Advanced bainitic steels with the multiphase structure of bainitic ferrite,retained austenite and martensite exhibit distinctive fatigue crack initiation behavior during high cycle fatigue/very high cycle fatigue(HCF/...Advanced bainitic steels with the multiphase structure of bainitic ferrite,retained austenite and martensite exhibit distinctive fatigue crack initiation behavior during high cycle fatigue/very high cycle fatigue(HCF/VHCF)regimes.The subsurface microstructural fatigue crack initiation,referred to as“non-inclusion induced crack initiation,NIICI”,is a leading mode of failure of bainitic steels within the HCF/VHCF regimes.In this regard,there is currently a missing gap in the knowledge with respect to the cyclic response of multiphase structure during VHCF failure and the underlying mechanisms of fatigue crack initiation during VHCF.To address this aspect,we have developed a novel approach that explicitly identifies the knowledge gap through an examination of subsurface crack initiation and interaction with the local microstructure.This was accomplished by uniquely combining electron microscopy,three-dimensional confocal microscopy,focused ion beam,and transmission Kikuchi diffraction.Interestingly,the study indicated that there are multiple micro-mechanisms responsible for the NIICI failure of bainitic steels,including two scenarios of transgranular-crack-assisted NIICI and two scenarios of intergranular-crack-assisted NIICI,which resulted in the different distribution of fine grains in the crack initiation area.The fine grains were formed through fragmentation of bainitic ferrite lath caused by localized plastic deformation or via local continuous dynamic recrystallization because of repeated interaction between slip bands and prior austenite grain boundaries.The formation of fine grains assisted the advancement of small cracks.Another important aspect discussed is the role of retained austenite(RA)during cyclic loading,on crack initiation and propagation in terms of the morphology,distribution and stability of RA,which determined the development of localized cyclic plastic deformation in multiphase structure.展开更多
GH4169 joints manufactured by Linear Friction Welding(LFW)are subjected to tensile test and stair-case method to evaluate the High Cycle Fatigue(HCF)performance at 650℃.The yield and ultimate tensile strengths are 58...GH4169 joints manufactured by Linear Friction Welding(LFW)are subjected to tensile test and stair-case method to evaluate the High Cycle Fatigue(HCF)performance at 650℃.The yield and ultimate tensile strengths are 582 MPa and 820 MPa,respectively.The HCF strength of joint reaches 400 MPa,which is slightly lower than that of Base Metal(BM),indicating reliable quality of this type of joint.The microstructure observation results show that all cracks initiate at the inside of specimens and transfer into deeper region with decrease of external stress,and the crack initiation site is related with microhardness of matrix.The Electron Backscattered Diffraction(EBSD)results of the observed regions with different distances to fracture show that plastic deformation plays a key role in HCF,and the Schmid factor of most grains near fracture exceeds 0.4.In addition,the generation of twins plays a vital role in strain concentration release and coordinating plastic deformation among grains.展开更多
Magnesium alloys with long-period stacking ordered(LPSO)structures are known for their impressive static mechanical strength,but the consistent occurrence of slip-cracking along the LPSO lamellae,which do not effectiv...Magnesium alloys with long-period stacking ordered(LPSO)structures are known for their impressive static mechanical strength,but the consistent occurrence of slip-cracking along the LPSO lamellae,which do not effectively impede the movement of basal dislocations,has prompted concerns about their very high cycle fatigue(VHCF)performance.In this study,an extruded Mg-Gd-Zn-Zr alloy was developed,showcasing exceptional VHCF resistance due to its bimodal structure comprisingfine grains and coarse grains consisting of curved LPSO lamellae.The investigation on the crack initiation mechanism revealed that slip-induced cracking predominantly occurs infine-grained regions rather than in the interior of coarse grains.The extrusion process aligns the basal planes of most coarse grains parallel to the axial direction,and the presence of curved LPSO lamellae acts as barriers to the movement of basal dislocations,thereby effectively increasing the threshold for slip-cracking along the basal plane.Consequently,fatigue damage manifests in the form of slip bands and micro-cracks within the interior offine grains,ultimately resulting in fatigue crack initiation,propagation andfinal fracture.展开更多
In this study,the efects of defect,mean stress and lower loading are investigated for high cycle(HCF)and very high cycle fatigue(VHCF)behavior of Ti-6Al-4V alloy.It indicates that the S-N curve of Ti-6Al-4V alloy exhi...In this study,the efects of defect,mean stress and lower loading are investigated for high cycle(HCF)and very high cycle fatigue(VHCF)behavior of Ti-6Al-4V alloy.It indicates that the S-N curve of Ti-6Al-4V alloy exhibits a linear decreasing trend or a plateau characteristic in HCF and VHCF regimes,which depends on the defect size and stress ratio.VHCF strength decreases with increasing the defect size,and it is irrespective of stress ratios.The fatigue crack initiates from specimen surface at R=−1 in both HCF and VHCF regimes.While the fatigue crack initiates from the subsurface or the interior of the specimen at R=0.1 in VHCF regime.A sequence of lower stress amplitude below the fatigue strength at 10^(9) cyc has no or negligible infuence on the fatigue life of 10^(5)-10^(9) cyc.The lower stress amplitude in variable amplitude loadings does not afect the failure mechanism.The residual compressive stress relaxation is not observed after a large number of lower loadings under ultrasonic frequency fatigue test.Gerber formula and Goodman formula give dangerous predictions of VHCF strength for both smooth specimens and specimens with defects.展开更多
