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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS2024

Effects of alloying elements on twinning in Ni-based superalloys

Micro-twinning is the major creep deformation mechanism in Ni-based superalloys at temperatures above 700 °C. Recent experiments suggest that superlattice stacking faults in y' phase may serve as the precursors to twin formation. Segregation of alloying elements to these precursors may have a significant effect on formation and extension of micro-twins. Using atomistic modeling we investigate and explain the effects of Nb and Cr alloying additions on these processes. The simulation shows that Nb increases the creep resistance which is mostly associated with impeding the reordering of the high energy double complex stacking fault. Cr, on the other hand, promotes twin growth, degrading the high temperature creep properties. These results can help to understand the effects of elemental composition of the alloy on creep resistance.

Ni-based superalloys↗
NASA NTRS1988

A study of microstructural characteristics of Ni-based superalloys at high temperatures

The microstructural characteristics of the Ni-based superalloy MAR-M246(Hf) which is used in manufacturing the components of the Space Shuttle's main engine have been studied. These superalloys need optimal heat treatment to get the best results. To find the optimum heat treatment the technique of differential thermal analysis and the optical photomicrographs are being planned to be utilized. In the first phase, the existing experimental equipment like cutting, grinding/polishing machines and metallurgical microscope have been set up to cut/polish and take the photomicrographs. In the beginning of this year an order was placed for the Leitz Mettalux-3 microscope with a hot stage for in-situ observation of the superalloy samples. The hot stage was tested for the first time, alloying the thermocouple with the Tantulum heating element and has not been installed finally by the supplier. A Perkin Elmer Differential Thermal Analyzer (DTA 1700) was procured in the first year of the project. Samples of MAR-M246(Hf), MAR-M247, Waspaloy, Udimet-41, CMSX-2 and CMSX-3 (polycrystalline and single crystals) have been studied using differential thermal analyzer.

Lal, R. B.↗
NASA NTRS2006

Preliminary Investigations of Joining Technologies for Attaching Refractory Metals to Ni-Based Superalloys

In this study, a range of joining technologies has been investigated for creating attachments between refractory metal and Ni-based superalloys. Refractory materials of interest include Mo-47%Re, T-111, and Ta-10%W. The Ni-based superalloys include Hastelloy X and MarM 247. During joining with conventional processes, these materials have potential for a range of solidification and intermetallic formation-related defects. For this study, three non-conventional joining technologies were evaluated. These included inertia welding, electro-spark deposition (ESD) welding, and magnetic pulse welding (MPW). The developed inertia welding practice closely paralleled that typically used for the refractory metals alloys. Metallographic investigations showed that forging during inertia welding occurred predominantly on the nickel base alloy side. It was also noted that at least some degree of forging on the refractory metal side of the joint was necessary to achieve consistent bonding. Both refractory metals were readily weldable to the Hastelloy X material. When bonding to the MarM 247, results were inconsistent. This was related to the higher forging temperatures of the MarM 247, and subsequent reduced deformation on that material during welding. ESD trials using a Hastelloy X filler were successful for all material combinations. ESD places down very thin (5- to 10- m) layers per pass, and interactions between the substrates and the fill were limited (at most) to that layer. For the refractory metals, the fill only appeared to wet the surface, with minimal dilution effects. Microstructures of the deposits showed high weld metal integrity with maximum porosity on the order of a few percent. Some limited success was also obtained with MPW. In these trials, only the T-111 tubes were used. Joints were possible for the T-111 tube to the Hastelloy X bar stock, but the stiffness of the tube (resisting collapse) necessitated the use of very high power levels. These power levels resulted in damage to the equipment (concentrator) during welding. It is of note that the joint made showed the typical wavy bond microstructure associated with magnetic pulse/explosion bond joints. Joints were not possible between the T-111 tube and the MarM 247 bar stock. In this case, the MarM 247 shattered before sufficient impact forces could be developed for bonding.

Gould, Jerry E.↗
NASA NTRSDate not supplied

Twin nucleation and growth mechanism in Ni-based superalloys

While micro-twinning is the dominant creep deformation mechanism in Ni-based superalloys at temperatures above 700 C, many aspects of twin nucleation and growth remain unexplored. Kolbe mechanism for micro-twinning, based on thermally activated reordering, is probably the only concept currently widely accepted by the scientific community. We propose a qualitatively different mechanism for nucleation and growth of twins. The mechanism can be briefly described as follows. Penetration of a   precipitate by two 1/2(110) edge dislocations travelling on adjacent {111} glide planes triggers nucleation (at the interface of the precipitate and the matrix) and emission of Shockley partial of screw character on the glide plane of the edge dislocation, which entered the precipitate first (generating trailing high-energy anti-phase boundary (APB)). Propagation of this Shockley partial into the precipitate converts the APB into super intrinsic stacking fault (SISF). The recurring arrival of additional edge dislocations on the glide planes adjacent to the configuration described above leads to formation of super extrinsic stacking fault SESF, subsequent micro-twin formation, and growth of the twinned region. We demonstrate the proposed mechanism via molecular dynamics simulations.

