The low-cycle fatigue (LCF) behavior of two kinds of A356 alloys produced by different titanium alloying methods was investigated and compared. The effect of titanium content and titanium alloying methods on LCF behav...The low-cycle fatigue (LCF) behavior of two kinds of A356 alloys produced by different titanium alloying methods was investigated and compared. The effect of titanium content and titanium alloying methods on LCF behavior is analyzed with plastic strain energy density. The results show that all alloys exhibit the cyclic hardening behavior. Raising Ti content can obviously increase the cyclic hardening ability. But the effect of Ti alloying method isn't distinct. Whether for the EA356 alloys or for MA356 alloys, the alloys with low titanium content have longer low-cycle fatigue life than that of the alloys with high titanium content. This is because that the alloys with low titanium content can consume higher cyclic plastic strain energy during cyclic deformation compared with alloys with high titanium content.展开更多
Ultrafine-grained(UFG)pure titanium was produced by equal channel angular pressing for 4 passes,followed by rotatory swaging at room temperature.The strain-controlled low-cycle fatigue tests of UFG and coarse-grained(...Ultrafine-grained(UFG)pure titanium was produced by equal channel angular pressing for 4 passes,followed by rotatory swaging at room temperature.The strain-controlled low-cycle fatigue tests of UFG and coarse-grained(CG)pure titanium were conducted by Instron electro-hydraulic servo fatigue testing machine in the strain amplitude range of 0.5%—1.1%at room temperature.Transmission electron microscope(TEM)and scanning electron microscope were used to investigate the microstructure and fracture surface of UFG pure titanium after fatigue tests.Results show that UFG pure titanium exhibits a longer low-cycle fatigue life,compared with the CG pure titanium.For example,at a total strain amplitude of 0.5%,UFG and CG pure titanium has fatigue life of 10850 and 4820 cycles,respectively.Significant cyclic softening occurs in UFG pure titanium,except in the case of a total strain amplitude of 0.5%.Hysteresis loop area is increased rapidly with the increase in strain amplitude.The fracture surface shows that the fatigue crack is initiated from the specimen surface.A series of fatigue striations and many microcracks exist in the propagation region.With the increase in strain amplitude,the predominant failure mode is transformed from ductile failure into quasi-cleavage failure.Dislocation slip is the main plastic deformation mechanism of UFG pure titanium during low-cycle fatigue deformation.展开更多
To overcome the challenges of limited experimental data and improve the accuracy of empirical formulas,we propose a low-cycle fatigue(LCF)life prediction model for nickel-based superalloys using a data augmentation me...To overcome the challenges of limited experimental data and improve the accuracy of empirical formulas,we propose a low-cycle fatigue(LCF)life prediction model for nickel-based superalloys using a data augmentation method.This method utilizes a variational autoencoder(VAE)to generate low-cycle fatigue data and form an augmented dataset.The Pearson correlation coefficient(PCC)is employed to verify the similarity of feature distributions between the original and augmented datasets.Six machine learning models,namely random forest(RF),artificial neural network(ANN),support vector machine(SVM),gradient-boosted decision tree(GBDT),eXtreme Gradient Boosting(XGBoost),and Categorical Boosting(CatBoost),are utilized to predict the LCF life of nickel-based superalloys.Results indicate that the proposed data augmentation method based on VAE can effectively expand the dataset,and the mean absolute error(MAE),root mean square error(RMSE),and R-squared(R^(2))values achieved using the CatBoost model,with respective values of 0.0242,0.0391,and 0.9538,are superior to those of the other models.The proposed method reduces the cost and time associated with LCF experiments and accurately establishes the relationship between fatigue characteristics and LCF life of nickel-based superalloys.展开更多
To achieve efficient connection and integrity in prefabricated frame structures,a novel beam-column connection between prefabricated steel-reinforced concrete(SRC)columns and cast-in-situ reinforced concrete(RC)beams ...To achieve efficient connection and integrity in prefabricated frame structures,a novel beam-column connection between prefabricated steel-reinforced concrete(SRC)columns and cast-in-situ reinforced concrete(RC)beams is proposed.Four connection specimens with varying thicknesses of connecting cover plates were designed and fabricated.Low-cycle reversed loading tests were conducted to obtain mechanical performance data,allowing for analysis of behavioral patterns,including load-displacement hysteresis curves,skeleton curves,load-carrying capacity,deformation performance,energy dissipation capacity,and stiffness degradation.Test results indicate that properly designed prefabricated connections exhibit mechanical properties comparable to those of cast-in-situ concrete connections.The hysteresis curves of the prefabricated connections are well-shaped,with nearly no reduction in load-carrying capacity and deformation performance,demonstrating excellent seismic behavior.The findings provide a theoretical basis and reference for the design of prefabricated concrete frame structures.展开更多