Improving the low-cycle fatigue(LCF)properties of additively manufactured Ti-5.6Al-3.8V alloy is critical in ensuring its service safety and represents a significant research challenge.This work discusses a so-lution ...Improving the low-cycle fatigue(LCF)properties of additively manufactured Ti-5.6Al-3.8V alloy is critical in ensuring its service safety and represents a significant research challenge.This work discusses a so-lution that optimizes the alloy's microstructure and ductility by precisely controlling the over-saturated strengthening elements and heat treatment.This was accomplished using selective laser melting(SLM),heat treatment at 800 ℃ for 2 h,and furnace cooling on a Ti-5.6Al-3.8V alloy with tightly controlled Al,V,and O concentrations in a lower range.The results showed that the SLM-fabricated Ti-5.6Al-3.8V alloy,post-heat treatment,exhibited α laths with a width of~1.4 μm and β columnar grains with a diameter of~126 μm,without experiencing coarsening or variant selection phenomena.The alloy bal-anced strength and ductility post-heat treatment with a UTS of 1015 MPa and an EL of 16.5%relative to the as-deposited state(UTS of 1199 MPa and EL of 11.9%).Notably,the LCF properties of the heat-treated SLM Ti-5.6Al-3.8V alloy are superior to those of other Ti-6Al-4V alloys produced by additive manu-facturing and comparable to traditional forgings.At high strain amplitudes(1-1.5%),the fatigue life of this alloy was twice that of the Ti-6Al-4V forgings.Furthermore,we comprehensively analyzed the mi-crostructure,strength,and ductility of the SLM Ti-5.6Al-3.8V alloy to elucidate the factors influencing its LCF properties.These findings provide a solid foundation for improving the LCF properties of additively manufactured Ti-6Al-4V alloy,thereby contributing to its safe and reliable use in critical applications.展开更多
In order to enhance the fatigue properties of metallic materials,a feasible rationale is to delay or prevent the interior and surface fatigue crack initiation.Based on this rationale,the study investigates the approac...In order to enhance the fatigue properties of metallic materials,a feasible rationale is to delay or prevent the interior and surface fatigue crack initiation.Based on this rationale,the study investigates the approach of improving the very high cycle fatigue properties of TC6 titanium alloys through near-βforging coupled with shot peening,conducted at 930℃and ambient temperature,respectively.To unveil the associated mechanisms,microstructure,microhardness,residual stress,and fatigue properties are thoroughly analyzed after each process.Results indicate a considerable refinement in microstructure and significant mitigation of the initially existed strong texture post near-βforging and annealing,efficiently delaying crack initiation and propagation.As a result,the very high cycle fatigue property of TC6 achieves remarkable enhancement after forging.Compared to near-βforging,shot peening might not necessarily improve the very high cycle fatigue performance,particularly beyond 10^(6)cycles.展开更多
Recently,a description on a practicability of the Wöhler Curve Method for low-cycle fatigue of metals was given by the author.By the description and the low cycle fatigue test data of 16 MnR steel,it is important...Recently,a description on a practicability of the Wöhler Curve Method for low-cycle fatigue of metals was given by the author.By the description and the low cycle fatigue test data of 16 MnR steel,it is important to show that,for low cycle fatigue of metals,such a way that a stress-based intensity parameter calculated by the linear-elastic analysis is taken to be a stress intensity parameter,S,to establish a relationship between the stress intensity parameter,S,and the fatigue life,N,is practicable.In this paper,many metallic materials from the literature are given to show that the Wöhler Curve Method is well suitable for low-cycle fatigue analysis of metals.展开更多
The second generation single crystal superalloy DD6 with 0.10%Hf and 0.34%Hf （in mass fraction） was subjected to high-cycle fatigue （HCF） loading at temperatures of 700 ℃ in ambient atmosphere. SEM was used to de...The second generation single crystal superalloy DD6 with 0.10%Hf and 0.34%Hf （in mass fraction） was subjected to high-cycle fatigue （HCF） loading at temperatures of 700 ℃ in ambient atmosphere. SEM was used to determine the initiation site and the failure mechanism. Evolution of the microstructure was investigated by TEM observation. The results show that fatigue limit of DD6 alloy with 0.34%Hf is a little smaller than that of the alloy with 0.10%Hf. The fatigue cracks initiated on the surface or near the surface of the specimens. The crack would propagate along { 111 } octahedral slip planes, rather than perpendicular to the loading axis of specimen. Typical fatigue striation formed in steady propagation of fatigue crack. The fracture mechanisms of the high cycle fatigue of DD6 alloys with 0.10%Hf and 0.34%Hf are quasi-cleavage fracture. Different types of dislocation structures were developed during high cycle fatigue deformation.展开更多
Gravity die casting(GC) and squeeze casting(SC) T4-treated Al-7.0Zn-2.5Mg-2.1Cu alloys were employed to investigate the microstructures,mechanical properties and low cycle fatigue(LCF) behavior.The results show that m...Gravity die casting(GC) and squeeze casting(SC) T4-treated Al-7.0Zn-2.5Mg-2.1Cu alloys were employed to investigate the microstructures,mechanical properties and low cycle fatigue(LCF) behavior.The results show that mechanical properties of SC specimens are significantly better than those of GC specimens due to less cast defects and smaller secondary dendrite arm spacing(SDAS).Excellent fatigue properties are obtained for the SC alloy compared with the GC alloy.GC and SC alloys both exhibit cyclic stabilization at low total strain amplitudes(less than 0.4%) and cyclic hardening at higher total strain amplitudes.The degree of cyclic hardening of SC samples is greater than that of GC samples.Fatigue cracks of GC samples dominantly initiate from shrinkage porosities and are easy to propagate along them,while the crack initiation sites for SC samples are slip bands,eutectic phases and inclusions at or near the free surface.展开更多
