Thermal fatigue behavior of K465 superalloy

The thermal fatigue behavior of K465 superalloy was investigated at the peak temperature of 1050℃. By scanning electron microscopy （SEM） and optical microscopy, the main crack length was observed and measured. The initiation sites of the tested alloys are different in as-cast （named as K465） and solution heat treatment （named as SK465） conditions. In K465 alloy, most thermal fatigue cracks nucleate at （Nb,W,Ti）C carbides. In SK465 alloy, thermal fatigue cracks initiate in interdendritic regions, MC-type carbides and some interfaces. Thermal fatigue cracks propagate in transdendritic mode, and M6C-type carbides could retard thermal fatigue crack growth for SK465 superalloy.     

Low-cycle fatigue behavior of DZ125 superalloy under prior exposure conditions

Low-cycle fatigue(LCF) behavior of the directionally solidified(DS) nickel-based DZ125 superalloy was studied at elevated temperature(980 ℃).Specimens were,respectively,exposed for 0,2,25,50,and 100 h in air.The fatigue life of pre-exposed specimens is lower than that of unexposed specimens.The result is closely associated with fatigue crack initiation and propagation due to oxygen embrittlement and cycle loading.Detailed fractographic evaluations indicate the fatigue life is closely related to ...     

新型抗热腐蚀镍基高温合金K44的高温低周疲劳行为

研究了K44合金900℃低周疲劳性能和断裂行为。研究结果表明,该合金在循环形变过程中,首先表现出起始循环硬化或软化,随后循环稳定及最终失稳断裂三阶段。高应变幅下位错切割γ′相形成层错,降低变形阻力,合金表现出循环软化行为;低应变幅下位错在γ′相前塞积造成位错可动性降低,合金表现出循环硬化行为。疲劳裂纹主要萌生于试样表面或近表面缺陷处,以穿晶方式扩展;合金基体中块状碳化物对裂纹扩展起阻滞作用。     

K40S钴基高温合金高温高周疲劳行为

研究了K40S合金700℃和900℃高周疲劳行为。结果表明：K40S合金具有低的高温疲劳缺口敏感性，合金在700℃和900℃温度条件下的疲劳缺口敏感性分别为0．025和0．035，合金低的缺口敏感性主要归因于合金良好的高温塑性。二次碳化物M23C6的高温动态时效强化有效地强化了合金基体，提高合金的形变抗力，使合金具有较高的疲劳强度。K40S合金700℃高周疲劳断裂机制主要为机械疲劳断裂，而在900℃温度条件下则为机械疲劳与高温环境氧化共同作用的结果。     

High-temperature low-cycle fatigue behavior of superalloy MA 760

The high-temperature cyclic behavior of oxide dispersion-strengthened superalloy MA 760 was studied at different temperatures and specimen orientations. The cyclic hardening curves at different strain am-plitudes were determined at 650, 900, and 950 °C, and the stress-strain response of the material was sub-stantiated by detailed fractographical scanning electron microscopy (SEM) and microstructuralJoptical microscopy, STEM) observations. During cycling, MA 760 did not exhibit marked hardening or soften-ing, and also the final fracture occurred very rapidly without a significant decrease in the cyclic strength of the material. The “brittle” nature of deformation behavior of MA 760, especially at lower test tempera-tures, as well as oxidation-induced secondary cracking along grain boundaries, was clearly revealed by microscopical studies of the fracture surfaces. At 650 °C, evidence of dislocation cutting of γ was ob-served in transmission micrographs, but at higher temperatures, dislocation climb and by-passing of γ particles were found to become more prominent features. At all test temperatures, dislocation climb over yttria particles, as well as the departure side pinning effect of dislocations at nonshearable particles, was frequently observed.     