The influence of gas high-temperature hot corrosion(HTHC) pre-exposure on low-cycle fatigue(LCF)behavior was characterized for the directionally solidified(DS) Ni-based superalloy DZ125. Fatigue tests were carried out...The influence of gas high-temperature hot corrosion(HTHC) pre-exposure on low-cycle fatigue(LCF)behavior was characterized for the directionally solidified(DS) Ni-based superalloy DZ125. Fatigue tests were carried out at 850 ℃ in the pre-exposed and unexposed specimens for 2, 15 and 25 h. Experimental results show that the porous corrosion scale and γ′-depleted layer formed in gas hot corrosion condition alter the crack initiation mechanisms of the superalloy. Fatigue cracks of the pre-exposed specimens originate from multiple surface locations where spalling of the corrosion products occur,while nucleation of unexposed specimen begins in the defects close to the surface. There is a significant reduction in LCF behavior for pre-exposed specimens in comparison with unexposed specimens.展开更多
Cyclic deformation and fatigue behaviors of Mg-12%Gd-3%Y-0.5%Zr （wt%, GW123K） alloy were investigated at room temperature under axial cyclic loading in strain controlled condition. It is shown that conventional extr...Cyclic deformation and fatigue behaviors of Mg-12%Gd-3%Y-0.5%Zr （wt%, GW123K） alloy were investigated at room temperature under axial cyclic loading in strain controlled condition. It is shown that conventional extruded GW123K alloy maintained cyclic stability at strain amplitudes ranging from 2 × 10^-3 to 10^-2. The pronounced symmetric hysteresis loops were also observed during cyclic loading. Fracture surface observations indicated that fatigue cracks mainly initiated at large Gd-riched phase or at inclusion clusters at surface or subsurface, and grain boundary （GB） and slip bands （SBs） are also preferential sites for micro-crack incubation.展开更多
Low-cycle fatigue behaviors of Ni-base single crystal superalloys containing different Re contents have been investigated at 760℃. During heat treatment. Reretards γ phases coarsening and equalizes the distribution ...Low-cycle fatigue behaviors of Ni-base single crystal superalloys containing different Re contents have been investigated at 760℃. During heat treatment. Reretards γ phases coarsening and equalizes the distribution of γ' phases. As Re content increases, fatigue life increases and slip bands distribute more inhomogeneously. Moreover, adding Re not only reduces stacking fault energy of the matrix, but also promotes the element segregation to increase the lattice misfit. However, the larger lattice misfit does not lead to the formation of dislocation networks, but which activates dislocation movement and promotes dislocations cross-slip and climbing movement under high temperature and applied stress. On the other hand, with the addition of Re, cyclic deformation behaviors change from cyclic hardening to cyclic stability, mainly depending on a transformation of deformation mechanisms from slip bands cutting through γ and γ' phases to stacking faults shearing.展开更多
The influence of crystal orientations on the low-cycle fatigue(LCF) behavior of a 3Re-bearing Ni-based single-crystal superalloy at 980 °C has been investigated. It is found that the orientation dependence of the...The influence of crystal orientations on the low-cycle fatigue(LCF) behavior of a 3Re-bearing Ni-based single-crystal superalloy at 980 °C has been investigated. It is found that the orientation dependence of the fatigue life not only depends on the elastic modulus, but also the number of active slip planes and the plasticity of materials determine the LCF life,especially for the [011] and [111] specimens. The [011] and [111] specimens with better plasticity withstand relatively concentrated inelastic deformation caused by fewer active slip planes, compared to the [001] specimens resisting widespread deformation caused by a higher number of active slip planes. Additionally, fatigue fracture is also influenced by cyclic plastic deformation mechanisms of the alloy with crystal orientations, and the [001] specimens are plastically deformed by wave slip mechanism and fracture along the non-crystallographic plane, while the [011] and [111] specimens are plastically deformed by planar slip mechanism and fracture along the crystallographic planes. Moreover, casting pores,eutectics, inclusions and surface oxide layers not only initiate the crack, but also reduce the stress concentration around crack tips. Our results throw light upon the effect of inelastic strain on the LCF life and analyze the cyclic plastic deformation for the alloy with different orientations.展开更多