An improved understanding of fatigue behavior of a cast aluminum alloy（2-AS5U3G-Y35）in very high cycle regime is developed through the ultrasonic fatigue test in axial and torsion loading.The new developed torsion f...An improved understanding of fatigue behavior of a cast aluminum alloy（2-AS5U3G-Y35）in very high cycle regime is developed through the ultrasonic fatigue test in axial and torsion loading.The new developed torsion fatigue system is presented.The effects of loading condition and frequency on the very high cycle fatigue（VHCF）are investigated.The cyclic loading in axial and torsion at 35 Hz and 20 kHz with stress ratio R=-1 is used respectively to demonstrate the effect of loading condition.S-N curves show that the fatigue failure occurs in the range of 105—1010 cycles in axial or torsion loading and the asymptote of S-N curve is inclined,but no fatigue limit exists under the torsion and axial loading condition.The fatigue fracture surface shows that the fatigue crack initiates from the specimen surface subjected to the cyclic torsion loading.It is different from the fatigue fracture characteristic in axial loading in which fatigue crack initiates from subsurface defect in very high cycle regime.The fatigue initiation is on the maximum shear plane,the overall crack orientation is on a typical spiral 45° to the fracture plane and it is the maximum principle stress plane.The clear shear strip in the torsion fatigue fracture surface shows that the torsion fracture is the shear fracture.展开更多
In recent years,(0001)twist grain boundaries(BTGBs)located in primary α grain clusters were identified as fatigue crack nucleation sites in different Ti alloys.In the present study,crack initiation was investigated i...In recent years,(0001)twist grain boundaries(BTGBs)located in primary α grain clusters were identified as fatigue crack nucleation sites in different Ti alloys.In the present study,crack initiation was investigated in a bimodal Ti-5Al-4 V alloy subjected to low-cycle fatigue and dwell-fatigue loadings at room temperature.The low fraction of primary α grains was not associated with a lack of sensitivity to BTGB cracking.Transmission electron microscopy and electron back-scattered diffraction were used to characterize BTGBs in the initial microstructure.The fatigue mechanisms were then analyzed with a focus on dislocation activity.α_(p) grains adjacent to cracked BTGBs contained a high dislocation density.It was primarily composed of planar slip bands of dislocations.In addition,<c+a>dislocations were noticed in the vicinity of cracked BTGBs.They supposedly pertain to crack tip plasticity during growth,and no evidence of a role of an incoming slip event in crack nucleation was obtained.Also,basal slip bands extending across adjacent grains were found to emerge from BTGBs.This feature provides an easier path for crack extension when growth along the grain boundary becomes difficult owing to a deviation from the basal plane.Atom probe tomography analyses evidenced V and Fe segregation at a grain boundary with a significant deviation from the BTGB configuration.This suggests a possible contribution of local solute segregation to the high cracking resistance of general α_(p)/α_(p) grain boundaries.This work provides new insights into the mechanisms involved in cracking of BTGB in Ti alloys subjected to cyclic loadings.展开更多
Low cycle fatigue （LCF） behavior of laser melting deposited （LMD） TC18 titanium alloy was studied at room temperature. Microstructure consisting of fine lamella-like primary α phase and transformed β matrix was ...Low cycle fatigue （LCF） behavior of laser melting deposited （LMD） TC18 titanium alloy was studied at room temperature. Microstructure consisting of fine lamella-like primary α phase and transformed β matrix was obtained by double annealed treatment, and inhomogeneous grain boundaryαphase was detected. Fatigue fracture surfaces and longitudinal sections of LCF specimens were examined by optical microscopy and scanning electron microscopy. Results indicate that more than one crack initiation site can be detected on the LCF fracture surface. The fracture morphology of the secondary crack initiation site is different from that of the primary crack initiation site. When the crack grows along the grain boundaryαphase, continuous grain boundaryαphase leads to a straight propagating manner while discontinuous grain boundaryαphase gives rise to flexural propagating mode.展开更多
The trade-off between mechanistic interpretability,operational convenience,and predictive accuracy is challenging for predicting the lifetime of lithium-ion batteries.To resolve this contradiction,we propose a damage ...The trade-off between mechanistic interpretability,operational convenience,and predictive accuracy is challenging for predicting the lifetime of lithium-ion batteries.To resolve this contradiction,we propose a damage model based on fatigue damage theory and electrochemical impedance spectroscopy.The causal relationship of“fatigue damage→resistance increase→capacity fading”is revealed to describe the underlying mechanism.Charge transfer resistance is chosen as the variable to ensure the convenience of data acquisition.To verify the accuracy of the model,the electrochemical impedance spectrum and capacity of a graphene-coated silicon electrode at two charging rates are collected and analyzed.50% and 75% of the measured data are utilized as inputs to compare the prediction capabilities of the proposed damage model and the existing empirical model.The particle filter algorithm is adopted to train the parameters of both models.The maximum prediction error of the damage model is less than 3%,showing better prediction accuracy and medium-term prediction stability than the empirical model.Our work demonstrates that the proposed damage model is an effective way to resolve contradictions in lifetime prediction.展开更多
Based on the failure model of building structural steels under earthquake loading, the low cycle fatigue test at constant strain, the stochastical fatigue test under real earthquake load spectrum and the structural fa...Based on the failure model of building structural steels under earthquake loading, the low cycle fatigue test at constant strain, the stochastical fatigue test under real earthquake load spectrum and the structural fatigue test are carried out. The experimental results show that microalloying of V Ti and Nb can improve the anti-seismic propersties of steel bars. In the high strain and shori life range, both the static strength and ductility of steels are very important to increasing the low cycle fatigue resistance of steels.展开更多
