镍基粉末冶金FGH95高温合金的拉伸及低周疲劳性能研究

在420～650℃的温度范围内,研究了PM FGH95高温合金在应变率0.0001～0.01 s-1范围内的拉伸性能及在不同应力比R=-1及R=0下的低周疲劳性能.拉伸试验结果表明:在此温度范围内,应变率对弹性模量、拉伸屈服强度及塑性模量的影响可以忽略不计;并且应变率自0.0001 s-1增大至0.01 s-1时,弹性模量、拉伸屈服强度及塑性模量受温度的影响也不明显.PM FGH95合金受材料微结构的影响很大,材料含有的微缺陷对其力学性能有非常不利的影响.LCF试验结果表明:PM FGH95合金是循环硬化材料.当应力比R=-1时,温度对LCF寿命的影响很小;但在应力比R=0时,LCF寿命受温度的影响很大,且随着温度的增加材料的低周疲劳寿命减小.SEM断口扫描发现,断裂表面有很多的解理面,但没有疲劳条带,且此断裂模式下没有明显的塑性变形,所以FGH95合金的低周疲劳寿命几乎是由裂纹萌生阶段决定的.     

Grain size based low cycle fatigue life prediction model for nickel-based superalloy

Nickel-based superalloys are easy to produce low cycle fatigue (LCF) damage when they are subjected to high temperature and mechanical stresses. Fatigue life prediction of nickel-based superalloys is of great importance for their reliable practical application. To investigate the effects of total strain and grain size on LCF behavior, the high temperature LCF tests were carried out for a nickel-based superalloy. The results show that the fatigue lives decreased with the increase of strain amplitude and grain size. A new LCF life prediction model was established considering the effect of grain size on fatigue life. Error analyses indicate that the prediction accuracy of the new LCF life model is higher than those of Manson-Coffin relationship and Ostergren energy method.     

Low-Cycle Fatigue and Creep-Fatigue Behaviors of a Second-Generation Nickel-Based Single-Crystal Superalloy at 760 ℃

Low-cycle fatigue (LCF) behaviors of a second-generation nickel-based single-crystal superalloys with [001] orientation at 760 °C 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 in γ channel 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.     

取向偏离度对镍基单晶高温合金DD10低周疲劳行为的影响

本文研究了特定试验条件下,镍基单晶DD10取向偏离[001]的程度对其低周疲劳行为的影响。结果表明：取向偏离度对DD10合金的低周疲劳行为影响明显。疲劳寿命随取向偏离度的增加而明显降低,在同等偏离度下,晶体取向靠近[001]-[011]的合金低周寿命优于取向靠近[001]-[11]的合金;分析发现：不同取向偏离度合金DD10低周疲劳寿命的差异源于其每循环周次的可累计塑性变形的差异。另一方面,取向偏离度对疲劳总应力幅和塑性应变幅的影响却截然相反,但对疲劳裂纹萌生和扩展方式影响不大。     

Effects of Crystal Orientations on the Low-Cycle Fatigue of a Single-Crystal Nickel-Based Superalloy at 980 °C

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.     

Corrosion of reinforcing steel and low cycle fatigue behaviour

Chloride induced corrosion of reinforcing steel can be highly detrimental and of great influence on the low cycle fatigue (LCF) characteristics. An experimental study conducted on BSt500_s reinforcing steel, showed that the LCF and life expectancy were reduced considerably according to the level of corrosion. Low cycle strain controlled fatigue testing under ± 1 and ± 2.5% constant amplitude strain indicated that the corroded steel bars exhibit gradual reduction in available energy, number of cycles to failure and the load bearing ability. Formation of irregularities such as pits, notches and cavities occurred on the corroded steel surface and stress concentration points were developed which are highly localized at imperfections and especially at the rib bases. The experimental investigation of the corroded specimens subjected to LCF showed that the life expectancy, the remaining energy density and the strength properties were reduced considerably as a result of these irregularities combined with the mass loss and reduction of the exterior hard layer of martensite. Structural design capable of resisting seismic activity that does not account for the reduction of the load bearing ability and life expectancy as well as the cumulative plastic deformation of the steel reinforcement due to corrosion and loading history that a structure will be subjected in harsh climatic environments and ground oscillating motion could lead to serious and unpredictable performance.     