Laser additive manufacturing(LAM)technique has unique advantages in producing geometrically complex metallic components.However,the poor low-cycle fatigue property(LCF)of LAM parts restricts its widely used.Here,the m...Laser additive manufacturing(LAM)technique has unique advantages in producing geometrically complex metallic components.However,the poor low-cycle fatigue property(LCF)of LAM parts restricts its widely used.Here,the microstructural features of a Ti-6 Al-4 V alloy manufactured via high power laser directed energy deposition subjected to low-cycle fatigue loading were studied.Before fatigue loading,the microstructure of the as-deposited parts was found to exhibit a non-homogeneous distribution of columnar prior-βgrains(200-4000μm)at various scanning velocities(300-1500 mm/min)and relatively coarseα-laths(1.0-4.5μm).Under cyclic loading,fatigue microcracks typically initiated within the alignedαphases in the preferred orientation(45°to the loading direction)at the surface of the fatigue specimens.Fatigued Ti-6 Al-4 V exhibited a single straight dislocation character at low strain amplitudes(<0.65%)and dislocation dipoles or even tangled dislocations at high strain amplitudes(>1.1%).In addition,dislocation substructure features,such as dislocation walls,stacking faults,and dislocation networks,were also observed.These findings may provide opportunities to understand the fatigue failure mechanism of additive manufactured titanium parts.展开更多
Cold-stretched pressure vessels from austenitic stainless steels （ASS） are widely used for storage and transportation of liquefied gases, and have such advantages as thin wall and light weight. Fatigue is an importa...Cold-stretched pressure vessels from austenitic stainless steels （ASS） are widely used for storage and transportation of liquefied gases, and have such advantages as thin wall and light weight. Fatigue is an important concern in these pressure vessels, which are subjected to alternative loads. Even though several codes and standards have guidelines on these pressure vessels, there are no relevant design methods on fatigue failure. To understand the fatigue properties of ASS 1.4301 （equivalents include UNS $30400 and AISI 304） in solution-annealed （SA） and cold-stretched conditions （9% strain level） and the response of fatigue properties to cold stretching （CS）, low-cycle fatigue （LCF） tests were performed at room temperature, with total strain amplitudes ranging from ：~0.4% to ＂0.8%. Martensite transformations were measured during the tests. Comparisons on cyclic stress response, cyclic stress-strain behavior, and fatigue life were carried out between SA and CS materials. Results show that CS reduces the initial hardening stage, but prolongs the softening period in the cyclic stress response. Martensite transformation helps form a stable regime and subsequent secondary hardening. The stresses of monotonic and cyclic stress-strain curves are improved by CS, which leads to a lower plastic strain and a much higher elastic strain. The fatigue resistance of the CS material is better than that of the SA material, which is approximately 1 - 103 to 2 - 104 cycles. The S-N curve of the ASME standard for ASS is compared with the fatigue data and is justified to be suitable for the fatigue design of cold-stretched pressure vessels. However, considering the CS material has a better fatigue resistance, the S-N curve will be more conservative. The present study would be helpful in making full use of the advantages of CS to develop a new S-N curve for fatigue design of cold-stretched pressure vessels.展开更多
The low-cycle fatigue （LCF） properties of DD10 （single-crystal） and DZ53 （columnar-grained） superalloys solidified by liquid-metal cooling （LMC） and high-rate solidification （HRS） processes have been systema...The low-cycle fatigue （LCF） properties of DD10 （single-crystal） and DZ53 （columnar-grained） superalloys solidified by liquid-metal cooling （LMC） and high-rate solidification （HRS） processes have been systematically investi- gated. It was found that the LCF life of DZ53 solidified by LMC was obviously better than that solidified by HRS. In contrast, for DD10, LMC showed no remarkable influences on LCF properties at high temperature and only improved LCF properties at intermediate temperature. Microstructure examination showed that the cracks generally initiated at microp- ores in the subsurface at intermediate temperature. However, the cracks occurred on the surface due to oxidation, or persistent slip bands near script-MC at high temperature. Therefore, the benefits of LMC technique can be attributed to both of the reduced casting defects which significantly affect the LCF properties at intermediate temperature and the improved microstructural homogeneity which was strongly correlated to the LCF properties of alloys at high temperature.展开更多
In as-cast Mg?2.1Gd?1.1Y?0.82Zn?0.11Zr(mole fraction,%)alloy,lamellar microstructures that extend from grain boundaries to the interior ofα-Mg grains are identified as clusters ofγ′using a scanning transmission ele...In as-cast Mg?2.1Gd?1.1Y?0.82Zn?0.11Zr(mole fraction,%)alloy,lamellar microstructures that extend from grain boundaries to the interior ofα-Mg grains are identified as clusters ofγ′using a scanning transmission electron microscope equipped with a high-angle annular dark-field detector.Under a total strain-controlled low-cyclic loading at573K,the mechanical response and failure mechanism of Mg?2.1Gd?1.1Y?0.82Zn?0.11Zr alloy(T6peak-aging heat treatment)were investigated.Results show that the alloy exhibits cyclic softening response at diverse total strain amplitudes and573K.The experimental observations using scanning electron microscopy show that the micro-cracks initiate preferentially at the interface between long-period stacking order structures andα-Mg matrix and extend along the basal plane ofα-Mg.The massive long-period stacking order structures distributed at grain boundaries impede the transgranular propagation of cracks.展开更多