This work aims to investigate the effect of main inclusions on crack initiation in bearing steel in the very high cycle fatigue（VHCF） regime. The size and type of inclusions in the steel were quantitatively analyzed...This work aims to investigate the effect of main inclusions on crack initiation in bearing steel in the very high cycle fatigue（VHCF） regime. The size and type of inclusions in the steel were quantitatively analyzed, and VHCF tests were performed. Some fatigue cracks were found to be initiated in the gaps between inclusions（Al2 O3, Mg O-Al2 O3） and the matrix, while other cracks originated from the interior of inclusions（Ti N, Mn S）. To explain the related mechanism, the tessellated stresses between inclusions and the matrix were calculated and compared with the yield stress of the matrix. Results revealed that the inclusions could be classified into two types under VHCF; of these two, only one type could be regarded as holes. Findings in this research provide a better understanding of how inclusions affect the high cycle fatigue properties of bearing steel.展开更多
Effect of revert cycles on microstructure and fatigue properties of cast cobalt base superalloy K640S has been investigated. The results show that: at 70 times of cool heat cycles, there were microcracks found in seve...Effect of revert cycles on microstructure and fatigue properties of cast cobalt base superalloy K640S has been investigated. The results show that: at 70 times of cool heat cycles, there were microcracks found in seven times revert and ten times revert. With the increasing of thermal fatigue cycles, the crack of revert grows a little faster than virgin. When the cycle time reaches 200, the crack length for both virgin and reverts have been as long as 2mm. The low cycle fatigue life has no remarkable change, with the increase of revert cycles at 815℃, 360MPa ,0 5Hz. With the times of cycles increasing, it is found that the content of impurity and gas in alloy change a little, and there is no obvious change for dendrite microstructure.展开更多
Low cycle fatigue behavior of a quenched and tempered high-strength steel(Q960 E) was studied in the strain amplitude ranging from ± 0.5% to ± 1.2% at room temperature. As a result of fatigue loading, the di...Low cycle fatigue behavior of a quenched and tempered high-strength steel(Q960 E) was studied in the strain amplitude ranging from ± 0.5% to ± 1.2% at room temperature. As a result of fatigue loading, the dislocation structural evolution and fracture mechanism were examined and studied by transmission electron microscopy and scanning electron microscopy(SEM). The results showed that this Q960 E steel showed cyclic softening at different strain amplitudes, and the softening tendency was more apparent at strain amplitude of ±(0.6–1.2)% than that at ± 0.5%. The reduction in dislocation density with increasing strain amplitude is responsible for the softening tendency of cyclic stress with the strain amplitude. The material illustrates near-Masing behavior at strain amplitude ranging from ± 0.6% to ± 1.2%. The near-Masing behavior of Q960 E high-strength steel can be the result of stability of martensite lath at different strain amplitudes. Partial transformation from martensite laths to dislocation cells is responsible for the derivation from ideal Masing behavior. In the SEM examination of fracture surfaces, transgranular cracks initiate on the sample surface. Striations can be found during the crack propagation stage.展开更多
The effects of recrystallization on low cycle fatigue behavior were investigated on directionally solidified Co-base superalloy DZ40M. Optical microscopy and SEM were used to examine the microstructure and fracture su...The effects of recrystallization on low cycle fatigue behavior were investigated on directionally solidified Co-base superalloy DZ40M. Optical microscopy and SEM were used to examine the microstructure and fracture surface of the specimens. The mechanical testing results demonstrated that the low cycle fatigue property of DZ40M significantly decreased with the partial recrystallization. Fatigue cracks initiate near the carbides and the grain boundaries with slip-bands. Both the fatigue crack initiation and propagation can be accelerated with the occurrences of recrystallized grain boundaries.展开更多
The influence of temperature and hardness level on the cyclic behavior of 55NiCrMoV7 steel, and the mierostrueture variation and hardness diminution during low cycle fatigue behavior were investigated. By means of SEM...The influence of temperature and hardness level on the cyclic behavior of 55NiCrMoV7 steel, and the mierostrueture variation and hardness diminution during low cycle fatigue behavior were investigated. By means of SEM and XRD, the modality of carbides and the full width half-maximum （FWHM） of martensite （211） [M（211）] of Xray diffraction spectrum in fatigue specimen were studied. The results showed that the cyclic stress response behav ior generally showed an initial exponential softening for the first few cycles, followed by a gradual softening without cyclic softening saturation. The fatigue behavior of the steel is closely related to the hardness level. The hardness diminution and the variation of half-width M（211） are remarkably influenced by the interaction between the cyclic plastic deformation and the thermal loading when the fatigue temperature exceeds the tempering temperature of the steel.展开更多
The ultra-high cycle fatigue behavior of a novel high strength steel with carbide-free bainite/martensite （CFB/M） complex microstructure was studied. The ultra-high cycle fatigue properties were measured by ultrason...The ultra-high cycle fatigue behavior of a novel high strength steel with carbide-free bainite/martensite （CFB/M） complex microstructure was studied. The ultra-high cycle fatigue properties were measured by ultrasonic fatigue testing equipment at a frequency of 20 kHz. It is found that there is no horizontal part in the S-N curve and fatigue fracture occurs when the life of specimens exceeds 10^7 cycles. In addition, the origination of fatigue cracks tends to transfer from the surface to interior of specimens as the fatigue cycle exceeds 10^7, and the fatigue crack originations of many specimens are not induced by inclusions, but by some kind of ＂soft structure＂. It is shown that the studied high strength steel performs good ultra-high cycle fatigue properties. The ultra-high fatigue mechanism was discussed and it is suggested that specific CFB/M complex microstructure of the studied steel contributes to its superior properties.展开更多
