The cause of the premature failure of 304 stainless steel tube heat exchanger was investigated.The unique skeleton structure inside the leakage point reveals that this is a new damage mechanism caused by a δ+γ two-p...The cause of the premature failure of 304 stainless steel tube heat exchanger was investigated.The unique skeleton structure inside the leakage point reveals that this is a new damage mechanism caused by a δ+γ two-phase structure and crevice corrosion.The three-dimensional structure of the leakage point was demonstrated using X-ray diffraction topography.The results of scanning electron microscope examination show the microstructure of the weld to be columnar and dendritic.It is found by electron probe microscope analysis and transmission electron microscopy that columnar dendrites consisted of γ-dendrite and an amount of δ-ferrite phases at the dendrite trunk.Simulated corrosion test results confirmed that the corrosion medium was the chloride ion.Crevice corrosion of chloride ions occurred at weld defects on the inner wall thus forming a concentration cell.Grains of columnar dendrites were then corroded by chloride ions and δ-ferrite phases on the grain boundaries were retained,which formed the particular skeleton corrosion structure.As a result,it led to leakage when the corrosion of weld occurred from the inner wall to the outer wall.展开更多
The high-cycle fatigue fracture characteristics and damage mechanism of nickel-based single crystal superalloys at 850℃ was investigated.The results indicate that high-cycle fatigue cracks in single crystal superallo...The high-cycle fatigue fracture characteristics and damage mechanism of nickel-based single crystal superalloys at 850℃ was investigated.The results indicate that high-cycle fatigue cracks in single crystal superalloys generally originate from defect locations on the subsurface or interior of the specimen at 850℃.Under the condition of stress ratio R=0.05,as the fatigue load decreases,the high-cycle fatigue life gradually increases.The high-cycle fatigue fracture is mainly characterized by octahedral slip mechanism.At high stress and low lifespan,the fracture exhibits single or multiple slip surface features.Some fractures originate along a vertical small plane and then propagate along the{111}slip surface.At low stress and high lifespan,the fracture surface tend to alternate and expand along multiple slip planes after originating from subsurface or internal sources,exhibiting characteristics of multiple slip planes.Through electron backscatter diffraction and transmission electron microscope analysis,there is obvious oxidation behavior on the surface of the high-cycle fatigue fracture,and the fracture section is composed of oxidation layer,distortion layer,and matrix layer from the outside to the inside.Among them,the main components of the oxidation layer are oxides of Ni and Co.The distortion layer is mainly distributed in the form of elongated or short rod-shaped oxides of Al,Ta,and W.The matrix layer is a single crystal layer.Crack initiation and propagation mechanism were obtained by systematical analysis of a large number of highcycle fatigue fractures.In addition,the stress ratio of 0.05 is closer to the vibration mode of turbine blades during actual service,providing effective guidance for the study of failure and fracture mechanisms of turbine blades.展开更多
Microstructure evolution of precision-cast Ti_(2)AlNb alloys after overtemperature treatments at 750 and 850℃ was investigated by optical microscope and scanning electron microscope,and Image J metallurgy analysis so...Microstructure evolution of precision-cast Ti_(2)AlNb alloys after overtemperature treatments at 750 and 850℃ was investigated by optical microscope and scanning electron microscope,and Image J metallurgy analysis software was used for quantitative analysis.The variation of mechanical properties before and after overtemperature treatment was tested.Results show that the Ti_(2)AlNb alloy consists of B2 phase and lamellar O phase before and after overtemperature treatment.After overtemperature treatment,partial B2 phase is transformed into the O phase,and the lamellar structure of O phase suffers discontinuous coarsening and spherolization.The coarsening degree is increased with the increase in temperature and prolongation of treatment time,and O phase content is also gradually increased.The microhardness and tensile properties of Ti_(2)AlNb alloys at room and high temperatures are decreased with the increase in temperature and prolongation of treatment time,and the high-temperature tensile strength exhibits relatively larger decrement.An obvious linear relationship exists between microhardness and O phase content.Both the microhardness and the O phase content are important parameters to evaluate the overtemperature service damage degree of precision-cast Ti_(2)AlNb alloy.展开更多