The effects of inclusions in powder superalloy FGH96 on low-cycle fatigue life were studied, and a low-cycle crack initiation life prediction model based on the theory of damage mechanics was proposed. The damage char...The effects of inclusions in powder superalloy FGH96 on low-cycle fatigue life were studied, and a low-cycle crack initiation life prediction model based on the theory of damage mechanics was proposed. The damage characterization parameter was proposed after the construction of damage evolution equations. Fatigue tests of the powder superalloy specimens with and without inclusion were conducted at 530 and 600 ℃, and the model verification was carried out for specimens with elliptical, semi-elliptical, polygon and strip-shaped surface/subsurface inclusion. The stress analysis was performed by finite element simulation and the predicted life was calculated. The results showed a satisfying agreement between predicted and experimental life.展开更多
The commercial AZ91 alloy and nonflammable SEN9(AZ91-0.3Ca-0.2Y,wt%)alloy are extruded at 300°C and 400°C.Their microstructure,tensile and compressive properties,and low-cycle fatigue(LCF)properties are inve...The commercial AZ91 alloy and nonflammable SEN9(AZ91-0.3Ca-0.2Y,wt%)alloy are extruded at 300°C and 400°C.Their microstructure,tensile and compressive properties,and low-cycle fatigue(LCF)properties are investigated,with particular focus on the influence of the extrusion temperature.In the AZ91 and SEN9 materials extruded at 300°C(300-materials),numerous fine Mg_(17)Al_(12)particles are inhomogeneously distributed owing to localized dynamic precipitation during extrusion,unlike those extruded at 400°C(400-materials).These fine particles suppress the coarsening of recrystallized grains,decreasing the average grain size of 300-materials.Although the four extruded materials have considerably different microstructures,the difference in their tensile yield strengths is insignificant because strong grain-boundary hardening and precipitation hardening effects in 300-materials are offset almost completely by a strong texture hardening effect in 400-materials.However,owing to their finer grains and weaker texture,300-materials have higher compressive yield strengths than400-materials.During the LCF tests,{10-12}twinning is activated at lower stresses in 400-materials than in 300-materials.Because the fatigue damage accumulated per cycle is smaller in 400-materials,they have longer fatigue lives than those of 300-materials.A fatigue life prediction model for the investigated materials is established on the basis of the relationship between the total strain energy density(ΔW_(t))and the number of cycles to fatigue failure(N_(f)),and it is expressed through a simple equation(ΔW_(t)=10·N_(f)-0.59).This model enables fatigue life prediction of both the investigated alloys regardless of the extrusion temperature and strain amplitude.展开更多
Low-cycle fatigue(LCF)behaviors of a second-generation nickel-based single-crystal superalloys with[001]orientation at 760℃ have been investigated.Different strain amplitudes were introduced to investigate the creep-...Low-cycle fatigue(LCF)behaviors of a second-generation nickel-based single-crystal superalloys with[001]orientation at 760℃ have been investigated.Different strain amplitudes were introduced to investigate the creep-fatigue effects.The LCF life of none tensile holding(NTH)was higher than that of the 60-s tensile hold(TH)at any strain amplitude.As the strain amplitude was 0.7%,the stacking and cross-slip dislocations appeared together at the γ/γ’coherent microstructure in both TH and NTH specimens.At the strain amplitude of 0.9%,plenty of the cross-slip dislocations appeared inγchannel and other dislocations were stacking at γ/γ’interfaces.However,the SFs still appeared in γ’phase with 60-s TH which caused cyclic softening.As the strain amplitude increased up to 1.2%,the dislocations are piling up at the γ/γ’interfaces and cutting through the γ’phase in both TH and NTH tests,which caused cyclic hardening.The influences of strain amplitude and holding time were complicated.Different stress response behaviors occurred in different loading conditions.The surface characteristic and fracture mechanism were observed by scanning electron microscopy.This result is helpful for building the relationship of various blade fatigue failure modes,cyclic stress response and microstructure deformation under different strain amplitudes.展开更多
文摘The low-cycle fatigue (LCF) behavior of two kinds of A356 alloys produced by different titanium alloying methods was investigated and compared. The effect of titanium content and titanium alloying methods on LCF behavior is analyzed with plastic strain energy density. The results show that all alloys exhibit the cyclic hardening behavior. Raising Ti content can obviously increase the cyclic hardening ability. But the effect of Ti alloying method isn't distinct. Whether for the EA356 alloys or for MA356 alloys, the alloys with low titanium content have longer low-cycle fatigue life than that of the alloys with high titanium content. This is because that the alloys with low titanium content can consume higher cyclic plastic strain energy during cyclic deformation compared with alloys with high titanium content.