基金the funding by National Key Technologies Research and Development Program of China(2017YFB0304500)the support from National Natural Science Foundation of China(No.51771014)Joint Funds of National Natural Science Foundation of China(No.U1834202)。
文摘Advanced bainitic steels with the multiphase structure of bainitic ferrite,retained austenite and martensite exhibit distinctive fatigue crack initiation behavior during high cycle fatigue/very high cycle fatigue(HCF/VHCF)regimes.The subsurface microstructural fatigue crack initiation,referred to as“non-inclusion induced crack initiation,NIICI”,is a leading mode of failure of bainitic steels within the HCF/VHCF regimes.In this regard,there is currently a missing gap in the knowledge with respect to the cyclic response of multiphase structure during VHCF failure and the underlying mechanisms of fatigue crack initiation during VHCF.To address this aspect,we have developed a novel approach that explicitly identifies the knowledge gap through an examination of subsurface crack initiation and interaction with the local microstructure.This was accomplished by uniquely combining electron microscopy,three-dimensional confocal microscopy,focused ion beam,and transmission Kikuchi diffraction.Interestingly,the study indicated that there are multiple micro-mechanisms responsible for the NIICI failure of bainitic steels,including two scenarios of transgranular-crack-assisted NIICI and two scenarios of intergranular-crack-assisted NIICI,which resulted in the different distribution of fine grains in the crack initiation area.The fine grains were formed through fragmentation of bainitic ferrite lath caused by localized plastic deformation or via local continuous dynamic recrystallization because of repeated interaction between slip bands and prior austenite grain boundaries.The formation of fine grains assisted the advancement of small cracks.Another important aspect discussed is the role of retained austenite(RA)during cyclic loading,on crack initiation and propagation in terms of the morphology,distribution and stability of RA,which determined the development of localized cyclic plastic deformation in multiphase structure.
基金supported by the National Natural Science Foundation of China(Nos.52074228,52305420,and 51875470)the China Postdoctoral Science Foundation(No.2023M742830)。
文摘GH4169 joints manufactured by Linear Friction Welding(LFW)are subjected to tensile test and stair-case method to evaluate the High Cycle Fatigue(HCF)performance at 650℃.The yield and ultimate tensile strengths are 582 MPa and 820 MPa,respectively.The HCF strength of joint reaches 400 MPa,which is slightly lower than that of Base Metal(BM),indicating reliable quality of this type of joint.The microstructure observation results show that all cracks initiate at the inside of specimens and transfer into deeper region with decrease of external stress,and the crack initiation site is related with microhardness of matrix.The Electron Backscattered Diffraction(EBSD)results of the observed regions with different distances to fracture show that plastic deformation plays a key role in HCF,and the Schmid factor of most grains near fracture exceeds 0.4.In addition,the generation of twins plays a vital role in strain concentration release and coordinating plastic deformation among grains.
基金supported by the National Natural Science Foundation of China(grant nos.12072212,12372185 and 12102280)the Sichuan Province Science and Technology Project(grant nos.2023NSFSC1918,and 2022JDJQ0011).
文摘Magnesium alloys with long-period stacking ordered(LPSO)structures are known for their impressive static mechanical strength,but the consistent occurrence of slip-cracking along the LPSO lamellae,which do not effectively impede the movement of basal dislocations,has prompted concerns about their very high cycle fatigue(VHCF)performance.In this study,an extruded Mg-Gd-Zn-Zr alloy was developed,showcasing exceptional VHCF resistance due to its bimodal structure comprisingfine grains and coarse grains consisting of curved LPSO lamellae.The investigation on the crack initiation mechanism revealed that slip-induced cracking predominantly occurs infine-grained regions rather than in the interior of coarse grains.The extrusion process aligns the basal planes of most coarse grains parallel to the axial direction,and the presence of curved LPSO lamellae acts as barriers to the movement of basal dislocations,thereby effectively increasing the threshold for slip-cracking along the basal plane.Consequently,fatigue damage manifests in the form of slip bands and micro-cracks within the interior offine grains,ultimately resulting in fatigue crack initiation,propagation andfinal fracture.
基金the support by the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB0620303)the Natural Science Basic Research Program of Shanxi(Program No.2023-JC-QN-0044).
文摘In this study,the efects of defect,mean stress and lower loading are investigated for high cycle(HCF)and very high cycle fatigue(VHCF)behavior of Ti-6Al-4V alloy.It indicates that the S-N curve of Ti-6Al-4V alloy exhibits a linear decreasing trend or a plateau characteristic in HCF and VHCF regimes,which depends on the defect size and stress ratio.VHCF strength decreases with increasing the defect size,and it is irrespective of stress ratios.The fatigue crack initiates from specimen surface at R=−1 in both HCF and VHCF regimes.While the fatigue crack initiates from the subsurface or the interior of the specimen at R=0.1 in VHCF regime.A sequence of lower stress amplitude below the fatigue strength at 10^(9) cyc has no or negligible infuence on the fatigue life of 10^(5)-10^(9) cyc.The lower stress amplitude in variable amplitude loadings does not afect the failure mechanism.The residual compressive stress relaxation is not observed after a large number of lower loadings under ultrasonic frequency fatigue test.Gerber formula and Goodman formula give dangerous predictions of VHCF strength for both smooth specimens and specimens with defects.