Effect of Ultrasonic Shot Peening on Microstructure and Mechanical Properties of High-Nitrogen Austenitic Stainless Steel

The effect of ultrasonic shot peening (USSP) was studied on microstructural modification and mechanical properties such as microhardness, yield strength, tensile strength, and low cycle fatigue (LCF) life of a nitrogen stabilized austenitic stainless steel, at room temperature. There was grain refinement up to nano scale in surface region of the shot peened specimens and the microhardness was increased markedly up to the depth of approximately 100 µm. There was insignificant increase in yield and tensile strength, but drastic reduction in LCF life, particularly at low strain amplitude, from USSP. The nominal increase in yield and tensile strength was due to grain refinement in the surface region and drastic fall in LCF life was due to surface cracking resulting from USSP.     

Low cyclic fatigue behavior of electron-beam-welded Ti–6Al–4V titanium joint

The low cycle fatigue(LCF) tests were carried out using symmetrical cyclic loading under total strain amplitude control conditions.The present paper is devoted to investigating the cyclic deformation response of Ti–6Al–4V titanium and the electron-beam-welded(EBW) joint in the following aspects,i.e.,cyclic deformation behavior,fatigue life and fatigue fracture behavior.The results show that the softening of the joint is significant at larger strain ranges,while not obvious at smaller strain ranges.The joint shows shorter fatigue life at larger strain ranges and equivalent fatigue life at smaller strain ranges compared with Ti–6Al–4V base metal.A fatigue crack of the joint not only originates at the surface or subsurface,but also at defects in the fusion zone(FZ).The crack propagation zone of Ti–6Al–4V base metal shows ductile fracture mechanism,while the joint shows brittle fracture mechanism.In all the fatigue fracture zones many dimples appear,showing the typical ductile fracture.     

变幅载荷作用下点焊焊接接头的疲劳损伤

对拉剪点焊试样进行了多级变幅疲劳加载,载荷块分别为低-高、高-低和低-高-低形式,测量了疲劳破坏过程中的固有频率.基于疲劳寿命试验结果和固有频率随疲劳寿命的变化,分析了点焊结构疲劳损伤累积的特点和加载次序对疲劳寿命的影响.结果表明,载荷块形状,即加载次序对疲劳寿命的影响并不明显,使用线性疲劳累积损伤理论预测的疲劳寿命接近实际疲劳寿命.依据固有频率的变化,可以监测疲劳寿命过程中出现的损伤,并确定损伤的程度.按照损伤力学中损伤随寿命的演化规律,可进行疲劳寿命预测,寿命预测结果与试验疲劳寿命吻合较好.     

Low cycle fatigue properties and cyclic deformation behavior of as-extruded AZ31 magnesium alloy

The low cycle fatigue(LCF)properties of as-extruded AZ31 Mg alloy were investigated under total strain amplitudes in the range of 0.4%-1.2%with strain rate of 1×10- 2s -1.Due to the twinning effect in compression during loading and the detwinning effect during unloading,the alloy showed an asymmetric hysteresis loop.The cyclic stress response exhibited cyclic hardening at high total strain amplitudes.The cyclic deformation behaviors were discussed using the Coffin-Manson plot,which divided the plastic strain amplitudes into the tension side and the compression side.Through the LCF tests that were started from either tension or compression under a total strain amplitude of 1.0%,the interaction between the twinning effect and dislocation was analyzed.The twinning effect during the LCF test and the variation of the dislocation density were investigated using optical microscopy and transmission electron microscopy,respectively.     

