Timothy P. Gabb

The role of interfacial dislocation networks in high temperature creep of superalloys

Abstract The role of interfacial dislocation networks around the γ′ precipitates during the high temperature creep of nickel-base superalloys is unclear. The networks have been shown to continually evolve during creep at relatively low temperatures or eventually reach a more stable configuration at high temperatures. The objective of this study was to examine the role of these networks in several nickel-base superalloys during creep at temperatures where directional coarsening of the γ′ precipitate occurs. It was found that dislocations were not located at the γ′ interfaces which joined together during directional coarsening. The results of this study combined with previous findings suggest that the directional coarsening process is strongly influenced by elastic strain energy. The dislocation networks formed during primary creep were stable during all subsequent creep stages. Aspects of these dislocation networks were determined to be a product of both the applied creep stress and the coherency strains caused by γ-γ′ lattice mismatch, The influence of the applied stress was seen through the prominence of octahedral slip dislocations in the interfacial networks, and the effect of lattice mismatch was manifested through an inverse dependence between dislocation spacing and lattice mismatch.

Orientation and temperature dependence of some mechanical properties of the single-crystal nickel-base superalloy René N4: Part II. Low cycle fatigue behavior

Single crystal specimens of a nickel-base superalloy with axes near [001], [O1l], and [112] were tested in tension at room temperature, 760, and 980 °C. The alloy, René* N-4, was developed for gas turbine engine blades and has the nominal composition 3.7 Al, 4.2 Ti, 4 Ta, 0.5 Nb, 6 W, 1.5 Mo, 9 Cr, 7.5 Co, balance Ni, all in weight percent. Analysis of slip band traces, specimen axis rotation, and dislocation Burgers vectors showed that at 760 and 980 °C primary cube slip supplanted normal octahedral slip for the [112]-oriented specimens. The other two orientations, which have lower resolved shear stresses on the cube system, exhibited octahedral slip at all three temperatures. The critical resolved shear stress is considerably greater on the cube system than on the octahedral system at room temperature. However, at 760 and 980 °C the critical resolved shear stresses on the two systems are about the same. While the room temperature and 980 °C yield strengths for the two orientations exhibiting octahedral slip could be rationalized on the basis of resolved shear stress, those at 760 °C could not. Such violations of Schmid’s law have previously been observed in other super-alloys and single phase γ’.

Orientation and temperature dependence of some mechanical properties of the single-crystal nickel-base superalloy René N4: Part III. Tension-compression anisotropy

Single crystal superalloy specimens with various crystallographic orientations were tested in compression at room temperature, 650, 760, 870, and 980 °C. These results are compared with the tensile behavior studied previously. The alloy, René* N4, was developed for gas turbine engine blades and has the nominal composition 3.7 Al, 4.2 Ti, 4 Ta, 0.5 Nb, 6 W, 1.5 Mo, 9 Cr, 7.5 Co, balance Ni, in weight percent. Slip trace analysis showed that primary cube slip had occurred even at room temperature for the [111] specimens. With increasing test temperature more orientations exhibited primary cube slip, until at 870 °C only the orientations near [001] and [011] exhibited normal octahedral slip. The yield strength for octahedral slip was numerically analyzed using a model proposed by Lall, Chin, and Pope to explain deviations from Schmid’s law in the yielding behavior of a single phase γ’ alloy, Ni3(Al,Nb). The Schmid’s law deviations in René N4 were found to be largely due to a tension-compression anisotropy. This is one of the sources of the Schmid’s law violations observed in Ni3(Al, Nb) which are rationalized by the model. A second effect, which increases the strength of orientations away from [001], was found to be small in René N4. Analysis of recently published data on the single crystal superalloy PWA 1480 yielded the same result.

Case Studies of Fatigue Life Improvement Using Low Plasticity Burnishing in Gas Turbine Engine Applications

Surface enhancement technologies such as shot peening, laser shock peening (LSP), and low plasticity burnishing (LPB) can provide substantial fatigue life improvement. However, to be effective, the compressive residual stresses that increase fatigue strength must be retained in service. For successful integration into turbine design, the process must be affordable and compatible with the manufacturing environment. LPB provides thermally stable compression of comparable magnitude and even greater depth than other methods, and can be performed in conventional machine shop environments on CNC machine tools. LPB provides a means to extend the fatigue lives of both new and legacy aircraft engines and ground-based turbines. Improving fatigue performance by introducing deep stable layers of compressive residual stress avoids the generally cost prohibitive alternative of modifying either material or design.

Low cycle fatigue behaviour of a plasma-sprayed coating material

Single crystal nickel-base superalloys employed in turbine blade applications are often used with a plasma spray coating for oxidation and hot corrosion resistance. These coatings may also affect fatigue life of the superalloy substrate. As part of a large program to understand the fatigue behavior of coated single crystals, fully reversed, total strain controlled fatigue tests were run on a free standing NiCoCrAlY coating alloy, PWA 276, at 0.1 Hz. Fatigue tests were conducted at 650 C, where the NiCoCrAlY alloy has modest ductility, and at 1050 C, where it is extremely ductile, showing tensile elongation in excess of 100 percent. At the lower test temperature, deformation induced disordering softened the NiCoCrAlY alloy, while at the higher test temperature cyclic hardening was observed which was linked to gradual coarsening of the two phase microstructure. Fatigue life of the NiCoCrAlY alloy was significantly longer at the higher temperature. Further, the life of the NiCoCrAlY alloy exceeds that of coated, /001/-oriented PWA 1480 single crystals at 1050 C, but at 650 C the life of the coated crystal is greater than that of the NiCoCrAlY alloy on a total strain basis.

