Indentation creep surface morphology of nickel-based single crystal superalloys

The crystallographic orientation dependence of surface morphology of indentation creep on a nickel-based single crystal superalloy is investigated by using crystal plasticity slip theory with a three-dimensional (3D) finite element model. The numerical results show that the pile-up patterns developed around the indentation imprint exhibit four-, two-, and threefold symmetry on the surfaces of [0 0 1]-, [0 1 1]-, and [1 1 1]-oriented single crystals, respectively. The evolution of radial and hoop stresses around the crater provides important information for possible radial crack nucleation, whose critical locations depend on crystallographic orientations. These characteristics can be well explained in the viewpoint of crystallographic anisotropy. The findings may shed some light on understanding of the crystal structures and its time-dependent deformation mechanisms with the indentation method.     

A complex procedure for describing porosity in precision cast elements of aircraft engines made of MAR-M 247 and MAR-M 509 superalloys

An analysis of the quality assurance systems applied in industry for an assessment of porosity in precision cast parts of aircraft engines has shown that they characterize with a lack of repeatability or reproducibility of the test results obtained. For this reason, a comprehensive procedure of porosity assessment has been developed. It has been shown in the research described here that the procedure guarantees obtaining an unambiguous, full characterization of porosity as well as accurate and repeatable test results.     

Oxygen enhanced crack growth in nickel-based superalloys and materials damage prognosis

This paper summarizes the results from a comprehensive multidisciplinary study to better understand the role of niobium and other strengthening elements in enhancing crack growth by oxygen in nickel-based superalloys at high temperatures, and considers its importance for materials damage prognosis and life cycle engineering in high temperature service. Three γ′ strengthened powder metallurgy (P/M) alloys, with 0, 2.5 and 5 wt pct Nb and comparable volume fractions (about 53 vol pct) of γ′′ precipitates, were specially designed for this study. Coordinated crack growth, microstructural and surface chemistry studies were conducted on the alloys. They were complemented by oxidation studies of Nb, Ni₃Nb, NbC, Ni₃Al and Ni₃Ti, and analyses of fracture surfaces of interrupted crack growth specimens by X-ray photoelectron spectroscopy (XPS). The findings taken in toto show that oxygen enhancement of crack growth is the result of the formation of a brittle film of surface oxides along grain boundaries and interfaces ahead of the crack tip by the preferential oxidation of Nb, Ti and Al in the Nb-rich carbides and Ni₃Al, Ni₃Ti and Ni₃Nb (in Inconel 718) precipitates. The results also showed that the oxidation of Nb-rich carbides alone can significantly enhance crack growth in oxygen. The findings are discussed in relation to the previously proposed crack growth mechanisms, and their applications.     

Fatigue growth of surface cracks in nickel-based superalloys

The fatigue growth of surface cracks in three nickel-base superalloys has been monitored using a DC electrical potential-drop technique at both 200°C and 600°C in air. A three dimensional finite element analysis stress intensity calibration has enabled these growth rates to be compared with standard data obtained from though-cracked compact tension specimens. Good agreement has been found between surface and through-crack growth rates over the range from threshold to $\text{～25}\text{MNm}{}^{\mathrm{<mtext>?3</mtext><mtext>2</mtext>}}$ although for applied stress intensity ranges in excess of this the surface cracks propagated more slowly than expected, particularly at 200°C. This retardation has been suggested to arise from the increased proportion of a plane stress crack growth for the shorter surface cracks leading to both increased closure effects and a change in the crack growth mechanism.     

On low cycle fatigue life of nickel-based superalloy valve membranes under non-proportional cyclic loading

A numerical model for low cycle fatigue damage analysis of valve membranes is presented in this paper. The isotropic and non-linear kinematic hardening laws of Lemaître–Chaboche, which define both the cyclic hardening and the ratchetting phenomenon, are adopted. The model for prediction of the number of cycles to crack initiation is based on a combination between Manson–Coffin relationship and Jiang–Sehitoglu fatigue parameter. The applied cyclic loading consists in a load pressure imposed to the internal face of the membrane and a vertical displacement of its hub section. The experimental results and numerical simulation predictions in terms of number of cycles to failure turned out to be in good concordance. Thus, numerical predictions are confirmed by microscopic observations made on membranes failed during testing.     