一种镍基高温合金的低周疲劳性能

研究了一种镍基高温合金在不同温度下的低周疲劳性能,分析了疲劳断口。结果表明,该合金循环应力响应行为表现出对温度和外加总应变幅很强的依赖关系,不同的循环应力响应行为可归因于位错、强化相和合金元素间复杂的交互作用。合金疲劳寿命与温度、外加总应变幅、氧化损伤程度有关。疲劳断裂行为受外加应变幅和氧化影响很大。     

Isothermal oxidation behavior of cast Ni-base superalloy K44

The oxidation behavior of a cast Ni-base superalloy K44 in air at 850-1 000℃ for 100 h was studied. The scales on the surface were determined by SEM and EPMA equipped with an EDXS. The results show that oxidation kinetics obey the parabolic law fi＇om which the values of activation energy Qp1=221.1 kJ/mol and Qp2=247.6 kJ/mol are estimated. The oxidation scales are composed of loose outer layer of TiO2/TiO-Cr2O3 and a small amount of NiCr204 and NiAl2O4, compact intermediate layer Cr2O3, and precipitate of internal oxides Al2O3.     

压铸模镶块的热疲劳失效行为

对工作1200模次发生热疲劳失效的压铸模镶块进行失效机理分析,同时建立了压铸过程的热-力耦合数值分析模型,研究了镶块在压铸服役过程中温度场和应力场的演变规律,并对其寿命进行预测。结果表明,镶块的失效是由于压铸过程中受到高温铝熔液的冲刷和冷却液的急冷引起的热疲劳行为所致;在循环压铸过程中,镶块凸台转角处出现应力集中,最大等效应力约为788 MPa,热疲劳裂纹在该处萌生和扩展;采用低周疲劳寿命预测模型对镶块热疲劳寿命进行预测,镶块热疲劳寿命约为1651次,与镶块实际热疲劳寿命非常吻合,这表明本文建立的热-力耦合数值模型可为压铸模镶块的可靠性设计提供借鉴。     

Crack growth behavior of SRR99 single crystal superalloy under thermal fatigue

The thermal fatigue behavior of a single crystal superalloy SRR99 was investigated.Specimens with V-type notch were tested at the peak temperatures of 900,1000,and 1100℃.The crack growth curves as a function of the number of cycles were plotted.With the increase of peak temperature,the crack initiation life was shortened dramatically.Through optical microscopy (OM) and scanning electron microscopy (SEM) observation,it was found that multiple small cracks nucleated at the notch tip region but only one or two of them continued to develop in the following thermal cycles.The primary cracks generally propagated along a preferential direction.Microstructure changes after thermal fatigue were also discussed on the basis of SEM observation.     

镍基粉末高温合金FGH96应变疲劳行为

对镍基粉末高温合金FGH96在750℃、应变比R=0.05下的应变疲劳循环应力响应曲线、循环应力-应变曲线和应变-寿命曲线进行分析,通过非线性回归拟合,得到基于Manson-Coffin公式及郑公式两种处理模型的应变—寿命曲线及相关参数。结果表明,FGH96合金在实验加载条件下,出现了循环软化—稳定—再软化断裂的应力响应特性,并随着加载应变幅的提高而愈加明显。与Manson-Coffin公式相比,郑公式在宏观预测应变疲劳寿命,尤其是确定材料疲劳极限问题上取得很好的预测效果。     

QT400球墨铸铁的热疲劳行为

采用V形缺口试样,通过上限温度为500~900 ℃的热疲劳试验研究了QT400球墨铸铁的热疲劳行为,并对不同上限温度下材料热疲劳裂纹的萌生与扩展以及试样的显微组织与硬度等进行了分析。结果表明,热疲劳裂纹主要从试样的V形缺口底部萌生,上限温度越高,裂纹萌生越早,萌生后扩展速率也越快,其热疲劳寿命越低;随着上限温度的升高,试验后试样基体的硬度逐渐增加,上限温度由700℃上升到800℃时,试样硬度快速上升;热疲劳试验过程中的氧化腐蚀、球墨形态以及组织变化等为球墨铸铁QT400的热疲劳破坏的主要影响因素。     

Effects of deformation behavior on fatigue fracture surface morphology in a nickel-base superalloy

Fatigue crack propagation fracture surface morphologies in nickel-base superalloys vary substantially with changes in loading parameters such as temperature, ΔK, load ratio, frequency, and additionally microstructure. Quantitative fracture surface roughness can vary from sub-micron levels to a maximum value of approximately half the grain size. Atomic Force Microscope studies of surface slip traces in compression specimens revealed a clear relationship between slip homogeneity in compression testing and fracture surface roughness under similar fatigue loading conditions. It has been shown in this study that changes in ΔK, strain level, temperature, grain size, and load ratio can all affect slip heterogeneity, which in turn controls the fracture surface roughness. Finally, a model is developed that quantitatively predicts fracture surface roughness and roughness-induced crack closure stress intensity values from measurements of slip line spacing in a compression specimen.     

