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《Acta Materialia》2003,51(12):3521-3535
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. 相似文献
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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。 相似文献
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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 eup, 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. 相似文献
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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。 相似文献
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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 BSt500s 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. 相似文献
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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. 相似文献
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Z.B. Bao 《Corrosion Science》2009,51(4):860-751
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. 相似文献
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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. 相似文献
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《Intermetallics》2006,14(10-11):1130-1135
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. 相似文献
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《Acta Materialia》2007,55(10):3375-3385
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. 相似文献
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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. 相似文献
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J. Strößner C.H. Konrad M. Brunner R. Völkl U. Glatzel 《Journal of Materials Processing Technology》2013,213(5):722-727
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. 相似文献
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Intergranular crack tip oxidation in a Ni-base superalloy 总被引:1,自引:0,他引:1
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. 相似文献
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Effects of recrystallization on the low cycle fatigue behavior of directionally solidified superalloy DZ40M 总被引:2,自引:0,他引:2
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. 相似文献
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《Scripta materialia》2004,50(6):849-854
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. 相似文献