The effect of surface recrystallization by heating after shot-peening on the creep rapture property and fracture behavior of a single-crystal superalloy was investigated. The results show that the creep rupture proper...The effect of surface recrystallization by heating after shot-peening on the creep rapture property and fracture behavior of a single-crystal superalloy was investigated. The results show that the creep rupture property of the single-crystal superalloy was greatly influenced by surface recrystallization. A recrystallized surface layer with a depth of 101 ~m resulted in a decrease in creep rupture life by nearly 50%, and an almost linear reduction of creep rupture life was observed with the increase of recrystallization depth. A lower strength of the recrystal- lized layer, inhomogeneous deformation between the recrystallized layer and the matrix, and stress concentration caused by notch effect resuited in the decrease in creep rupture life of the single-crystal superalloy.展开更多
A series of experiments of investigating the recrystallization of single crystal DD3 superalloy were carried out. The threshold temperature for recrystallization and the effect of annealing temperature on recrystalliz...A series of experiments of investigating the recrystallization of single crystal DD3 superalloy were carried out. The threshold temperature for recrystallization and the effect of annealing temperature on recrystallization were studied. The results show that the threshold temperature for recrystallization of the shot-peened DD3 samples is between 1 000 ℃ and 1 050 ℃ under the condition of annealing for 2 h, and the recrystallization depth increases with the rise of the annealing temperature. Below 1 150 ℃, the recrystallization depth increases slowly with the temperature climbing, while above 1 150 ℃, the recrystallization depth increases quickly with the rise of the temperature. The solution of the γ＇ phase is a critical factor of the recrystallization behavior of DD3 superalloy. In addition, the kinetics and microstructural evolution of recrystallization at 1 200 ℃ were also studied. It is found that the recrystallization progresses rapidly at 1 200 ℃ through the growth of fully developed recrystallized grains, and the recrystallization process on the shot-peened surface is similar to that of wrought materials, including nucleation of recrystallization, growth of new grains into the matrix, and growth of new grains by swallowing up each other.展开更多
As-cast SRR99 specimens were shot-peened and then annealed at 1250 and 1300°C for different times to investigate the kinetics of recrystallization.It was found that the relationship between annealing time and vol...As-cast SRR99 specimens were shot-peened and then annealed at 1250 and 1300°C for different times to investigate the kinetics of recrystallization.It was found that the relationship between annealing time and volume fraction of recrystallized grains could be described by the Johnson-Mehl-Avrami equation.Based on the kinetics analysis of the recrystallization process,the apparent activation energy for recrystallization was determined.In addition,the microstructural evolution during the recrystallization process at 1300°C was studied.Inwards,recrystallization first occurs in the dendritic core regions at the shot-peened surface.With the dissolution of coarse γ＇ particles in the interdendritic regions,the recrystallized grain boundaries move through the interdendritic regions.Finally,the fully developed grains nearly have a uniform depth.The recrystallization process at the shot-peened surface is similar to that of wrought materials,which includes nucleation of recrystallization,growth of recrystallized nuclei into the matrix,and growth of recrystallized grains by swallowing up each other.展开更多