Effects of Ni,Cr and W on the microstructural stability of multicomponent CoNi-base superalloys studied using CALPHAD and diffusion-multiple approaches

The alloying effects of Ni,W and Cr on the microstructural stability of CoNi-base alloys were investigated using a multicomponent diffusion multiple after being aged at 1000 ℃ for 1000 h.The diffusion multiple was carefully designed based on thermodynamic calculations.The relationships between alloy compo-sitions and microstructural characteristics were established over a large compositional range using this single sample,and the alloying effects of Ni,W and Cr on the elemental partitioning behaviors between γ and γ'two phases were thermodynamically analyzed using high-throughput calculation.The results together show that an increase of Ni content increases the γ'volume faction in the long-term aged microstructures.However,the higher Ni content leads to the precipitation of the x phase by promoting the partitioning of W from the γ'phase to the γphase.The decrease of W content dramatically reduces the γ'volume faction,but the addition of Cr can properly counteract this effect by promoting the partitioning of Al and Ti from the γphase to the γ'phase.This study will be helpful for accelerating the development of novel γ'-strengthened multicomponent CoNi-base alloys,as well as providing experimental data to improve the thermodynamic database.     

Plasma-carburization of nickel-based self-fluxing alloy

Surface hardening of Ni-based self-fluxing alloy was carried out using plasma-carburization. The carburization was performed utilizing methane glow discharge plasma on sintered Ni-based self-fluxing alloy (METCO 16C) at 977 °C and 1065 °C for 3.6 ks. The carburized surfaces were characterized by employing optical microscopy, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy and Vickers micro-hardness testing. The SEM and EPMA micrographs show that the thickness of the carburized hard layer increased with increasing temperature, to a depth of approximately 120-200 μm. Hard chromium carbide (Cr₃C₂) was found to precipitate near the surface on carburization at 977 °C more than at 1065 °C. Cr₃C₂ contributes to the surface hardening.     

Niobium in superalloys: a perspective

The metallurgical behaviour of niobium (columbium &) as an alloying element in austenitic superalloys is characterized. First, superalloys are defined. Next, the properties of nio bium which are useful for superalloy con stitution are reviewed. Finally, the specific function of niobium as an additive to nickel, cobalt and iron sup eralloy s is established and its broad effects summarized.     

Quench crack behavior of nickel-base disk superalloys

There is a need to increase the temperature capability of superalloy turbine disks to allow higher operating temperatures in advanced aircraft engines. When modifying processing and chemistry of disk alloys to achieve this capability, it is important to preserve the ability to use rapid cooling during supersolvus heat treatments to achieve coarse grain, fineγ′ microstructures. An important step in this effort is an understanding of the key variables controlling the cracking tendencies of nickel-base disk alloys during quenching from supersolvus heat treatments. The objective of this study was to investigate the quench cracking tendencies of several advanced disk superalloys during simulated heat treatments. Miniature disk specimens were rapidly quenched after solution heat treatments. The responses and failure modes were compared and related to the quench cracking tendencies of actual disk forgings. Cracking along grain boundaries was generally observed to be operative. For the alloys examined in this study, the solution temperature, not alloy chemistry, was found to be the primary factor controlling quench cracking. Alloys with high solvus temperatures show greater tendency for quench cracking.     

Performance and thermal stability of Pt-modified Al-diffusion coatings for superalloys under cyclic and isothermal conditions

Abstract

In this paper, the influence of the platinum content on the cyclic oxidation resistance at 1150°C of Pt-aluminide coatings deposited on a single crystal Ni-based superalloy is reported. The different coatings involve the well-known bi-phase (Ni,Pt)Al and PtAl₂ as well as single phase (Ni,Pt)Al coatings with various compositions and microstructures. The results showed that the cyclic oxidation performance depends strongly on the average Pt-content present in the coating and, to a lesser extent, on the coating microstructure. Moreover, no relevant effect of the Al content could be evidenced. Surface deformation occurred during exposure and was attributed to phase transformations within the coating. This assumption was confirmed by isothermal experiments between 1050 and 1200°C. Post-experimental investigations of tested specimens were performed by optical and scanning electron microscopy, X-ray diffraction and EPMA.     

Micromechanisms of fatigue crack growth in a forged Inconel 718 nickel-based superalloy

The micromechanisms of fatigue crack propagation in a forged, polycrystalline IN 718 nickel-based superalloy are evaluated. Fracture modes under cyclic loading were established by scanning electron microscopy analysis. The results of the fractographic analysis are presented on a fracture mechanism map that shows the dependence of fracture modes on the maximum stress intensity factor, K_max, and the stress intensity factor range, ΔK. Plastic deformation associated with fatigue crack growth was studied using transmission electron microscopy. The effects of ΔK and K_max on the mechanisms of fatigue crack growth in this alloy are discussed within the context of a two-parameter crack growth law. Possible extensions to the Paris law are also proposed for crack growth in the near-threshold and high ΔK regimes.     