Ni-based superalloys↗
NASA NTRS2019

The Effect Stacking Fault Segregation and Phase Transformations have on Creep Strength in Ni-based Superalloys

In this study, two commercially similar polycrystalline Ni-based disk superalloys (LSHR and ME3) were creep tested at 760C and 552MPa to approximately 0.3% plastic strain. LSHR consistently displayed superior creep properties at this stress/temperature regime even though the microstructural characteristics between the two alloys were comparable. High resolution structural and chemical analysis, however, revealed significant differences between the two alloys among active gamma prime shearing modes involving superlattice intrinsic and extrinsic stacking faults. In ME3, Co and Cr segregation and Ni and Al depletion were observed along the intrinsic faults - revealing a gamma prime -to- gamma phase transformation. Conversely in LSHR, an alloy with a higher W content, Co and W segregation was observed along the intrinsic faults. This observation combined with scanning transmission electron microscopy (STEM) simulations confirm a gamma prime-to-D019 phase transformation along the intrinsic faults in LSHR. Using experimental observations and density functional theory calculations, a novel local phase transformation strengthening mechanism is proposed that could be further utilized to improve the high temperature creep capabilities of Ni-base disk alloys.

Superalloy↗
NASA NTRSDate not supplied

Effect of Alloying Additions on Twinning in Ni-based Superalloys

Micro-twinning is the dominant creep deformation mechanism in Ni-based superalloys at temperatures above 700 °C. We use atomistic simulations to study two mechanisms of twin nucleation and growth that are characterized by qualitatively different rate limiting processes. In case of the mechanism proposed by Kolbe, the rate limiting process is diffusion-mediated atomic reshuffling. In case of the other mechanism, we proposed recently, the rate limiting process is nucleation of Shockley partial dislocation. We demonstrate the effects of alloying additions on functionality of these mechanisms.

Valery V Borovikov↗
NASA NTRS1985

Rapid solidification characteristics in melt spinning a Ni-base superalloy

The solidification kinetics involved in the process of melt spinning a Ni-base superalloy have been characterized. Through a correlation of ribbon thickness to melt puddle residence time, it was found that the solidification front velocity, V, is typically about 100 mm/sec at the ribbon surface not in contact with the spinning wheel. The rate of solidification varies within the ribbon, increasing with decreasing distance, S, from the wheel-contact surface as V = 3.65/s. Ribbon microstructure and texture characteristics are discussed in light of this kinetics result. The thickness-vs-time correlation was further analyzed to yield information about thermal history during ribbon formation. These thermal results are generally consistent with those deduced from dendrite arm spacing measurements.

Huang, S. C.↗
NASA NTRS1984

The effects of Cr, Co, Al, Mo and Ta on the cyclic oxidation behavior of a prototype cast Ni-base superalloy based on a 2(5) composite statistically designed experiment

A series of cast Ni-base superalloys were systematically varied at selected levels of Co, Cr, Mo, Ta, and Al. The elemental levels varied were Mo, 0 to 4 percent; Cr, 6 to 18 percent; Co, 0 to 20 percent, Ta, 0 to 8 percent; and Al, 3.25 to 6.25 percent. The cyclic oxidation resistance was determined from specific weight change data as a function of time for 1 hr cycles in static air at 1100 C. The significant terms in decreasing order of their importance were Al, Ta, Cr2, Al-Cr, Cr-Co, Co2, Al-Mo, Cr-Mo, Al-Al, and Mo-Ta. The Al term alone accounted for close to 82 percent of the explained variability. The estimating equation showed that the Al level was the most important and should be at its 6.25 wt % maximum value. The Mo and Ta levels should also be at their maximum 4 and 8 wt % respectively. The cobalt composition should be as low as possible, i.e., 0 wt%. The Cr level optimum varies depending on the other 4 levels. The X-ray diffaction results indicate the most protective scales are alumina/aluminate spinel stabilizized with a tri-rutile oxide high in Ta and Mo.

Barrett, C. A.↗
NASA NTRS1989

A study of microstructural characteristics and differential thermal analysis of Ni-based superalloys

The objective of this work is to correlate the mechanical properties of the Ni-based superalloy MAR M246(Hf) used in the Space Shuttle Main Engine with its structural characteristics by systematic study of optical photomicrographs and differential thermal analysis. The authors developed a method of predicting the liquidus and solidus temperature of various nickel based superalloys (MAR-M247, Waspaloy, Udimet-41, polycrystalline and single crystals of CMSX-2 and CMSX-3) and comparing the predictions with the experimental differential thermal analysis (DTA) curves using Perkin-Elmer DTA 1700. The method of predicting these temperatures is based on the additive effect of the components dissolved in nickel. The results were compared with the experimental values.

