Effect of a large population of seeded alumina inclusions on crack initiation and small crack fatigue crack growth in Udimet 720 nickel-base disk superalloy (original) (raw)
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The Influence of Inclusions on Low Cycle Fatigue Life in a P/M Nickel-Base Disk Superalloy
Superalloys 1996 (Eighth International Symposium), 1996
The high alloy content of advanced nickel-base disk superalloys calls for powder metallurgy (P/M) processing to minimize segregation produced by conventional cast & wrought processing. Although the technology has developed to allow reliable application of P/M materials, of concern are ceramic inclusions which are intrinsic to the process due to the use of ceramic crucibles in producing the raw meltstock and in the powder atomization process itself. For robust disk design, the impact of ceramics on Low Cycle Fatigue (LCF) must be assessed. Actual production material can be evaluated but the low frequency of larger inclusions means that impracticably large volumes of material must be tested. To address this problem, tests were run on powder seeded with controlled distributions of ceramics. The alloy was the current GEAE disk material Rene' 88 DT (R'X8DT). Two types of ceramic seeds were added at two size/density combinations. Small seeds (-270/+325 mesh) were added to-270 mesh powder at rates (numbers per unit volume) predicted via probabilistic calculations to cause surface initiations. Large seeds (-SO/+100 mesh) were added to the powder at rates predicted to cause internal initiations. The powder was consolidated and processed using production parameters. LCF tests were made on samples taken from fully heat treated forgings at 204' C and 649" C at two stress levels. Roughly half of the bars were shotpeened to study the ability of this processing to suppress surface initiations. The impact of the seeds was significant and was a function of seed type and size, temperature, and bar surface condition. At 204" C life decreased with increasing seed size up to a maximum life reduction of 33-50%. Shotpeening only slightly improved the lives of small seeded bars. At 649" C, the seeds had dramatic impacts. Small seeds reduced life by 1-2 orders of magnitude and large seeds by a further l-2 orders of magnitude. Shotpeening did suppress surface initiations and significantly improved life. The relative impacts between 204" C and 649" C and the impact of shotpeening depended on failure mechanisms-at 204" C most of the failures initiated at facets, explaining a relatively minor seeding impact at this temperature while at 649" C almost all failures initiated at the seeds (or at intrinsic inclusions in the unseeded material).
Metallurgical and Materials Transactions A, 2018
Cyclic fatigue experiments in the high and very high cycle fatigue regimes have been performed on a Rene´88DT polycrystalline nickel-based superalloy. The microstructural configurations that favor early strain localization and fatigue crack initiation at high temperature from 400°C to 650°C have been investigated. Competing failure modes are observed in the high to the very high cycle fatigue regime. Fatigue cracks initiate from non-metallic inclusions and from intrinsic internal microstructural features. Interestingly, as stresses are reduced into the very high cycle regime, there is a transition to initiation only at crystallographic facets. At higher stress in the high cycle fatigue regime, a significant fraction of specimens initiate cracks at non-metallic inclusions. This transition is analyzed with regard to microstructural features that favor strain localization and accumulate damage early during cycling.
Materials Science and Engineering: A, 2008
The fatigue lives of modern powder metallurgy disk alloys can be reduced by over an order of magnitude by surface cracking at inherent non-metallic inclusions. The objective of this work was to study the effectiveness of shot peening in suppressing LCF crack initiation and growth at surface nonmetallic inclusions. Inclusions were carefully introduced at elevated levels during powder metallurgy processing of the nickel-base disk superalloy Udimet ® 720. Multiple strain-controlled fatigue tests were then performed on machined specimens at 427 and 650 °C in peened and unpeened conditions. Analyses were performed to compare the low cycle fatigue lives and failure initiation sites as a function of inclusion content, shot peening, and fatigue conditions. A large majority of the failures in as-machined specimens with introduced inclusions occurred at cracks initiating from inclusions intersecting the specimen surface. The inclusions could reduce fatigue life by up to 100X. Large inclusions had the greatest effect on life in tests at low strain ranges and high strain ratios. Shot peening can be used to improve life in these conditions by reducing the most severe effects of inclusions.
Fatigue crack growth in nickel-based superalloys at elevated temperatures
Materials & Design, 2011
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.► Crack growth measurements are better estimated by the potential drop technique. ► Crack propagation is faster at 550 than 450 °C for all the materials. ► The crack propagates faster in the U720LC than in the U720PM mono-material. ► The content of U720PM and U720LC mono-materials is very stochastic at the interface. ► The heterogeneity at the interface affects the behaviour of the joint material.