基金Natural Science Foundation of Shaanxi Province (2023-JC-YB-312)。
文摘Ultrafine-grained(UFG)pure titanium was produced by equal channel angular pressing for 4 passes,followed by rotatory swaging at room temperature.The strain-controlled low-cycle fatigue tests of UFG and coarse-grained(CG)pure titanium were conducted by Instron electro-hydraulic servo fatigue testing machine in the strain amplitude range of 0.5%—1.1%at room temperature.Transmission electron microscope(TEM)and scanning electron microscope were used to investigate the microstructure and fracture surface of UFG pure titanium after fatigue tests.Results show that UFG pure titanium exhibits a longer low-cycle fatigue life,compared with the CG pure titanium.For example,at a total strain amplitude of 0.5%,UFG and CG pure titanium has fatigue life of 10850 and 4820 cycles,respectively.Significant cyclic softening occurs in UFG pure titanium,except in the case of a total strain amplitude of 0.5%.Hysteresis loop area is increased rapidly with the increase in strain amplitude.The fracture surface shows that the fatigue crack is initiated from the specimen surface.A series of fatigue striations and many microcracks exist in the propagation region.With the increase in strain amplitude,the predominant failure mode is transformed from ductile failure into quasi-cleavage failure.Dislocation slip is the main plastic deformation mechanism of UFG pure titanium during low-cycle fatigue deformation.
基金Financial support from the Fundamental Research Funds for the Central Universities(ZJ2022-003,JG2022-27,J2020-060,and J2021-060)Sichuan Province Engineering Technology Research Center of General Aircraft Maintenance(GAMRC2021YB08)the Young Scientists Fund of the National Natural Science Foundation of China(No.52105417)is acknowledged.
文摘To overcome the challenges of limited experimental data and improve the accuracy of empirical formulas,we propose a low-cycle fatigue(LCF)life prediction model for nickel-based superalloys using a data augmentation method.This method utilizes a variational autoencoder(VAE)to generate low-cycle fatigue data and form an augmented dataset.The Pearson correlation coefficient(PCC)is employed to verify the similarity of feature distributions between the original and augmented datasets.Six machine learning models,namely random forest(RF),artificial neural network(ANN),support vector machine(SVM),gradient-boosted decision tree(GBDT),eXtreme Gradient Boosting(XGBoost),and Categorical Boosting(CatBoost),are utilized to predict the LCF life of nickel-based superalloys.Results indicate that the proposed data augmentation method based on VAE can effectively expand the dataset,and the mean absolute error(MAE),root mean square error(RMSE),and R-squared(R^(2))values achieved using the CatBoost model,with respective values of 0.0242,0.0391,and 0.9538,are superior to those of the other models.The proposed method reduces the cost and time associated with LCF experiments and accurately establishes the relationship between fatigue characteristics and LCF life of nickel-based superalloys.
基金supported by High-level Talents Program of Hainan Basic and Applied Basic Research Program of China(520RC543)Crosswise Tasks of Enterprise Entrusted(JG-ZH-A-202411-003).