Aggarwal, M. D.↗
NASA NTRS1982

Reduction of chromium in Ni-base superalloys through element substitution and rapid solidification processing

The reduction in the use of Cr in Ni base superalloys by the combined approaches of both elemental substitution and rapid solidification processing is studied. The elements Si, Zr, Y and Hf were chosen as potential partial substitutes for Cr in Waspaloy and IN 713LC sine their separate addition to other alloys has previously resulted in enhanced oxidation resistance. The roles of Cr and these replacement elements in determining the microstructure and properties are evaluated. The elements Si, Zr, and Y and Hf are used as partial replacements for Cr in the base superalloys and these resultant alloys are processed using rapid solidification techniques. The mechanical properties and oxidation resistance of the processed materials are evaluated. The microstructure is characterized using state of the art techniques (e.g. analytical transmission electron microscopy), and the mechanism by which these structures are produced is determined.

Fraser, H. D.↗
NASA NTRS2018

Segregation and Phase Transformations Along Superlattice Intrinsic Stacking Faults in Ni-Based Superalloys

In this study, local chemical and structural changes along superlattice intrinsic stacking faults combine to represent an atomic-scale phase transformation. In order to elicit stacking fault shear, creep tests of two different single crystal Ni-base superalloys, ME501 and CMSX-4, were performed near 750 degrees Centigrade using stresses of 552 megapascals and 750 megapascals, respectively. Through high resolution Scanning Transmission Electron Microscopy (STEM) and state-of-the-art energy dispersive x-ray spectroscopy, ordered compositional changes were measured along SISFs (Superlattice Intrinsic Stacking Faults) in both alloys. For both instances, the elemental segregation and local crystal structure present along the SISFs are consistent with a nanoscale to D019 (ordering of a hexagonal close-packed crystal) phase transformation. Another notable observation is prominent Cr- and Co-rich Cottrell atmospheres and new evidence of more complex reordering processes responsible for the formation of these faults. These findings are further supported using density functional theory calculations and High Angle Annular Dark Field (HAADF) STEM image simulations.

STEM-EDS↗
NASA NTRS1986

The effect of variations of cobalt content on the cyclic oxidation resistance of selected Ni-base superalloys

Cobalt levels were systematically varied in the Ni-base turbine alloys U-700 (cast), U-700m(PM/HIP), Waspaloy, Mar-M-247, In-738, Nimonic-115, U-720, and SX-R-150. The cobalt levels ranged from 0 wt % to the nominal commercial content in each alloy. The alloys were tested in cyclic oxidation in static air at 1000, 1100 and 1150 C for 500, 200 and 100 hr respectively. An oxidation attack parameter, Ka derived from the specific weight change versus time data was used to evaluate the oxidation behavior of the alloys along with X-ray diffraction analysis of the surface oxides. The alloys tend to form either Cr2O3/chromite spinel or Al2O3/aluminate spinel depending on the CR/Al ratio in the alloys. Alloys with a ratio of 3.5 or higher tend to favor the Cr oxides while those under 3.0 form mostly Al oxides. In general the Al2O3/aluminate spinel forming alloys have the better oxidation resistance. Increased cobalt content lowers the scaling resistance of the higher Cr alloys while a 5.0 wt % Co content is optimum for the Al controlling alloys. The refractory metals, particularly Ta, appear beneficial to both types of oxides perhaps due to the formation of the omni-present trirutile Ni(Ta,Cb,Mo,W)2O6. Both scales break down as increasing amounts of NiO is formed.

Barrett, C. A.↗
NASA NTRS1987

The effect of variations of cobalt content on the cyclic oxidation resistance of selected Ni-base superalloys

Cobalt levels were systematically varied in the Ni-base turbine alloys U-700 (cast), U-700m (PM/HIP), Waspaloy, Mar-M-247, In-738, Nimonic-115, U-720, and SX-R-150. the cobalt levels ranged from 0 wt pct to the nominal commercial content in each alloy. the alloys were tested in cyclic oxidation in static air at 1000, 1100 and 1150 C for 500, 200, and 100 hr, respectively. An oxidation attack parameter, Ka, derived from the specific weight change versus time data was used to evaluate the oxidation behavior of the alloys along with X-ray diffraction analysis of the surface oxides. The alloys tend to form either Cr2O3/chromite spinel or Al2O3/aluminate spinel depending on the Cr/Al ratio in the alloys. Alloys with a ratio of 3.5 or higher tend to favor the Cr oxides while those under 3.0 form mostly Al oxides. In general the Al2O3/aluminate spinel forming alloys have the better oxidation resistance. Increased cobalt content lowers the scaling resistance of the higher Cr allys while a 5.0 wt pct Co content is optimum for the Al controlling alloys. The refractory metals, particularly Ta, appear beneficial to both types of oxides, perhaps due to the formation of the omnipresent trirutile Ni(Ta, Cb, Mo, W)2O6. Both scales break down as increasing amounts of NiO are formed.