基金Key Research and Development Projects of Shaanxi Province(No.2023-YBGY-359)China CEEC University Joint Education Project(2021108)+2 种基金Science and Technology Plan of Xi'an City(Nos.21ZCZZHXJS-QCY6-0001,21CXLHTJSGG-QCY8-0003)Open Fund of Key Laboratory of Plasma Dynamics of Air Force Engineering University(No.6142202210203)“Qin Chuang Yuan”Project of Shaanxi Province(No.2023KXJ-272).
文摘Improving the low-cycle fatigue(LCF)properties of additively manufactured Ti-5.6Al-3.8V alloy is critical in ensuring its service safety and represents a significant research challenge.This work discusses a so-lution that optimizes the alloy's microstructure and ductility by precisely controlling the over-saturated strengthening elements and heat treatment.This was accomplished using selective laser melting(SLM),heat treatment at 800 ℃ for 2 h,and furnace cooling on a Ti-5.6Al-3.8V alloy with tightly controlled Al,V,and O concentrations in a lower range.The results showed that the SLM-fabricated Ti-5.6Al-3.8V alloy,post-heat treatment,exhibited α laths with a width of~1.4 μm and β columnar grains with a diameter of~126 μm,without experiencing coarsening or variant selection phenomena.The alloy bal-anced strength and ductility post-heat treatment with a UTS of 1015 MPa and an EL of 16.5%relative to the as-deposited state(UTS of 1199 MPa and EL of 11.9%).Notably,the LCF properties of the heat-treated SLM Ti-5.6Al-3.8V alloy are superior to those of other Ti-6Al-4V alloys produced by additive manu-facturing and comparable to traditional forgings.At high strain amplitudes(1-1.5%),the fatigue life of this alloy was twice that of the Ti-6Al-4V forgings.Furthermore,we comprehensively analyzed the mi-crostructure,strength,and ductility of the SLM Ti-5.6Al-3.8V alloy to elucidate the factors influencing its LCF properties.These findings provide a solid foundation for improving the LCF properties of additively manufactured Ti-6Al-4V alloy,thereby contributing to its safe and reliable use in critical applications.
基金financial support from the National Science and Technology Major Project(Grant No.J2019-IV-0010-0078)the National Natural Science Foundation of China(Grant Nos.12172139 and 11972012)funding from the Fundamental Research Funds for the Central Universities(Grant No.2019kfyXJJS141).
文摘In order to enhance the fatigue properties of metallic materials,a feasible rationale is to delay or prevent the interior and surface fatigue crack initiation.Based on this rationale,the study investigates the approach of improving the very high cycle fatigue properties of TC6 titanium alloys through near-βforging coupled with shot peening,conducted at 930℃and ambient temperature,respectively.To unveil the associated mechanisms,microstructure,microhardness,residual stress,and fatigue properties are thoroughly analyzed after each process.Results indicate a considerable refinement in microstructure and significant mitigation of the initially existed strong texture post near-βforging and annealing,efficiently delaying crack initiation and propagation.As a result,the very high cycle fatigue property of TC6 achieves remarkable enhancement after forging.Compared to near-βforging,shot peening might not necessarily improve the very high cycle fatigue performance,particularly beyond 10^(6)cycles.
文摘Recently,a description on a practicability of the Wöhler Curve Method for low-cycle fatigue of metals was given by the author.By the description and the low cycle fatigue test data of 16 MnR steel,it is important to show that,for low cycle fatigue of metals,such a way that a stress-based intensity parameter calculated by the linear-elastic analysis is taken to be a stress intensity parameter,S,to establish a relationship between the stress intensity parameter,S,and the fatigue life,N,is practicable.In this paper,many metallic materials from the literature are given to show that the Wöhler Curve Method is well suitable for low-cycle fatigue analysis of metals.
文摘The second generation single crystal superalloy DD6 with 0.10%Hf and 0.34%Hf （in mass fraction） was subjected to high-cycle fatigue （HCF） loading at temperatures of 700 ℃ in ambient atmosphere. SEM was used to determine the initiation site and the failure mechanism. Evolution of the microstructure was investigated by TEM observation. The results show that fatigue limit of DD6 alloy with 0.34%Hf is a little smaller than that of the alloy with 0.10%Hf. The fatigue cracks initiated on the surface or near the surface of the specimens. The crack would propagate along { 111 } octahedral slip planes, rather than perpendicular to the loading axis of specimen. Typical fatigue striation formed in steady propagation of fatigue crack. The fracture mechanisms of the high cycle fatigue of DD6 alloys with 0.10%Hf and 0.34%Hf are quasi-cleavage fracture. Different types of dislocation structures were developed during high cycle fatigue deformation.