文摘To achieve efficient connection and integrity in prefabricated frame structures,a novel beam-column connection between prefabricated steel-reinforced concrete(SRC)columns and cast-in-situ reinforced concrete(RC)beams is proposed.Four connection specimens with varying thicknesses of connecting cover plates were designed and fabricated.Low-cycle reversed loading tests were conducted to obtain mechanical performance data,allowing for analysis of behavioral patterns,including load-displacement hysteresis curves,skeleton curves,load-carrying capacity,deformation performance,energy dissipation capacity,and stiffness degradation.Test results indicate that properly designed prefabricated connections exhibit mechanical properties comparable to those of cast-in-situ concrete connections.The hysteresis curves of the prefabricated connections are well-shaped,with nearly no reduction in load-carrying capacity and deformation performance,demonstrating excellent seismic behavior.The findings provide a theoretical basis and reference for the design of prefabricated concrete frame structures.
基金financially supported by the National Natural Science Foundation of China (No. 51571010)the National Basic Research Program of China (No. 2015CB057400)
文摘The influence of gas high-temperature hot corrosion(HTHC) pre-exposure on low-cycle fatigue(LCF)behavior was characterized for the directionally solidified(DS) Ni-based superalloy DZ125. Fatigue tests were carried out at 850 ℃ in the pre-exposed and unexposed specimens for 2, 15 and 25 h. Experimental results show that the porous corrosion scale and γ′-depleted layer formed in gas hot corrosion condition alter the crack initiation mechanisms of the superalloy. Fatigue cracks of the pre-exposed specimens originate from multiple surface locations where spalling of the corrosion products occur,while nucleation of unexposed specimen begins in the defects close to the surface. There is a significant reduction in LCF behavior for pre-exposed specimens in comparison with unexposed specimens.
基金supported by the National Natural Science Foundation of China under Grant No.50571102
文摘Cyclic deformation and fatigue behaviors of Mg-12%Gd-3%Y-0.5%Zr （wt%, GW123K） alloy were investigated at room temperature under axial cyclic loading in strain controlled condition. It is shown that conventional extruded GW123K alloy maintained cyclic stability at strain amplitudes ranging from 2 × 10^-3 to 10^-2. The pronounced symmetric hysteresis loops were also observed during cyclic loading. Fracture surface observations indicated that fatigue cracks mainly initiated at large Gd-riched phase or at inclusion clusters at surface or subsurface, and grain boundary （GB） and slip bands （SBs） are also preferential sites for micro-crack incubation.
基金supported financially by the National Natural Science Foundation of China (Nos. 51671188, 51571196 and 51601192)the Shenyang Science and Technology Project (No. 17-101-2-00)
文摘Low-cycle fatigue behaviors of Ni-base single crystal superalloys containing different Re contents have been investigated at 760℃. During heat treatment. Reretards γ phases coarsening and equalizes the distribution of γ' phases. As Re content increases, fatigue life increases and slip bands distribute more inhomogeneously. Moreover, adding Re not only reduces stacking fault energy of the matrix, but also promotes the element segregation to increase the lattice misfit. However, the larger lattice misfit does not lead to the formation of dislocation networks, but which activates dislocation movement and promotes dislocations cross-slip and climbing movement under high temperature and applied stress. On the other hand, with the addition of Re, cyclic deformation behaviors change from cyclic hardening to cyclic stability, mainly depending on a transformation of deformation mechanisms from slip bands cutting through γ and γ' phases to stacking faults shearing.
基金supported by the National Natural Science Foundation of China (Nos. 51571196, 51671188 and 5160119)Shenyang Science and Technology Project (No. 17-101-2-00)
文摘The influence of crystal orientations on the low-cycle fatigue(LCF) behavior of a 3Re-bearing Ni-based single-crystal superalloy at 980 °C has been investigated. It is found that the orientation dependence of the fatigue life not only depends on the elastic modulus, but also the number of active slip planes and the plasticity of materials determine the LCF life,especially for the [011] and [111] specimens. The [011] and [111] specimens with better plasticity withstand relatively concentrated inelastic deformation caused by fewer active slip planes, compared to the [001] specimens resisting widespread deformation caused by a higher number of active slip planes. Additionally, fatigue fracture is also influenced by cyclic plastic deformation mechanisms of the alloy with crystal orientations, and the [001] specimens are plastically deformed by wave slip mechanism and fracture along the non-crystallographic plane, while the [011] and [111] specimens are plastically deformed by planar slip mechanism and fracture along the crystallographic planes. Moreover, casting pores,eutectics, inclusions and surface oxide layers not only initiate the crack, but also reduce the stress concentration around crack tips. Our results throw light upon the effect of inelastic strain on the LCF life and analyze the cyclic plastic deformation for the alloy with different orientations.