Vibration fatigue is the main failure mode of compressor blade. Evaluating the vibration stress of blade that leads to cracking is very useful for analysis of vibration fatigue. In this paper, fatigue stress estimatio...Vibration fatigue is the main failure mode of compressor blade. Evaluating the vibration stress of blade that leads to cracking is very useful for analysis of vibration fatigue. In this paper, fatigue stress estimation methods by quantitative fractography were studied through experimental blade and in-service first-stage compressor blade in order to evaluate the initiation vibration stress of in-service blade. The analysis process of initiation vibration stress was established. The evaluating result of vibration stress of in-service blade subjected to centrifugal force and bending vibration stress agrees with aero engine test result. It is shown that the evaluation method can not only evaluate the equivalent fatigue stresses of different crack depths but also yield the initiation equivalent fatigue stress.展开更多
Low-cycle fatigue crack initiation behavior of nickel-based single crystal superalloy at 530℃ was investigated.Results show that the behavior of crack initiation is closely related to the maximum strain.When the maxi...Low-cycle fatigue crack initiation behavior of nickel-based single crystal superalloy at 530℃ was investigated.Results show that the behavior of crack initiation is closely related to the maximum strain.When the maximum strain is 2.0%,the fatigue crack is originated at the position of persistent slip bands on the surface of specimen,which is located on the{111}slip plane.No defects are observed at the crack initiation position.When the maximum strain is lower than 1.6%,the cracks are initiated at the casting defects on sub-surface or at interior of the specimen.The casting defects are located on the{100}slip plane vertical to the axial force.The crack is initiated along the{100}slip plane and then expanded along different{111}slip planes after a short stage of expansion.As the maximum strain decreases,the position of crack initiation gradually changes from the surface to the interior.Moreover,the secondary cracks extending inward along the fracture surface appear in the crack initiation area,and there is obvious stress concentration near the secondary cracks.The dislocation density is high near the fracture surface in the crack initiation zone,where a lot of dislocations cutting into the γ'phase exist.An oxide layer of 50‒100 nm is presented on the fracture surface,and Ni,Al,Cr and Co elements are mainly segregated into the oxide layer of the surface.展开更多
The effect of frequency and sample shape on fatigue behaviors of DZ125 superalloy are systematically studied.The results show that fatigue fracture still occurs above the cycle of 10~8 for tests carried out at the fre...The effect of frequency and sample shape on fatigue behaviors of DZ125 superalloy are systematically studied.The results show that fatigue fracture still occurs above the cycle of 10~8 for tests carried out at the frequency of f=20 kHz and stress ratio R =-1,so the traditional fatigue limit at cycle of 10~7 is not appropriate for fatigue design.Fatigue fracture surfaces are perpendicular to stress axis for cylindrical and plate specimens,and the fatigue cracks originate from the extra surface of the specimens.Fatigue crack is apt to propagate from cutting direction to forward direction,which occurs mainly in the second propagation stage at higher stress amplitude. There is an obvious frequency effect for DZ125 superalloy.The higher the test frequency is,the more serious the effect of frequency on fatigue behaviors of the alloy.After the frequency correction,the ultra-high cycle fatigue S-N curve well coincide with the traditional fatigue S-N curve.展开更多
文摘The cause of the premature failure of 304 stainless steel tube heat exchanger was investigated.The unique skeleton structure inside the leakage point reveals that this is a new damage mechanism caused by a δ+γ two-phase structure and crevice corrosion.The three-dimensional structure of the leakage point was demonstrated using X-ray diffraction topography.The results of scanning electron microscope examination show the microstructure of the weld to be columnar and dendritic.It is found by electron probe microscope analysis and transmission electron microscopy that columnar dendrites consisted of γ-dendrite and an amount of δ-ferrite phases at the dendrite trunk.Simulated corrosion test results confirmed that the corrosion medium was the chloride ion.Crevice corrosion of chloride ions occurred at weld defects on the inner wall thus forming a concentration cell.Grains of columnar dendrites were then corroded by chloride ions and δ-ferrite phases on the grain boundaries were retained,which formed the particular skeleton corrosion structure.As a result,it led to leakage when the corrosion of weld occurred from the inner wall to the outer wall.