基金Project(2015A030312003)supported by the Guangdong Natural Science Foundation for Research Team,ChinaProject(51374110)supported by the National Natural Science Foundation of China
文摘Gravity die casting(GC) and squeeze casting(SC) T4-treated Al-7.0Zn-2.5Mg-2.1Cu alloys were employed to investigate the microstructures,mechanical properties and low cycle fatigue(LCF) behavior.The results show that mechanical properties of SC specimens are significantly better than those of GC specimens due to less cast defects and smaller secondary dendrite arm spacing(SDAS).Excellent fatigue properties are obtained for the SC alloy compared with the GC alloy.GC and SC alloys both exhibit cyclic stabilization at low total strain amplitudes(less than 0.4%) and cyclic hardening at higher total strain amplitudes.The degree of cyclic hardening of SC samples is greater than that of GC samples.Fatigue cracks of GC samples dominantly initiate from shrinkage porosities and are easy to propagate along them,while the crack initiation sites for SC samples are slip bands,eutectic phases and inclusions at or near the free surface.
基金Supported by the National Natural Science Foundation of China(50775182)the Scientific Research Foundation for the Returned Scholars of the Ministry of Education of China~~
文摘An improved understanding of fatigue behavior of a cast aluminum alloy（2-AS5U3G-Y35）in very high cycle regime is developed through the ultrasonic fatigue test in axial and torsion loading.The new developed torsion fatigue system is presented.The effects of loading condition and frequency on the very high cycle fatigue（VHCF）are investigated.The cyclic loading in axial and torsion at 35 Hz and 20 kHz with stress ratio R=-1 is used respectively to demonstrate the effect of loading condition.S-N curves show that the fatigue failure occurs in the range of 105—1010 cycles in axial or torsion loading and the asymptote of S-N curve is inclined,but no fatigue limit exists under the torsion and axial loading condition.The fatigue fracture surface shows that the fatigue crack initiates from the specimen surface subjected to the cyclic torsion loading.It is different from the fatigue fracture characteristic in axial loading in which fatigue crack initiates from subsurface defect in very high cycle regime.The fatigue initiation is on the maximum shear plane,the overall crack orientation is on a typical spiral 45° to the fracture plane and it is the maximum principle stress plane.The clear shear strip in the torsion fatigue fracture surface shows that the torsion fracture is the shear fracture.
基金financially supported by the National Natural Science Foundation of China(No.U21A2050)support of the Chinese Scholarship Council(No.202006290165).
文摘In recent years,(0001)twist grain boundaries(BTGBs)located in primary α grain clusters were identified as fatigue crack nucleation sites in different Ti alloys.In the present study,crack initiation was investigated in a bimodal Ti-5Al-4 V alloy subjected to low-cycle fatigue and dwell-fatigue loadings at room temperature.The low fraction of primary α grains was not associated with a lack of sensitivity to BTGB cracking.Transmission electron microscopy and electron back-scattered diffraction were used to characterize BTGBs in the initial microstructure.The fatigue mechanisms were then analyzed with a focus on dislocation activity.α_(p) grains adjacent to cracked BTGBs contained a high dislocation density.It was primarily composed of planar slip bands of dislocations.In addition,<c+a>dislocations were noticed in the vicinity of cracked BTGBs.They supposedly pertain to crack tip plasticity during growth,and no evidence of a role of an incoming slip event in crack nucleation was obtained.Also,basal slip bands extending across adjacent grains were found to emerge from BTGBs.This feature provides an easier path for crack extension when growth along the grain boundary becomes difficult owing to a deviation from the basal plane.Atom probe tomography analyses evidenced V and Fe segregation at a grain boundary with a significant deviation from the BTGB configuration.This suggests a possible contribution of local solute segregation to the high cracking resistance of general α_(p)/α_(p) grain boundaries.This work provides new insights into the mechanisms involved in cracking of BTGB in Ti alloys subjected to cyclic loadings.
基金Project(2011CB606305)supported by the National Basic Research Program of ChinaProject(IRT0805)supported by the Cheung Kong Scholars Innovation Research Team Program of Ministry of Education,China
文摘Low cycle fatigue （LCF） behavior of laser melting deposited （LMD） TC18 titanium alloy was studied at room temperature. Microstructure consisting of fine lamella-like primary α phase and transformed β matrix was obtained by double annealed treatment, and inhomogeneous grain boundaryαphase was detected. Fatigue fracture surfaces and longitudinal sections of LCF specimens were examined by optical microscopy and scanning electron microscopy. Results indicate that more than one crack initiation site can be detected on the LCF fracture surface. The fracture morphology of the secondary crack initiation site is different from that of the primary crack initiation site. When the crack grows along the grain boundaryαphase, continuous grain boundaryαphase leads to a straight propagating manner while discontinuous grain boundaryαphase gives rise to flexural propagating mode.
基金supported by the National Natural Science Foundation of China(12021002,12472183,and 12041201).
文摘The trade-off between mechanistic interpretability,operational convenience,and predictive accuracy is challenging for predicting the lifetime of lithium-ion batteries.To resolve this contradiction,we propose a damage model based on fatigue damage theory and electrochemical impedance spectroscopy.The causal relationship of“fatigue damage→resistance increase→capacity fading”is revealed to describe the underlying mechanism.Charge transfer resistance is chosen as the variable to ensure the convenience of data acquisition.To verify the accuracy of the model,the electrochemical impedance spectrum and capacity of a graphene-coated silicon electrode at two charging rates are collected and analyzed.50% and 75% of the measured data are utilized as inputs to compare the prediction capabilities of the proposed damage model and the existing empirical model.The particle filter algorithm is adopted to train the parameters of both models.The maximum prediction error of the damage model is less than 3%,showing better prediction accuracy and medium-term prediction stability than the empirical model.Our work demonstrates that the proposed damage model is an effective way to resolve contradictions in lifetime prediction.
