽  
Binary Eutectic

ABSTRACTThe stoichiometric intermetallic compound NiAl with B2 superlattice structure exhibits superior physical and high-temperature mechanical properties, and excellent oxidation resistance. The main disadvantages of polycrystalline NiAl are the lack in plasticity and fracture toughness below the brittle-to-ductile-transition temperature of about 550°C. Insufficient high-temperature strength and creep resistance occur at temperatures above 800°C. Despite these facts NiAl-based alloys are still considered as promising structural materials for high-temperature applications. The refractory metals Cr, Mo, and Re with b.c.c. and h.c.p. lattice structures form with NiAl quasi-binary eutectic systems, showing high melting temperatures and thermally stable microstructures. Elasticity, solid solution hardening, fibre reinforcement, and creep properties were investigated in view of the constitutional defect structure and microstructural features. Especially the fibre reinforced NiAl matrix composites possess optimum high-temperature strength up to 1200 °C, and improved creep resistance as well.

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