Statistical aspects of fatigue behaviour in a PM Ni-base superalloy Udimet 720

Fatigue growth of naturally-nucleated surface cracks was investigated in air under four-point bending for the PM Ni-base superalloy Udimet 720. Cracks were found to nucleate from surface defects. Statistical aspects of the fatigue crack nucleation and fatigue crack growth resistance were investigated in terms of probabilistic modelling.     

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。     

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。     

Influence of ageing treatment on work hardening behaviour of a Ni-base superalloy

Thus it may be summarised that work hardening behaviour of the alloy superni 263 can not be analysed using the simple power law equation. The work hardening behaviour is satisfactorily analysed using the modified power law equation (Ludwigson equation). There is systematic variation in the different work hardening parameters K₁, n₁, K₂, n₂ and _C with the period of ageing at 800 °C. The drastic lowering of the parameters n₁ and e_up, from ageing even for a short duration, suggests that this material should be formed in fully solution treated condition and any precipitation of γ′ must be avoided for good formability.     

CMSX-4单晶高温合金在不同温度下的低周疲劳行为

研究了单晶高温合金CMSX-4在中温760 ℃和高温950 ℃下的低周疲劳行为。结果表明：在760 ℃下合金具有较长的疲劳寿命和较高的疲劳强度,断裂后断面高度差大并与应力轴方向呈45°角,裂纹沿着{111}面扩展;而在950 ℃下合金具有较短的疲劳寿命和较低的疲劳强度,断面与应力轴垂直,裂纹沿着{001}面扩展。低周疲劳断口的扫描电镜结果表明, 760 ℃试样表面的微孔是主要的疲劳源,而950 ℃试样表面的氧化层是主要的疲劳源且呈现多源开裂。低周疲劳断口的透射电镜结果表明,中温760 ℃下位错具有的平面滑移和波状滑移的变形机制,是由位错的平面滑移向波状滑移转变的过程;而高温950 ℃下位错主要通过交滑移和攀移进行运动。     

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.     

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。     

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.     

Preparation and hot corrosion behaviour of an Al-gradient NiCoCrAlYSiB coating on a Ni-base superalloy

A gradient NiCoCrAlYSiB coating was prepared on a Ni-base superalloy using arc ion plating (AIP) and subsequent gaseous phase aluminisation techniques. Hot corrosion of normal NiCoCrAlYSiB and the gradient coating in pure Na₂SO₄ and Na₂SO₄/NaCl (75:25, wt./wt.) salts was performed at 900 °C in static air. The corrosion results indicated an enhanced corrosion resistance to both salts for the gradient NiCoCrAlYSiB coating, which the improved performance of it should be attributed to the β aluminide ‘‘pool” at the surface layer. By partially sacrificing Al₂O₃ (i.e. Al), the gradient NiCoCrAlYSiB coating specimen behaved excellently in the two kinds of salts. The grain growth during the gaseous phase aluminisation and the corrosion mechanism, including the role NaCl played in the mixture salt corrosion, are discussed.     

第二代单晶高温合金的低周疲劳行为

研究了含0.34%Hf DD6单晶高温合金在760℃下的低周疲劳行为,对其疲劳裂纹的萌生与扩展进行了分析。结果表明,含0.34%Hf DD6合金比含0.10%Hf的疲劳寿命稍有降低;其损伤以弹性损伤为主。疲劳裂纹萌生于试样表面、亚表面或试样内部后,先沿垂直应力轴方向扩展,后沿{111}面扩展。其断裂机制为类解理断裂。断口上有典型的疲劳条带。断裂组织分析表明,位错主要在基体通道中扩展,变形后期切过γ'相,形成滑移带。     

小角度晶界对单晶高温合金高周疲劳性能的影响

用双籽晶法制备了带有9°小角度晶界的DD6单晶高温合金试板,研究了小角度晶界对合金700℃高周疲劳性能的影响,并与[001]取向合金的疲劳性能进行了对比分析,用扫描电镜和透射电镜分析了其断口形貌和断裂机制。结果表明,带有9°小角度晶界合金的高周疲劳极限比[001]取向合金的稍有降低。疲劳裂纹萌生于试样表面或亚表面,而不在晶界上形成。小角度晶界试样的疲劳断裂机制有两种情况,大部分试样为类解理断裂,其它试样为类解理与沿晶混合断裂。     

Microstructural evolution in a strip-cast Ni-base superalloy

Microstructure and mechanical properties of the strip-cast Ni-base superalloy(Hastelloy-X) alloy have been compared with those of a conventionally ingot-cast alloy. As-cast strip shows a fine columnar dendritic structure as a result of a relatively high cooling rate of 10² -10⁴ K/sec during the process. The alloying elements, such as Cr and Fe, are homogeneously distributed in the γ matrix of the as-cast strip. Whereas, Mo is segregated at the center part of the as-cast strip. Mo distribution, however, becomes homogeneous after cold rolling and subsequent heat treatment at 1175°C. Two types of carbide, a Mo-rich M₆C type and a Cr-rich M₂₃C₆ type, are present in the γ matrix of the as-cast and rolled strips. Rolling and subsequent heat treatment of the strip-cast alloy results in a fine grain structure compared to the ingot-cast alloy. The γ matrix grains in stripcast and ingot-cast alloy are approximately 14 μm and 34 μm in diameter respectively. Superior tensile properties in the strip-cast alloy is due to the fine microstructural evolution during the process.     