基金National Science and Technology Major Project(J2019-VI-0022-0138)。
文摘The high-cycle fatigue fracture characteristics and damage mechanism of nickel-based single crystal superalloys at 850℃ was investigated.The results indicate that high-cycle fatigue cracks in single crystal superalloys generally originate from defect locations on the subsurface or interior of the specimen at 850℃.Under the condition of stress ratio R=0.05,as the fatigue load decreases,the high-cycle fatigue life gradually increases.The high-cycle fatigue fracture is mainly characterized by octahedral slip mechanism.At high stress and low lifespan,the fracture exhibits single or multiple slip surface features.Some fractures originate along a vertical small plane and then propagate along the{111}slip surface.At low stress and high lifespan,the fracture surface tend to alternate and expand along multiple slip planes after originating from subsurface or internal sources,exhibiting characteristics of multiple slip planes.Through electron backscatter diffraction and transmission electron microscope analysis,there is obvious oxidation behavior on the surface of the high-cycle fatigue fracture,and the fracture section is composed of oxidation layer,distortion layer,and matrix layer from the outside to the inside.Among them,the main components of the oxidation layer are oxides of Ni and Co.The distortion layer is mainly distributed in the form of elongated or short rod-shaped oxides of Al,Ta,and W.The matrix layer is a single crystal layer.Crack initiation and propagation mechanism were obtained by systematical analysis of a large number of highcycle fatigue fractures.In addition,the stress ratio of 0.05 is closer to the vibration mode of turbine blades during actual service,providing effective guidance for the study of failure and fracture mechanisms of turbine blades.
文摘Microstructure evolution of precision-cast Ti_(2)AlNb alloys after overtemperature treatments at 750 and 850℃ was investigated by optical microscope and scanning electron microscope,and Image J metallurgy analysis software was used for quantitative analysis.The variation of mechanical properties before and after overtemperature treatment was tested.Results show that the Ti_(2)AlNb alloy consists of B2 phase and lamellar O phase before and after overtemperature treatment.After overtemperature treatment,partial B2 phase is transformed into the O phase,and the lamellar structure of O phase suffers discontinuous coarsening and spherolization.The coarsening degree is increased with the increase in temperature and prolongation of treatment time,and O phase content is also gradually increased.The microhardness and tensile properties of Ti_(2)AlNb alloys at room and high temperatures are decreased with the increase in temperature and prolongation of treatment time,and the high-temperature tensile strength exhibits relatively larger decrement.An obvious linear relationship exists between microhardness and O phase content.Both the microhardness and the O phase content are important parameters to evaluate the overtemperature service damage degree of precision-cast Ti_(2)AlNb alloy.
文摘The effect of surface recrystallization by heating after shot-peening on the creep rapture property and fracture behavior of a single-crystal superalloy was investigated. The results show that the creep rupture property of the single-crystal superalloy was greatly influenced by surface recrystallization. A recrystallized surface layer with a depth of 101 ~m resulted in a decrease in creep rupture life by nearly 50%, and an almost linear reduction of creep rupture life was observed with the increase of recrystallization depth. A lower strength of the recrystal- lized layer, inhomogeneous deformation between the recrystallized layer and the matrix, and stress concentration caused by notch effect resuited in the decrease in creep rupture life of the single-crystal superalloy.
基金Item Sponsored by Open Foundation of Nondestructive Test Key Laboratory of Chinese Ministry of Education(ZD200729011)
文摘A series of experiments of investigating the recrystallization of single crystal DD3 superalloy were carried out. The threshold temperature for recrystallization and the effect of annealing temperature on recrystallization were studied. The results show that the threshold temperature for recrystallization of the shot-peened DD3 samples is between 1 000 ℃ and 1 050 ℃ under the condition of annealing for 2 h, and the recrystallization depth increases with the rise of the annealing temperature. Below 1 150 ℃, the recrystallization depth increases slowly with the temperature climbing, while above 1 150 ℃, the recrystallization depth increases quickly with the rise of the temperature. The solution of the γ＇ phase is a critical factor of the recrystallization behavior of DD3 superalloy. In addition, the kinetics and microstructural evolution of recrystallization at 1 200 ℃ were also studied. It is found that the recrystallization progresses rapidly at 1 200 ℃ through the growth of fully developed recrystallized grains, and the recrystallization process on the shot-peened surface is similar to that of wrought materials, including nucleation of recrystallization, growth of new grains into the matrix, and growth of new grains by swallowing up each other.