Barrett, Charles A.↗
NASA NTRS1977

Estimate of conjugate gamma and gamma prime compositions in Ni-base superalloys

Approaches for estimating the composition of the matrix phase of alloys from the melt composition are reviewed. The first method is based on assigning essentially fixed stoichiometry to precipitating phases and is typified by PHACOMP. The second method uses analytical geometry to interpret phase diagrams and is applicable to a two-phase region of a six-component Ni-base system. The geometric method is also applicable to commercial Ni-base superalloys.

Dreshfield, R. L.↗
NASA NTRS1977

Estimation of conjugate gamma and gamma-prime compositions in Ni-base superalloys

To control the formation of unwanted phases, superalloy metallurgists have developed methods of estimating the composition of the matrix phase of alloys. That composition is then used to estimate the alloy's propensity toward sigma and other unwanted phase formations upon prolonged exposure to elevated temperatures in service. This paper reviews two approaches for estimating phase composition from the melt composition. One method is based on assigning essentially fixed stoichiometry to precipitating phases and is typified by 'PHACOMP'. The second method uses analytical geometry to interpret phase diagrams and is shown to be applicable to a two-phase region of a six-component Ni-base system. The geometric method is also shown to be applicable to commercial Ni-base superalloys.

Dreshfield, R. L.↗
NASA NTRS1986

The effect of Cr, Co, Al, Mo and Ta on a series of cast Ni-base superalloys on the stability of an aluminide coating during cyclic oxidation in Mach 0.3 burner rig

The influence of varying the content of Co, Cr, Mo, Ta, and Al in a series of cast Ni-based gamma/gamma'superalloys on the behavior of aluminide coatings was studied in burner rig cyclic oxidation tests at 1100 C. The alloys had nominally fixed levels of Ti, W, Cb, Zr, C, and B. The alloy compositions were based on a full 2(sup 5)-fractional statistical design supplemented by 10 star point alloys and a center point alloy. This full central composite design of 43 alloys plus two additional alloys with extreme Al levels allowed a complete second degree estimating equation to be derived from the 5-compositional variables. The weight change/time data for the coated samples fitted well to the paralinear oxidation model and enabled a modified oxidation attack parameter, K'(sub a) to be derived to rank the alloys and log K' (sub a ) to be used as the dependent variable in the estimating equation to determine the oxidation resistance of the coating as a function of the underlying alloy content. The most protective aluminide coatings are associated with the highest possible base ally contents of CR and Al and at a 4 percent Ta level. The Mo and Co effects interact but at fixed levels of 0, 5, or 10% Co. A 4% Mo level is optimum.

Zaplatynsky, I.↗
NASA NTRS2016

NASA/ORNL/AFRL Project Work on EBM LSHR: Additive Manufacturing of High-Temperature Gamma-Prime Strengthened Ni-Based Superalloys

Powder-bed fabrication of aerospace alloys may revolutionize production by eliminating the need for extensive machining and expensive tooling. Heated-bed electron-beam melting (EBM) offers advantages over non-heated laser additive manufacturing (AM) methods, including lower residual stress, reduced risk of contamination, slower cooling rates, and faster build times. NASA Glenn Research Center has joint project work with Oak Ridge National Lab and the Air Force Research Laboratory to explore the feasibility of fabricating advanced Ni-based gamma-prime superalloys with EBM AM.

additive manufacturing↗
NASA NTRS1983

The effect of microstructure on the fatigue behavior of Ni base superalloys

Nickel-base superalloys are used in jet engine components such as disks, turbine blades, and vanes. Improvements in the fatigue behavior will allow the life to be extended or the payloads to be increased. The first part of the present investigation deals primarily with the effects of microstructural variations on the fatigue crack propagation (FCP) behavior of nickel-base alloys, while the second part is concerned with low-cycle fatigue (LCF) behavior of Ni base systems. Waspaloy at low temperature is considered, taking into account material heat treatment and test procedures, a composite plot of Waspaloy FCP data, Paris law fatigue crack propagation constants, monotonic tensile data, and overload FCP test results for Waspaloy. It is found that the FCP and overload behavior of nickel-base alloys may be markedly improved by heat treating. Attention is given to effects of cyclic deformation on microstructure and substructure, environmental damage, and an environmental/deformation model of high temperature LCF.

Antolovich, S. D.↗