Microstructure–low cycle fatigue behaviour relationships in a PM γ-TiAl alloy

The low cycle fatigue (LCF) behaviour has been investigated in gas atomized powder compacts of a Ti–47.2Al–2.1Cr–2.1Nb alloy. The Cyclic Stress–Strain (CSS) behaviour at 20 °C and at 500 °C was found to be strongly dependent upon the microstructure. An increasing amount of lamellar colonies, a finer lamellar spacing and a smaller γ grain size were shown to markedly decrease the cyclic strain hardening rate. For the different microstructures involved, a correlation between the capability of developing a vein-like structure and the cyclic strain hardening rate has been established. The more prone the microstructure to develop a vein-like structure, the higher the cyclic strain hardening rate. Therefore, the present study helped determine a compromise heat treatment that can ensure an improved fatigue behaviour for such a γ-TiAl alloy.     

Corrosion kinetics of a directionally solidified Ni-base superalloy

A variety of experiments were carried out to characterize the corrosion kinetics of a longitudinally oriented directionally solidified Ni-base superalloy, DS GTD-111, commonly applied as a first- and second-stage blading material in electric power generation gas-powered turbines. Under operating environments, the airfoil sections of turbine blades sustain surface-initiated damage due to the superimposed centrifugal stresses, elevated temperature and presence of corrosive reactants in the environment. As a consequence, surface cracking curtails the service lives of such components. To thoroughly characterize the stress-free and stress-assisted kinetics of diffusion and cyclic oxide rupture, several types of experiments are conducted: low cycle fatigue, thermomechanical fatigue, and thermogravimetric analysis, among others. A key goal of this study is to provide data necessary for the development of diffusion kinetics models. Accordingly, the study is divided into two parts: stress-free diffusion and stress-assisted rupture. Models are developed for each of these conditions.     

High-temperature oxidation of a single crystal Ni-base superalloy

The isothermal oxidation behaviour of a single crystal Ni-base superalloy (CMSX2) between 900 and 1300°C in air at atmospheric pressure or in pure static oxygen at 0.2 atm. pressure has been studied for test durations between 8 to 1000 hours. The extremely low oxidation rate did not permit accurate measurements of oxidation kinetics. However, oxide scale examinations by means of SEM, EDS, TEM, X-ray diffraction, RHEED, SIMS and metallography allowed to discuss the oxidation mechanisms of the studied alloy. At temperature higher than 1100°C, the very good oxidation resistance is due to the growth of a thin Al₂O₃ scale while at lower temperature it may be attributed to the growth of a subscale of a complex oxide (Cr, Ti, Al) (Ti, Ta)O₄ possessing a rutile structure. The effect of chemical homogeneity was studied in comparing the behaviour of as-cast and homogenized specimens.     

Influence of casting surface on creep behaviour of thin-wall Ni-base superalloy Inconel100

The conventionally cast nickel-base superalloy Inconel100 was investigated with the focus on the influence of the surface condition and the thin-section size on the creep properties. Flat samples with 0.9 mm and 1.3 mm thickness were wire eroded from precision cast ingots. Thin cast specimens with a remaining casting surface were compared with machined specimens which flat surfaces were subsequently ground to the final thicknesses. Microstructures were examined by optical and scanning electron microscopy. Creep tests in air at 980 °C under a constant load of 150 MPa revealed decreasing creep strength with decreasing sample thickness. However, thin cast samples and samples with a machined surface having the same thickness showed similar creep properties.     

Intergranular crack tip oxidation in a Ni-base superalloy

High-temperature intergranular crack tip oxidation under a single 600 s long sustained tensile load at 700 °C was studied for the Ni-base superalloy Allvac 718Plus. High-resolution analytical techniques showed oxidation to take place at and immediately ahead of the tip of an open crack, forming a closed but layered oxide structure about the prior (now oxidized) grain boundary. Near the prior grain boundary the oxide is Ni-rich, with a Co-enriched layer furthest away from the metal and a Fe-enriched region below this. A Cr-rich oxide is present below the outer Ni-rich oxides throughout the crack, also in the direction of crack growth. This is believed to have a hindering effect on oxidation ahead of the crack. Ni₃(Nb,Al) γ′ precipitates close to the grain boundaries were found to oxidize and form regions of near-stoichiometric NiO within the oxide layers. Remaining constituents of γ′ (e.g. Al and Nb) were found to be enriched in the surrounding oxidized matrix and also to produce thin oxide layers near the interface between the unoxidized metal and the Cr-rich oxide. The formation of the crack tip oxides is discussed with regard to thermodynamics, kinetics and the influence of applied mechanical load.     

Effects of recrystallization on the low cycle fatigue behavior of directionally solidified superalloy DZ40M

The effects of recrystallization on low cycle fatigue behavior were investigated on directionally solidified Co-base superalloy DZAOM.Optical microscopy and SEM were used to examine the mierostructure and fracture surface of the specimens.The mechanical testing results demonstrated that the low cycle fatigue property of DZ40M significantly decreased with the partial reerystallization.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.     

Observation of a Ru-rich Heusler phase in a multicomponent Ni-base superalloy

A new, ordered L2₁ Heusler phase based on Ru₂AlTa was identified in a heat-treated multicomponent Ni-base superalloy containing high ruthenium. Such a phase has not previously been observed in muticomponent Ni-base superalloys or in quaternary Ni–Al–Ta–Ru systems. The influence of solidification-induced microsegregation on the formation and distribution of the Heusler phase is discussed.