基金supported by the National Key Research and Development Plan of China(2016YFB1100104)National Natural Science Foundation of China(Grant No.51971166)。
文摘Laser additive manufacturing(LAM)technique has unique advantages in producing geometrically complex metallic components.However,the poor low-cycle fatigue property(LCF)of LAM parts restricts its widely used.Here,the microstructural features of a Ti-6 Al-4 V alloy manufactured via high power laser directed energy deposition subjected to low-cycle fatigue loading were studied.Before fatigue loading,the microstructure of the as-deposited parts was found to exhibit a non-homogeneous distribution of columnar prior-βgrains(200-4000μm)at various scanning velocities(300-1500 mm/min)and relatively coarseα-laths(1.0-4.5μm).Under cyclic loading,fatigue microcracks typically initiated within the alignedαphases in the preferred orientation(45°to the loading direction)at the surface of the fatigue specimens.Fatigued Ti-6 Al-4 V exhibited a single straight dislocation character at low strain amplitudes(<0.65%)and dislocation dipoles or even tangled dislocations at high strain amplitudes(>1.1%).In addition,dislocation substructure features,such as dislocation walls,stacking faults,and dislocation networks,were also observed.These findings may provide opportunities to understand the fatigue failure mechanism of additive manufactured titanium parts.
基金Project supported by the National Key Technology R&D Program (No.2011BAK06B0205)the International Science and Technology Cooperation Project (No. 2010DFB42960)the Key Technology Innovation Team of Zhejiang Province (No. 2010R50001),China
文摘Cold-stretched pressure vessels from austenitic stainless steels （ASS） are widely used for storage and transportation of liquefied gases, and have such advantages as thin wall and light weight. Fatigue is an important concern in these pressure vessels, which are subjected to alternative loads. Even though several codes and standards have guidelines on these pressure vessels, there are no relevant design methods on fatigue failure. To understand the fatigue properties of ASS 1.4301 （equivalents include UNS $30400 and AISI 304） in solution-annealed （SA） and cold-stretched conditions （9% strain level） and the response of fatigue properties to cold stretching （CS）, low-cycle fatigue （LCF） tests were performed at room temperature, with total strain amplitudes ranging from ：~0.4% to ＂0.8%. Martensite transformations were measured during the tests. Comparisons on cyclic stress response, cyclic stress-strain behavior, and fatigue life were carried out between SA and CS materials. Results show that CS reduces the initial hardening stage, but prolongs the softening period in the cyclic stress response. Martensite transformation helps form a stable regime and subsequent secondary hardening. The stresses of monotonic and cyclic stress-strain curves are improved by CS, which leads to a lower plastic strain and a much higher elastic strain. The fatigue resistance of the CS material is better than that of the SA material, which is approximately 1 - 103 to 2 - 104 cycles. The S-N curve of the ASME standard for ASS is compared with the fatigue data and is justified to be suitable for the fatigue design of cold-stretched pressure vessels. However, considering the CS material has a better fatigue resistance, the S-N curve will be more conservative. The present study would be helpful in making full use of the advantages of CS to develop a new S-N curve for fatigue design of cold-stretched pressure vessels.
基金financially supported by the National Natural Science Foundation of China under Grant Nos.51631008,51101160 and 2010CB631201
文摘The low-cycle fatigue （LCF） properties of DD10 （single-crystal） and DZ53 （columnar-grained） superalloys solidified by liquid-metal cooling （LMC） and high-rate solidification （HRS） processes have been systematically investi- gated. It was found that the LCF life of DZ53 solidified by LMC was obviously better than that solidified by HRS. In contrast, for DD10, LMC showed no remarkable influences on LCF properties at high temperature and only improved LCF properties at intermediate temperature. Microstructure examination showed that the cracks generally initiated at microp- ores in the subsurface at intermediate temperature. However, the cracks occurred on the surface due to oxidation, or persistent slip bands near script-MC at high temperature. Therefore, the benefits of LMC technique can be attributed to both of the reduced casting defects which significantly affect the LCF properties at intermediate temperature and the improved microstructural homogeneity which was strongly correlated to the LCF properties of alloys at high temperature.