Low cycle fatigue modeling for nickel-based single crystal superalloy

The low cycle fatigue (LCF) characteristics of nickel-based single crystal (SC) superalloy have been experimentally and numerically investigated. The effects of crystallographic orientation, load ratio and stress concentration are studied. In order to model the effect of crystallographic orientation, a new orientation factor, which is relevant to the yield strength, is constructed. On the other hand, a new asymmetrical loading factor is introduced to describe the effect of load ratio. The LCF model for SC superalloy smooth specimen is established with these new damage parameters. The effect of the strain gradient on the LCF life of SC superalloy is further studied, which is applied to the evaluation of the LCF life of SC superalloy notched specimen. The LCF model proposed is validated by the experimental data of SC superalloy DD3 and PWA1480.     

Residual stress assessment of nickel-based alloy 690 welding parts

Failures of welding parts in nuclear power plants have increased significantly. One of main degradation mechanisms was initiation and growth of primary water stress corrosion cracking accelerated by residual stress at dissimilar metal welds of piping, reactor head penetrations and nozzles connecting major components. Therefore, estimation of the welding residual stress became an important issue for ensuring reliability of them. The present study deals with verification of welding simulation techniques through sensitivity analyses of bottom-mounted instrumentation nozzle mock-up and control rod drive mechanism (CRDM) nozzle mock-up, examination of residual stress effects for a welded cylindrical block and application of the residual stress evaluation method to real penetration nozzles of CRDM made of nickel-based alloy 690.     

Fatigue crack growth in nickel-based superalloys at elevated temperatures

In the present work, fatigue crack growth in two nickel-base mono-materials and one bi-material has been investigated at 450 and 550 °C. The electric potential drop technique was found to better estimate the crack length during cycling as compared to the compliance method. This finding is supported by microscopic observations of the fracture surface and also by the numerical simulation using finite element code Castem2000. The crack was found to grow faster in the coarse grained material than in the fine grained one. The fracture surface observation showed that the performance of the bi-material is linked to the mono-material content at the interface. In addition, the content of each mono-material at the interface was found to be very stochastic. This heterogeneity, due to the assembly process, strongly affects the behaviour of the biomaterial. Finite element computation showed a good agreement between numerical and experimental results in term of stress intensity factor.     

Dynamic recrystallization behavior of a typical nickel-based superalloy during hot deformation

The dynamic recrystallization (DRX) behavior of a typical nickel-based superalloy is investigated by the hot compression tests. Based on the conventional DRX kinetics model, the volume fractions of DRX are firstly estimated. Results show that there is an obvious deviation between the experimental and predicted volume fractions of DRX when the forming temperature is below 980 °C, which is induced by the slow dynamic recrystallization rate under low forming temperatures. Therefore, the segmented models are proposed to describe the kinetics of DRX for the studied superalloy. Comparisons between the experimental and predicted results indicate that the proposed segmented models can give an accurate and precise estimation of the volume fractions of DRX for the studied superalloy. In addition, the optical observation of the deformed microstructure confirms that the dynamically recrystallized grain size can be well characterized by a power function of Zener–Hollumon parameter.     

Techniques for High Temperature Fatigue Testing

The influence of small concentrations of SO₂, HCl or Na₂SO₄ vapour in air on the corrosion of the uncoated nickel-based superalloys IN 792 DS + Hf, CMSX-6 and MA 760 ODS and the coatings RT 22 and LCO 22 has been investigated at 1000 and 1100°C. A level of 1% SO₂ in air somewhat increased the oxidation of IN 792 DS + Hf and favoured the scale spallation of CMSX-6 at 1000°C. Some precipitates of Ti sulphide were found in the subsurface zones of both alloys. No influence was observed at 1100°C. The oxidation of MA 760 ODS and the two coatings was not affected by SO₂ at 1000 and 1100°C. The addition of 100ppm HCl to air favoured the spallation of the scales of IN 792 DS + Hf and CMSX-6 at 1000°C. Again, no influence could be observed with these alloys at 1100°C nor with MA 760 ODS or the coatings at either of these temperatures. Contrary to expectations, no synergistic effect was found in air with 1% SO₂ and 100ppm HCl, but SO₂ reduced the negative effect of HCl. Catastrophic corrosion occurred with IN 792 DS + Hf and CMSX-6 in the presence of Na₂SO₄ vapour in air at 1000 and 1100°C after an incubation period of more than 100 h. The mechanism of this rapid corrosion could not be clarified and it is still an open question whether the corrosion is caused by acidic fluxing of a liquid Na₂SO₄?MoO₃ film or by the reaction between Na₂SO₄ vapour and the scale, thus altering the transport properties of the oxides. MA 760 ODS and the coatings were not tested. Constant extension rate tests with IN 792 DS + Hf and CMSX-6 at 1000°C did not reveal any influence of 100 ppm HCl on the mechanical properties at strain rates of 1 × 10^?6 and 3 × 10^?8 S^?1.     