文摘As-cast SRR99 specimens were shot-peened and then annealed at 1250 and 1300°C for different times to investigate the kinetics of recrystallization.It was found that the relationship between annealing time and volume fraction of recrystallized grains could be described by the Johnson-Mehl-Avrami equation.Based on the kinetics analysis of the recrystallization process,the apparent activation energy for recrystallization was determined.In addition,the microstructural evolution during the recrystallization process at 1300°C was studied.Inwards,recrystallization first occurs in the dendritic core regions at the shot-peened surface.With the dissolution of coarse γ＇ particles in the interdendritic regions,the recrystallized grain boundaries move through the interdendritic regions.Finally,the fully developed grains nearly have a uniform depth.The recrystallization process at the shot-peened surface is similar to that of wrought materials,which includes nucleation of recrystallization,growth of recrystallized nuclei into the matrix,and growth of recrystallized grains by swallowing up each other.
基金financially supported by Equipment Advance Research Fund of China (No. 9140A1211HK51)
文摘Vibration fatigue is the main failure mode of compressor blade. Evaluating the vibration stress of blade that leads to cracking is very useful for analysis of vibration fatigue. In this paper, fatigue stress estimation methods by quantitative fractography were studied through experimental blade and in-service first-stage compressor blade in order to evaluate the initiation vibration stress of in-service blade. The analysis process of initiation vibration stress was established. The evaluating result of vibration stress of in-service blade subjected to centrifugal force and bending vibration stress agrees with aero engine test result. It is shown that the evaluation method can not only evaluate the equivalent fatigue stresses of different crack depths but also yield the initiation equivalent fatigue stress.
基金National Science and Technology Major Project(J2019-VI-0022-0138)。
文摘Low-cycle fatigue crack initiation behavior of nickel-based single crystal superalloy at 530℃ was investigated.Results show that the behavior of crack initiation is closely related to the maximum strain.When the maximum strain is 2.0%,the fatigue crack is originated at the position of persistent slip bands on the surface of specimen,which is located on the{111}slip plane.No defects are observed at the crack initiation position.When the maximum strain is lower than 1.6%,the cracks are initiated at the casting defects on sub-surface or at interior of the specimen.The casting defects are located on the{100}slip plane vertical to the axial force.The crack is initiated along the{100}slip plane and then expanded along different{111}slip planes after a short stage of expansion.As the maximum strain decreases,the position of crack initiation gradually changes from the surface to the interior.Moreover,the secondary cracks extending inward along the fracture surface appear in the crack initiation area,and there is obvious stress concentration near the secondary cracks.The dislocation density is high near the fracture surface in the crack initiation zone,where a lot of dislocations cutting into the γ'phase exist.An oxide layer of 50‒100 nm is presented on the fracture surface,and Ni,Al,Cr and Co elements are mainly segregated into the oxide layer of the surface.
文摘The effect of frequency and sample shape on fatigue behaviors of DZ125 superalloy are systematically studied.The results show that fatigue fracture still occurs above the cycle of 10~8 for tests carried out at the frequency of f=20 kHz and stress ratio R =-1,so the traditional fatigue limit at cycle of 10~7 is not appropriate for fatigue design.Fatigue fracture surfaces are perpendicular to stress axis for cylindrical and plate specimens,and the fatigue cracks originate from the extra surface of the specimens.Fatigue crack is apt to propagate from cutting direction to forward direction,which occurs mainly in the second propagation stage at higher stress amplitude. There is an obvious frequency effect for DZ125 superalloy.The higher the test frequency is,the more serious the effect of frequency on fatigue behaviors of the alloy.After the frequency correction,the ultra-high cycle fatigue S-N curve well coincide with the traditional fatigue S-N curve.