International Journal of Fatigue, 1995
Background: Nickel-based superalloys are typically used as blades and discs in the hot section of gas turbine engines, which are subjected to cyclic loading at high temperature during service. Understanding fatigue crack deformation and growth in these alloys at high temperature is crucial for ensuring structural integrity of gas turbines. Methods: Experimental studies of crack growth were carried out for a three-point bending specimen subjected to fatigue at 725°C. In order to remove the influence of oxidation which can be considerable at elevated temperature, crack growth was particularly tested in a vacuum environment with a focus on dwell effects. For simulation, the material behaviour was described by a cyclic viscoplastic model with nonlinear kinematic and isotropic hardening rules, calibrated against test data. In combination with the extended finite element method (XFEM), the viscoplasticity model was further applied to predict crack growth under dwell fatigue. The crack was assumed to grow when the accumulated plastic strain ahead of the crack tip reached a critical value which was back calculated from crack growth test data in vacuum. Results: Computational analyses of a stationary crack showed the progressive accumulation of strain near the crack tip under fatigue, which justified the strain accumulation criterion used in XFEM prediction of fatigue crack growth. During simulation, the crack length was recorded against the number of loading cycles, and the results were in good agreement with the experimental data. It was also shown, both experimentally and numerically, that an increase of dwell period leads to an increase of crack growth rate due to the increased creep deformation near the crack tip, but this effect is marginal when compared to the dwell effects under fatigue-oxidation conditions. Conclusion: The strain accumulation criterion was successful in predicting both the path and the rate of crack growth under dwell fatigue. This work proved the capability of XFEM, in conjunction with advanced cyclic viscoplasticity model, for predicting crack growth in nickel alloys at elevated temperature, which has significant implication to gas turbine industries in terms of "damage tolerance" assessment of critical turbine discs and blades.
Distribution of Inclusion-Initiated Fatigue Cracking in Powder Metallurgy Udimet 720 Characterized
2004
In the absence of extrinsic surface damage, the fatigue life of metals is often dictated by the distribution of intrinsic defects. In powder metallurgy (PM) alloys, relatively large defects occur rarely enough that a typical characterization with a limited number of smallvolume fatigue test specimens will not adequately sample inclusion-initiated damage. Counterintuitively, inclusion-initiated failure has a greater impact on the distribution in PM alloy fatigue lives because they tend to have fewer defects than their cast and wrought counterparts. Although the relative paucity of defects in PM alloys leads to higher mean fatigue lives, the distribution in observed lives tends to be broader. In order to study this important failure initiation mechanism without expending an inordinate number of specimens, a study was undertaken at the NASA Glenn Research Center where known populations of artificial inclusions (seeds) were introduced to production powder. Map of a low-cycle fatigue (LCF) specimen surface showing the relative sizes of observed surface inclusions and crack lengths observed at intervals during interrupted fatigue testing; da/dN, crack growth rate. Long description. Illustration of surface specimen and graph of axial location in inches versus angle in degrees for C-L17 interruptions from 250 to 2402 cycles.
2002
The fatigue li ves of modern powder metallurgy (PM) di sk alloys are influenced by variabilities in alloy rnicrostmcture and mechanical properties. These propelties can vary due to the different steps of materials/component processing and machining. One of these variables, the presence of nonmetallic inclusions, has been shown to significantly degrade low-cycle fatigue (LCF) life (refs. 5 to 8). Nonmetallic inclusions are inherent defects in powder alloys that are a by-product of powder-processing techniques. Contamination of the powder can occur in the melt, during powder atomization, or dwing any of the various handling processes through consolidation. In modern nickel disk powder processing facilities, the level of inclusion contamination have been reduced to less than 1 part per million by weight. Despite the efforts of manufacturers to ensure the cleanliness of their powder production processes, the presence of inclusions remains a so urce of great concern for the designer. The objective of this study was to investigate the effects on fatigue life of these inclusions. Since natural inclusions occur so infrequentl y, elevated levels of inclusions were carefully introduced in a nickel-based disk superalloy, Udimet® 720 (registered trademark of Special Metals Corporation), produced using PM processing. Multiple strain-controlled fatigue tests were then performed on this material at 650°C. Analyses were performed to compare the LCF lives and failure initiation sites as functions of inclusion content and fatigue condition. A large majority of the failures in specimens with introduced inclusions occulTed at cracks initiating from inclusions at the specimen smface. The inclusions could reduce fatigue life by up to lOO X. These effects were found to be dependent on strain range and strain ratio. Test at lower strain ranges and higher strain ratios produced larger effects of inclusions on life.
Micromechanisms of fatigue crack growth in a single crystal Inconel 718 nickel-based superalloy
Acta Materialia, 1999
ÐThe fatigue crack growth behavior of an experimental, single crystal alloy, of equivalent nominal chemical composition to Inconel 718 is presented. Fracture modes under cyclic loading were determined by scanning electron microscopy. The results of the fractographic analyses are presented on a fracture mechanism map that shows the dependence of the fatigue fracture mechanisms on the maximum stress intensity factor, K max , and the stress intensity factor range, DK. Crack-tip deformation mechanisms associated with fatigue crack growth were studied using transmission electron microscopy. The relative eects of DK and K max on the fatigue crack growth behavior of this material are discussed within the context of a two-parameter crack growth law. The in¯uence of grain boundaries on the fatigue crack growth resistance of materials such as Inconel 718 is also discussed in light of the results of this investigation.
Superalloys 2016, 2016
Both environmental embrittlement and crack tip visco-plastic stress relaxation play a significant role in determining the dwell fatigue crack growth (DFCG) resistance of nickel-based disk superalloys. In the current study performed on the Low Solvus High Refractory (LSHR) disk alloy, the influence of these two mechanisms were separated so that the effects of each could be quantified and modeled. the formulation the remaining stress level concept as measured by simple stress relaxation tests. The newly proposed parameter, Ksrf, did an excellent job in correlating the dwell crack growth rates for the four heat treatments which were shown to have similar intrinsic environmental cyclic fatigue crack growth resistance.