LOW CYCLE FATIGUE AND DEFORMATION BEHAVIOR IN AN 8090 Al-Li ALLOY

1.IntroductionAl-Liall0ysrepresentanewclass0flightweight,highmodulusandhighstrengtheconomica1structuralmaterials.Theref0re,theseall0ysareespeciallyattractivet0aero-nauticalandaer0spaceindustries.Tensilepr0pertiesandfracturet0ughnessinAl-Liall0ysaswellaseffectofprecipitates0nthemhavebeenextensivelyinvestigated[1w7].Fatiguefracture0fmaterialsacc0untsf0rthemaj0rity0fin-servicefailureinengineeringc0mpo-nents,andl0wcyclefatiguebehaviorisimportantforlifetimepredicti0ninpracticalalloys-Severalresea…     

应变速率对IC10合金1100℃低周疲劳性能的影响

研究了不同应变速率（1×10^-2s^-1,1×10^-3s^-1,1.6×10^-4s^-1）下IC10合金1100℃下的低周疲劳性能、弹性模量、屈服强度和循环应力响应行为。研究结果表明：随应变速率的增加,材料的弹性模量没有变化,而材料的屈服强度明显增大,材料的屈服强度与应变速率的对数符合线性关系;材料在3种应变速率下都表现为明显的循环软化,随着应变水平的降低,该材料的循环软化行为逐渐地变弱。应变速率对低周疲劳寿命的影响规律与应变水平的大小有关：当总应变范围大于1.2%时,应变速率为1.6×10^-4s^-1的低周疲劳寿命最长,应变速率为1×10^-3s^-1的疲劳寿命最短,而应变速率为1×10^-2s^-1的疲劳寿命界于二者之间;当总应变范围为0.8%时,随着应变速率的减小,疲劳寿命明显缩短。     

复杂应力状态下镍基单晶合金低周疲劳寿命预测模型

镍基单晶合金的低周疲劳（ＬＣＦ）晶体取向相关性和其弹塑性的晶体取向相关性有密切关系,基于所建晶体取向各向异性弹塑性晶体滑移模型,本文推导了复杂应力状态下镍基单晶合金低周疲劳寿命预测模型,结论为ＬＣＦ寿命与修正的滑移应变呈指数关系。在详细分析薄壁圆筒试样受拉－扭载荷下应力应变分布规律的基础上,利用不同拉－扭载茶下的低周疲劳寿命数据,对本文所提模型进行了成功的考核。     

基于局部应变法的焊接微区低周疲劳性能

针对忽略焊接力学不均匀性的名义应变法评价焊接接头低周疲劳性能不准确的问题,提出一种考虑焊接接头力学不均匀性、基于弹塑性有限元计算的局部应变法,对焊接接头的低周疲劳性能进行准确评价.通过耐热钢电弧焊接头光滑试样和焊缝光滑试样的低周疲劳试验,以及焊接接头光滑试样在加载过程中的弹塑性有限元计算,采用名义应变法和局部应变法分别对耐热钢电弧焊接头光滑试样进行低周疲劳性能评价.结果表明,耐热钢电弧焊接头在低周疲劳载荷下断裂于焊缝区域.采用名义应变法评价该焊接接头时,结果偏保守.由于局部应变法考虑了焊接接头的力学不均匀性,基于断裂微区的局部应变进行疲劳性能评价,发现采用局部应变法得到的焊接接头应变—寿命曲线与断裂微区焊缝的应变—寿命曲线一致.     

Fatigue response,fracture characteristic and life modeling of a near-alpha titanium alloy under typical cyclic loadings in service

Experimental investigations on the fatigue behavior of a near-alpha titanium alloy under typical cyclic loadings were carried out to simulate the service loading states applied on the engine blades.The axial stress-controlled tension–tension low-cycle fatigue(LCF) tests were carried out over a range of maximum stresses and stress ratios.The rotary bending tests were conducted using a step-loading procedure to reveal the high-cycle fatigue(HCF) limit stresses.The cyclic softening effect is observed in this material,and the strain ratcheting occurs obviously at the maximum LCF loading of 900 MPa.The LCF resistance is found to be dependent on both the maximum loading and the stress ratio.The HCF limit stresses for 1 9 107 and 1 9 106 cycles are determined as405.7 and 457.6 MPa,respectively.The macroscopic fatigue fracture mode and the failure features on fracture surfaces were analyzed by scanning electron microscope(SEM).     