Thermal stability of a platinum aluminide coating on nickel-based superalloys

An investigation was carried out to determine the thermal stability of a platinum aluminide coating on the directionally solidified alloy MAR M 002 and its single-crystal version alloy, SRR 99, at 800, 1000 and 1100°C. The morphology, structure and microchemical composition of the coating were characterized using scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. In the as-deposited condition, the coating was found to consist of two layers. Most of the platinum was concentrated in the outer coating layer which consisted of a fine dispersion of PtAl₂ in a matrix of β-(Ni, Pt)Al containing other elements in solid solution, such as cobalt and chromium. The inner coating layer was relatively free of platinum and consisted essentially of β-NiAl. Exposure at 800°C was found to have no significant effect on the structure and composition of the coating on each alloy. At temperatures ?1000°C, however, PtAl₂ became thermodynamically unstable and significant interdiffusion occurred between the coating and alloy substrate. After exposure at 1000°C, the components of the outer coating layer were NiAl and Ni₃Al. However, after exposure at 1100°C, the outer coating layer consisted only of Ni₃Al. Also, after exposure at both temperatures, the composition of the outer coating layer approached that of the inner layer due to interdiffusion. Although the coating on both alloys exhibited similar structural stability at all temperatures investigated, the coating on alloy MAR M 002 was found to develop a more protective scale. This behaviour was correlated with differences in alloy substrate composition particularly rare-earth elements such as hafnium.     

High-temperature mechanical properties of nickel-based superalloys manufactured by additive manufacturing

Mechanical behaviour of Haynes 230 and Inconel 625 manufactured by selective laser melting (SLM) has been experimentally investigated in 870^oC. Significant embrittlement of SLM Inconel 625 was found, accompanied by hardening behaviour during the test. Moreover, due to inhomogeneous grain deformation, the hardness dispersion of the two SLM alloys increased significantly. Employing a systematic research with scanning electron microscopy and energy-dispersive spectrometer, intergranular cracking of two SLM alloys can be observed from the surface cracks and fracture appearances, respectively. In addition, carbides along grain boundaries of the two nickel-based superalloys were found and affected the strength of the grain boundary at 870^oC. Therefore, it resulted in different fracture mechanisms.     

结构材料喷射成形技术与雾化沉积高温合金

喷射成形是利用快速凝固方法直接制备金属材料坯料或半成品的先进材料制造技术 ,喷射沉积高温结构材料的冶金性能好、生产效率高、成本低 ,因而在近几年得到了迅速发展 .本项研究的主要目的是要通过喷射成形工艺参数的调整、最大限度地直接减少喷射成形坯中的孔隙度 ,进而得到优质坯料 .利用优化的雾化喷射沉积技术制备了多种高温合金沉积坯 ,沉积坯整体致密、晶粒细小、组织均匀、无宏观偏析、含气量低、力学性能提高 .还简要地比较了喷射成形高温合金与用常规铸锭冶金工艺和粉末冶金工艺制备高温合金的异同 ;总结了航空材料研究院喷射成形高温材料近年来的研究状况 ,包括专用高温材料喷射成形装置和技术及其应用 .     

Small crack behavior and fracture of nickel-based superalloy under ultrasonic fatigue

Fracture and small crack behavior in the very high cycle domain of 10⁹ cycles were investigated with a nickel-based superalloy under ultrasonic fatigue in ambient air at room temperature. The influence of ultrasonic frequency is examined by comparing the results with those in conventional low frequency fatigue. It is found that fatigue strength increases as frequency is raised up to 19.5 kHz and the most of fatigue life is consumed in nucleating and propagating small cracks up to 100 μm. Transition of fracture mode from transgranular ductile fracture to cleavage-dominated fracture occurs beyond a critical stress intensity factor range of approximately , leading to the catastrophic failure under ultrasonic fatigue.