文摘Based on the failure model of building structural steels under earthquake loading, the low cycle fatigue test at constant strain, the stochastical fatigue test under real earthquake load spectrum and the structural fatigue test are carried out. The experimental results show that microalloying of V Ti and Nb can improve the anti-seismic propersties of steel bars. In the high strain and shori life range, both the static strength and ductility of steels are very important to increasing the low cycle fatigue resistance of steels.
基金financially supported by the State Key Laboratory for Advanced Metallurgy Foundation (No.41614014)the National Natural Science Foundation of China (No.51774031)
文摘This work aims to investigate the effect of main inclusions on crack initiation in bearing steel in the very high cycle fatigue（VHCF） regime. The size and type of inclusions in the steel were quantitatively analyzed, and VHCF tests were performed. Some fatigue cracks were found to be initiated in the gaps between inclusions（Al2 O3, Mg O-Al2 O3） and the matrix, while other cracks originated from the interior of inclusions（Ti N, Mn S）. To explain the related mechanism, the tessellated stresses between inclusions and the matrix were calculated and compared with the yield stress of the matrix. Results revealed that the inclusions could be classified into two types under VHCF; of these two, only one type could be regarded as holes. Findings in this research provide a better understanding of how inclusions affect the high cycle fatigue properties of bearing steel.
文摘Effect of revert cycles on microstructure and fatigue properties of cast cobalt base superalloy K640S has been investigated. The results show that: at 70 times of cool heat cycles, there were microcracks found in seven times revert and ten times revert. With the increasing of thermal fatigue cycles, the crack of revert grows a little faster than virgin. When the cycle time reaches 200, the crack length for both virgin and reverts have been as long as 2mm. The low cycle fatigue life has no remarkable change, with the increase of revert cycles at 815℃, 360MPa ,0 5Hz. With the times of cycles increasing, it is found that the content of impurity and gas in alloy change a little, and there is no obvious change for dendrite microstructure.
基金financial supports of the National Natural Science Foundation of China (No. 51674079)Anhui Provincial Natural Science Foundation (Nos. KJ2018A0062, KJ2017A128 and KJ2017A066)
文摘Low cycle fatigue behavior of a quenched and tempered high-strength steel(Q960 E) was studied in the strain amplitude ranging from ± 0.5% to ± 1.2% at room temperature. As a result of fatigue loading, the dislocation structural evolution and fracture mechanism were examined and studied by transmission electron microscopy and scanning electron microscopy(SEM). The results showed that this Q960 E steel showed cyclic softening at different strain amplitudes, and the softening tendency was more apparent at strain amplitude of ±(0.6–1.2)% than that at ± 0.5%. The reduction in dislocation density with increasing strain amplitude is responsible for the softening tendency of cyclic stress with the strain amplitude. The material illustrates near-Masing behavior at strain amplitude ranging from ± 0.6% to ± 1.2%. The near-Masing behavior of Q960 E high-strength steel can be the result of stability of martensite lath at different strain amplitudes. Partial transformation from martensite laths to dislocation cells is responsible for the derivation from ideal Masing behavior. In the SEM examination of fracture surfaces, transgranular cracks initiate on the sample surface. Striations can be found during the crack propagation stage.
文摘The effects of recrystallization on low cycle fatigue behavior were investigated on directionally solidified Co-base superalloy DZ40M. Optical microscopy and SEM were used to examine the microstructure and fracture surface of the specimens. The mechanical testing results demonstrated that the low cycle fatigue property of DZ40M significantly decreased with the partial recrystallization. Fatigue cracks initiate near the carbides and the grain boundaries with slip-bands. Both the fatigue crack initiation and propagation can be accelerated with the occurrences of recrystallized grain boundaries.
基金Item Sponsored by Scientific Research Foundation for Returned Overseas Chinese Scholars ,State Education Ministry(2004176)
文摘The influence of temperature and hardness level on the cyclic behavior of 55NiCrMoV7 steel, and the mierostrueture variation and hardness diminution during low cycle fatigue behavior were investigated. By means of SEM and XRD, the modality of carbides and the full width half-maximum （FWHM） of martensite （211） [M（211）] of Xray diffraction spectrum in fatigue specimen were studied. The results showed that the cyclic stress response behav ior generally showed an initial exponential softening for the first few cycles, followed by a gradual softening without cyclic softening saturation. The fatigue behavior of the steel is closely related to the hardness level. The hardness diminution and the variation of half-width M（211） are remarkably influenced by the interaction between the cyclic plastic deformation and the thermal loading when the fatigue temperature exceeds the tempering temperature of the steel.
基金supported by the National Key Fundamental Research and Development Program of China (No.2004CB619105)
文摘The ultra-high cycle fatigue behavior of a novel high strength steel with carbide-free bainite/martensite （CFB/M） complex microstructure was studied. The ultra-high cycle fatigue properties were measured by ultrasonic fatigue testing equipment at a frequency of 20 kHz. It is found that there is no horizontal part in the S-N curve and fatigue fracture occurs when the life of specimens exceeds 10^7 cycles. In addition, the origination of fatigue cracks tends to transfer from the surface to interior of specimens as the fatigue cycle exceeds 10^7, and the fatigue crack originations of many specimens are not induced by inclusions, but by some kind of ＂soft structure＂. It is shown that the studied high strength steel performs good ultra-high cycle fatigue properties. The ultra-high fatigue mechanism was discussed and it is suggested that specific CFB/M complex microstructure of the studied steel contributes to its superior properties.