基金Project(2015TP1035)supported by the Science and Technology Planning Project of Hunan Province,ChinaProject(531107040183)supported by the Fundamental Research Funds for the Central Universities,China
文摘In as-cast Mg?2.1Gd?1.1Y?0.82Zn?0.11Zr(mole fraction,%)alloy,lamellar microstructures that extend from grain boundaries to the interior ofα-Mg grains are identified as clusters ofγ′using a scanning transmission electron microscope equipped with a high-angle annular dark-field detector.Under a total strain-controlled low-cyclic loading at573K,the mechanical response and failure mechanism of Mg?2.1Gd?1.1Y?0.82Zn?0.11Zr alloy(T6peak-aging heat treatment)were investigated.Results show that the alloy exhibits cyclic softening response at diverse total strain amplitudes and573K.The experimental observations using scanning electron microscopy show that the micro-cracks initiate preferentially at the interface between long-period stacking order structures andα-Mg matrix and extend along the basal plane ofα-Mg.The massive long-period stacking order structures distributed at grain boundaries impede the transgranular propagation of cracks.
基金sponsored by AECC Beijing Institute of Aeronautical Materialsfunded by National High-tech R&D Program of China (863 Program) (No. 2015AA034401)。
文摘The effects of inclusions in powder superalloy FGH96 on low-cycle fatigue life were studied, and a low-cycle crack initiation life prediction model based on the theory of damage mechanics was proposed. The damage characterization parameter was proposed after the construction of damage evolution equations. Fatigue tests of the powder superalloy specimens with and without inclusion were conducted at 530 and 600 ℃, and the model verification was carried out for specimens with elliptical, semi-elliptical, polygon and strip-shaped surface/subsurface inclusion. The stress analysis was performed by finite element simulation and the predicted life was calculated. The results showed a satisfying agreement between predicted and experimental life.
基金supported by the National Research Foundation(NRF)Grant(No.2019R1A2C1085272)the National Research Council of Science and Technology(NST)Grant(No.CRC-15-06-KIGAM)funded by the Korean government(MSIP,South Korea)
文摘The commercial AZ91 alloy and nonflammable SEN9(AZ91-0.3Ca-0.2Y,wt%)alloy are extruded at 300°C and 400°C.Their microstructure,tensile and compressive properties,and low-cycle fatigue(LCF)properties are investigated,with particular focus on the influence of the extrusion temperature.In the AZ91 and SEN9 materials extruded at 300°C(300-materials),numerous fine Mg_(17)Al_(12)particles are inhomogeneously distributed owing to localized dynamic precipitation during extrusion,unlike those extruded at 400°C(400-materials).These fine particles suppress the coarsening of recrystallized grains,decreasing the average grain size of 300-materials.Although the four extruded materials have considerably different microstructures,the difference in their tensile yield strengths is insignificant because strong grain-boundary hardening and precipitation hardening effects in 300-materials are offset almost completely by a strong texture hardening effect in 400-materials.However,owing to their finer grains and weaker texture,300-materials have higher compressive yield strengths than400-materials.During the LCF tests,{10-12}twinning is activated at lower stresses in 400-materials than in 300-materials.Because the fatigue damage accumulated per cycle is smaller in 400-materials,they have longer fatigue lives than those of 300-materials.A fatigue life prediction model for the investigated materials is established on the basis of the relationship between the total strain energy density(ΔW_(t))and the number of cycles to fatigue failure(N_(f)),and it is expressed through a simple equation(ΔW_(t)=10·N_(f)-0.59).This model enables fatigue life prediction of both the investigated alloys regardless of the extrusion temperature and strain amplitude.
文摘Low-cycle fatigue(LCF)behaviors of a second-generation nickel-based single-crystal superalloys with[001]orientation at 760℃ have been investigated.Different strain amplitudes were introduced to investigate the creep-fatigue effects.The LCF life of none tensile holding(NTH)was higher than that of the 60-s tensile hold(TH)at any strain amplitude.As the strain amplitude was 0.7%,the stacking and cross-slip dislocations appeared together at the γ/γ’coherent microstructure in both TH and NTH specimens.At the strain amplitude of 0.9%,plenty of the cross-slip dislocations appeared inγchannel and other dislocations were stacking at γ/γ’interfaces.However,the SFs still appeared in γ’phase with 60-s TH which caused cyclic softening.As the strain amplitude increased up to 1.2%,the dislocations are piling up at the γ/γ’interfaces and cutting through the γ’phase in both TH and NTH tests,which caused cyclic hardening.The influences of strain amplitude and holding time were complicated.Different stress response behaviors occurred in different loading conditions.The surface characteristic and fracture mechanism were observed by scanning electron microscopy.This result is helpful for building the relationship of various blade fatigue failure modes,cyclic stress response and microstructure deformation under different strain amplitudes.