多轴载荷下ZL101铝合金的低周疲劳行为

研究不同等效应变幅下ZL101铝合金在多轴比例和非比例载荷下的低周疲劳行为,并用透射电镜观察合金的疲劳行为中的位错结构。结果表明：合金在两种加载方式下均表现为循环硬化;在非比例载荷下合金表现出附加强化,但程度不明显;合金的疲劳寿命随等效应变幅的增加而降低,合金在非比例加载下的疲劳寿命低于比例加载时的疲劳寿命。对位错结构的观察表明,随等效应变幅度的提高,合金的低周疲劳位错结构从交叉位错带转化为位错胞,合金在非比例加载下更易形成位错胞结构。     

A Multiaxial Low Cycle Fatigue Life Prediction Model for Both Proportional and Non-proportional Loading Conditions

This paper has presented a life prediction model in the field of multiaxial low-cycle fatigue. The proposed model is generally applied for constant amplitude multiaxial proportional and non-proportional loading. Depending upon applied strain path the equivalent strain varies within a cycle. Equivalent average strain amplitude is considered as fatigue damage parameter in the proposed model. The model has requirement of only two material constants and no other tuning parameters. The model is examined by the proportional and non-proportional low-cycle fatigue life experimental data for eight different types of materials. The model is successfully correlated with multiaxial fatigue lives of eight different materials.     

2205DSS焊接接头腐蚀疲劳性能分析

对川东船厂制造化学品船所用的国产2205DSS常用焊接接头在模拟海水环境中的腐蚀疲劳性能进行了试验研究.通过Origin软件对试验数据进行非线性拟合得到了环境温度分别在24,30和40℃、加载应力比为0.1、加载频率为20 Hz条件下的腐蚀疲劳寿命S-N曲线,得到焊接接头在该环境下的疲劳极限,建立了有温度参数表示的S-N数学模型.结果表明,在模拟海水中焊接接头发生腐蚀疲劳断裂的位置在母材区,环境温度升高对低应力水平作用下的试件疲劳寿命有明显影响.     

P92钢630 ℃下的高温低周疲劳行为分析

P92钢作为新一代耐热钢,其服役温度正随着超超临界机组的不断推广而逐渐提高,而高温疲劳性能对保证电厂的安全运行具有重大作用.文中通过P92钢630℃下的疲劳试验,研究了其在不同应变幅和应变速率下的疲劳寿命以及应力—应变关系,并结合断口形貌分析了P92钢的断裂机理.结果表明,P92钢属于高温循环软化材料,其疲劳寿命随塑性应变幅的增大而呈指数下降,同时应变速率的增大会导致其疲劳寿命的增大.P92钢疲劳断口分为疲劳源区、裂纹扩展区以及断裂区.高应变幅下蠕变孔洞和二次裂纹的增加是导致其疲劳寿命降低的主要原因.     

LOW CYCLE FATIGUE BEHAVIORS AND MICROSTRUCTURES OF Ti–2Al–2.5Zr WITH FINE GRAIN AT RT AND 77 K

对细晶Ti--2Al--2.5Zr合金进行了室温/低温(77 K)疲劳实验及微观组织观察. 结果表明: 室温低应变幅Δε_t/2(=0.5%, 1.0%)下,合金表现为循环软化; 室温高应变幅(1.5%, 2.0%)下, 则表现为循环应力饱和; 77 K时, 不同应变幅下均表现为循环硬化, 且随应变幅升高, 循环硬化程度增强. 疲劳寿命测试结果表明: 低温疲劳寿命始终高于室温. 断口SEM观察表明, 室温和低温下, 疲劳裂纹扩展区均有明显的疲劳条纹,疲劳裂纹以穿晶方式扩展, 室温下伴随有大量二次裂纹, 低温下的二次裂纹数量明显减少. TEM观察表明: 低温下孪生是合金主要的变形方式, 包括{1011}和{1121}型孪晶. 疲劳变形位错组态为: 室温较低应变幅(0.5%, 1.0%)下, 形成位错线和局部位错缠结; 室温下应变幅提高到1.5%和2.0%时,\{1010}柱面和{1121}锥面滑移同时开动, 位错组态演化为亚晶和明显的位错胞. 77 K下, 应变幅2.0%时形成沿 柱面平行分布的位错带; 77 K下应变幅升高到4.5%时, 多滑移形成相互垂直的位错线. 低温诱发形变孪晶是Ti--2Al--2.5Zr低温疲劳寿命升高的原因.