The Coming ICME Data Tsunami and What Can be Done

With the increased emphasis on reducing the cost and time to market of new materials the need for robust automated materials information management system(s) enabling sophisticated data mining tools is increasing, as evidenced by the emphasis on Integrated Computational Materials Engineering (ICME) and the recent establishment of the Materials Genome Initiative (MGI). Further, this need is also fueled by the demands for higher efficiency in material testing; consistency, quality and traceability of data; product design; engineering analysis; as well as control of access to proprietary or sensitive information. Further, the use of increasingly sophisticated nonlinear, anisotropic and/or multi-scale models requires both the processing of large volumes of test data and complex materials data necessary to establish processing-microstructure-property-performance relationships. Fortunately material information management systems have kept pace with the growing user demands and evolved to enable the: (i) capture of both point wise data and full spectrum of raw data curves, (ii) data management functions such as access, version, and quality controls; (iii) a wide range of data import, export and analysis capabilities; (iv) data “pedigree” traceability mechanisms; (v) data searching, reporting and viewing tools; and (vi) access to the information via a wide range of interfaces. This paper discusses key principles for the development of a robust materials information management system to enable the data mining of microstructure / property/ performance relationships critical to ICME. Furthermore NASA Glenn’s attempt at establishing such a database, for a model turbine disk Ni-based alloy (i.e., ME3) is articulated.

The low cycle fatigue deformation response of a single-crystal superalloy at 650 °C

Abstract The purpose of this study was to characterize the cyclic stress-strain response, the corresponding deformation structure and their relationships in the single crystal nickel-base superalloy PWA 1480. The isothermal low cycle fatigue response and deformation structures of specimens oriented near the [001], [ 2 5 20], [ 3 6 10], [011], [ 2 34] and [ 1 11] crystallographic orientations were characterized at an intermediate temperature (650 °C). The initial yield strength of all these specimens was controlled by the shearing of the γ′ precipitates by dislocation pairs. The low cycle fatigue tests exhibited cyclic hardening, which was associated with dislocation interactions in the γ matrix. In specimens deforming by slip on a single slip system, dislocations of the primary slip system accumulated in the γ matrix and formed sessile entanglements. In specimens deforming by slip on several slip systems, the dislocations of the different operative slip systems intersected in the γ matrix and formed sessile arrangements.

Impulse excitation study of elasticity of different precipitated microstructures in IN738LC at high temperatures

The elastic modulus of the cast superalloy IN738LC in various heat-treated conditions was determined with multiple specimens for each microstructure using the impulse excitation technique and the resonant frequencies while heating and cooling. Whereas the second and higher order harmonics were also excited in the high temperature range 700–1000 °C in 50 mm long specimens during controlled heating, analogous specimens 35 mm in length, impacted in similar fashion, did not excite the higher harmonics. Also, the 50 mm long specimens became excited and stayed in the second harmonic over broader temperature ranges during uncontrolled cooling inside the closed furnace. All precipitated conditions had nearly similar elastic data, varying from about 200 to 115 GPa, with small deviations, within 5%, found among multiple specimens of similar microstructures tested. Specimens with fine nano-size precipitates had a distinctly smaller rate of decrease in elastic modulus with increasing temperature, in contrast to a somewhat larger and nearly similar rate of decrease in specimens with coarse or medium-sized precipitates. This behavior is indicative of a larger average cohesive strength between the atoms and/or between the matrix and the precipitate particles in the former microstructure. The duplex size precipitate microstructure seemed to have both small and large drops in different specimens.

Fatigue-environment interactions in a SiC/Ti-15-3 composite

Abstract Load-controlled isothermal and non-isothermal fatigue lives of a [0°] 8 SiC/Ti-15-3 were evaluated at temperatures between 150 and 550 °C and a target strain range of about 0.45%. In non-isothermal fatigue tests, load was first cycled at minimum temperature and then temperature was cycled at zero load. For fatigue tests with peak temperatures at or above 300 °C, fatigue life was dramatically reduced compared to that at 150 °C. The shortest life was produced by the non-isothermal test with the greatest temperature range (Δ T = 400 °C) and highest peak temeperature ( T max = 550 °C). Vacuum testing showed that much of the life reduction under isothermal and non-isothermal conditions was related to environmental effects, although the nature of the fatigue-environment interaction was decidedly different for the isothermal and non-isothermal test cycles which were studied.

Creep Property Characterization of Potential Brayton Cycle Impeller and Duct Materials

This paper represents a status report documenting the work on creep of superalloys performed under Project Prometheus. Cast superalloys have potential applications in space as impellers within closed-loop Brayton cycle nuclear power generation systems. Likewise wrought superalloys are good candidates for ducts and heat exchangers transporting the inert working gas in a Brayton-based power plant. Two cast superalloys, Mar-M247LC and IN792, and a NASA GRC powder metallurgy superalloy, LSHR, are being screened to compare their respective capabilities for impeller applications. Several wrought superalloys including Hastelloy X, (Haynes International, Inc., Kokomo, IN), Inconel 617, Inconel 740, Nimonic 263, and Incoloy MA956 (Special Metals Corporation, Huntington, WV) are also being screened to compare their capabilities for duct applications. These proposed applications would require sufficient strength and creep resistance for long term service at temperatures up to 1200 K, with service times to 100,000 h or more. Conventional tensile and creep tests were performed at temperatures up to 1200 K on specimens extracted from the materials. Initial microstructure evaluations were also undertaken.