第二代单晶高温合金DD6的高周疲劳行为

研究Hf含量为0.34%和0.10%Hf的第二代单晶高温合金DD6在700℃的高周疲劳行为:采用扫描电镜对疲劳裂纹的萌生与失效机制进行分析,采用透射电镜研究变形后的显微组织.结果表明:含0.34%Hf的DD6合金比含0.10%Hf的合金的疲劳寿命稍有降低;疲劳裂纹萌生于试样表面、亚表面,沿{111}面扩展,而并不沿垂直应力轴方向扩展;断口上有典型的疲劳条带;Hf含量为0.34%和0.10%的DD6合金的高周疲劳断裂机制为类解理断裂.透射电镜分析表明,合金在循环变形中形成了不同形式的位错.     

Thermo-mechanical fatigue behavior of nickel-base powder metallurgy superalloy FGH96 under tension-tension loading

Thermo-mechanical fatigue (TMF) behavior of FGH96, a nickel-base powder metallurgy superalloy, has been studied under tension-tension loading at the temperature range from 550℃ to 720℃. The results show that TMF fracture mode is intergranular for the in-phase (IP), but transgranular cleavage-like for the out-of-phase (OP) samples. The total content of Al, Ti and Nb in the γ' phases for the IP or OP samples and the partitioning ratio of γ'/γ in these elements for the IP samples are relatively higher at the lower strain amplitude, which is consistent with the case of the γ' size that is larger at the lower strain amplitude, the lattice parameter misfit is negative and the absolute value is lower at the lower strain amplitude that is correlative with the change of the γ' morphology. The deformation at the lower strain amplitude is mainly dominated by the dislocation lines and dislocation pairs in the matrix channels, at the higher strain amplitude dominated by the large numbers of superlattice stacking faults within the γ' phases.     

Constitutive modeling of a directionally solidified nickel-based superalloy DZ125 subjected to thermal mechanical creep fatigue loadings

A transversely isotropic continuum elasto-viscoplasticity model,which was developed from Chaboche's unified constitutive model,was formulated to capture the thermal mechanical creep fatigue deformation behavior of a directionally solidified nickel-based superalloy.A fourthorder tensor was introduced to model material anisotropy.In order to model the tertiary creep behavior,the Kachanov damage evolution equation was coupled into the stress tensor.Based on the test results of uniaxial tensile,fatigue,and creep loadings at isothermal temperature conditions,the material parameters are obtained.Thermal mechanical fatigue(TMF) and creep-fatigue interaction test results were used to verify the robustness of the model.Additionally,strain-temperature-dependent stress-strain responses under TMF loadings were analyzed using the present model.Under strain-controlled conditions,both of the stress ranges and mean stresses are strongly influenced by the strain-temperature phases,a key parameter for TMF tests.     

Fatigue crack propagation behavior of K40S cobalt-base superalloy at elevated temperature

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Low-cycle fatigue behavior of a directionally solidified Ni-based superalloy subjected to gas hot corrosion pre-exposure

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.     

合金元素对Ni基高温合金1223K氧化行为的影响

研究了不同Cr、Ti、Ta含量定向凝固Ni基高温合金的1223 K恒温氧化行为。结果表明：Cr、Ti、Ta对合金的抗氧化性能在氧化的不同阶段作用各异。在氧化初期,由于Ti元素与O的亲和力很强,Ti含量高的No.3合金在氧化曲线上增重较为明显,而随着氧化的进行,Cr含量高的No.1合金增重较大,这是由于Cr的选择性氧化所致。在1223 K,No.1合金中Ti和Ta含量较低无法形成有效弥散分布于Cr2O3之中,难以形成连续的保护性氧化膜,从而不能有效地阻碍Cr3＋向合金内部持续扩散。而No.2和No.3合金中Ti和Ta含量适中,形成连续膜可延缓Cr3＋的持续向外扩散,对改善合金的抗氧化能力有积极作用。