Engineering topics

Stephens, J. R.

Publications and source records attributed to Stephens, J. R..

At least 37 records · Page 2

NASA NTRS1988

Influence of alloying elements on the oxidation behavior of NbAl3

NbAL3 is one candidate material for advanced aeropropulsion systems because of its high melting point, low density, and good oxidation resistance. Although NbAl3 has the lowest oxidation rate among the binary Nb-Al alloys, it does not form exclusive layers of protective Al2O3 scales. Recently Perkin et al., have shown the feasibility of forming alumina scales on Nb-Al alloys at greatly reduced Al contents. However, the objective was to maintain the high Al content, and hence low density, while achieving the capability of growing protective alumina scales. Alloy development followed approaches similar to those used successfully for superalloys and oxidation resistant MCrAlY coatings. Among the three elements examined (Ti, Si, and Cr) as ternary additions to Nb-Al3, Cr was the most effective in favoring the selective oxidation of Al. Nb-41Al-8Cr formed exclusive layers of alumina and had a k sub p value of 0.22 mg squared/cm (sup 4)/hr at 1200 C. The addition of 1 wt percent Y to this alloy was also beneficial, resulting in nearly an order of magnitude decrease in K sub p at 1200 C. Further improvements were achieved by adding about 1 wt percent Si to the quaternary alloy. The k sub p value of 0.012 mg squared/cm (sup 4)/hr for Nb-40Al-8Cr-1Y-1Si at 1200 C was identical to the best NiAl + Zr alloys. These NbAl3 alloys also exhibited excellent cyclic oxidation resistance for 100 hr at 1200 C, being nearly equivalent to NiAl + Zr.

Hebsur, M. G.↗

NASA NTRS1987

B2 aluminides for high temperature applications

The structure, phase composition, and mechanical properties of B2 FeAl and NiAl aluminides are investigated. It is noted that, in both alloys, the room temperature brittleness is of primary concern. In the case of FeAl, significant tensile ductilities of up to 8 percent have been obtained at room temperature in iron-rich alloys. In the case of polycrystalline NiAl, attempts to improve ductility have not so far been successfull. Possible solutions to this problem are examined.

Vedula, K.↗

NASA NTRS1987

Powder processing of nickel and other aluminides by hot consolidation

Alloys based on FeAl, NiAl, and Ni3Al have been processed by extrusion of powders over a range of compositions, powder characteristics, and extrusion parameters. It is found that extrusion leads to dynamic recrystallization and that the grain size of the extrusions depends on the powder size, extrusion temperature, and extrusion ratio. Prior particle boundary oxides play an important role in controlling the grain size and uniformity by acting as obstacles to grain growth. Hot isostatic pressing and vacuum hot pressing can be used for powder densification, but prior particle oxides are not effectively broken up due to insufficient shearing deformation.

Vedula, K.↗

NASA NTRS1986

High-Temperature Alloys for Automotive Stirling Engines

Stirling engine is external-combustion engine that offers fuel economy, low emissions, low noise, and low vibrations. One of most critical areas in engine development concerns material selection for component parts. Alloys CG-27 and XF-818 identified capable of withstanding rigorous requirements of automotive Stirling engine. Alloys chosen for availability, performance, and manufacturability. Advanced iron-base alloys have potential for variety of applications, including stationary solar-power systems.

Stephens, J. R.↗

NASA NTRS1986

Fiber-reinforced superalloy composites provide an added performance edge

Fiber reinforcements are being explored as a means to increasing the performance of superalloys past 980 C. Fiber-reinforced superalloys (FRS), particularly tungsten FRS (TFRS) are candidate materials for rocket-engine turbopump blades for advanced Shuttle engines and in airbreathing and other rocket engines. Refractory metal wires are the reinforcement of choice due to tolerance to fiber/matrix interactions. W alloy fibers have a maximum tensile strength of 2165 MPa at 1095 C and a 100 hr creep rupture strength at stresses up to 1400 MPa. A TFRS has the potential of a service temperature 110 C over the strongest superalloy. Manufacturing processes being evaluated to realize the FRS components are summarized, together with design features which will be introduced in turbine blades to take advantage of the FRS materials and to extend their surface life.

Petrasek, D. W.↗

NASA NTRS1986

Thermal aging effects in refractory metal alloys

The alloys of niobium and tantalum are attractive from a strength and compatibility viewpoint for high operating temperatures required in materials for fuel cladding, liquid metal transfer, and heat pipe applications in space power systems that will supply from 100 kWe to multi-megawatts for advanced space systems. To meet the system requirements, operating temperatures ranging from 1100 to 1600 K have been proposed. Expected lives of these space power systems are from 7 to 10 yr. A program is conducted at NASA Lewis to determine the effects of long-term, high-temperature exposure on the microstructural stability of several commercial tantalum and niobium alloys. Variables studied in the investigation include alloy composition, pre-age annealing temperature, aging time, temperature, and environment (lithium or vacuum), welding, and hydrogen doping. Alloys are investigated by means of cryogenic bend tests and tensile tests. Results show that the combination of tungsten and hafnium or zirconium found in commercial alloys such as T-111 and Cb-752 can lead to aging embrittlement and increased susceptibility to hydrogen embrittlement of ternary and more complex alloys. Modification of alloy composition helps to eliminate the embrittlement problem.

Stephens, J. R.↗

NASA NTRS1986

Alloy chemistry and microstructural control to meet the demands of the automotive Stirling engine

The automotive Stirling engine now under development by DOE/NASA as an alternative to the internal combustion engine, imposes severe materials requirements for the hot portion of the engine. Materials selected must be low cost and contain a minimum of strategic elements so that availability is not a problem. Heater head tubes contain high pressure hydrogen on the inside and are exposed to hot combustion gases on the outside surface. The cylinders and regenerator housings must be readily castable into complex shapes having varying wall thicknesses and be amenable to brazing and welding operations. Also, high strength, oxidation resistance, resistance to hydrogen permeation, cyclic operation, and long-life are required. A research program conducted by NASA Lewis focused on alloy chemistry and microstructural control to achieve the desired properties over the life of the engine. Results of alloy selection, characterization, evaluation, and actual engine testing of selected materials are presented.

Stephens, J. R.↗

NASA NTRS1986

Preparation, analysis and release of simulated interplanetary grains into low Earth orbit

Astronomical observations which reflect the optical and dynamical properties of interstellar and interplanetary grains are the primary means of identifying the shape, size, and the chemistry of extraterrestrial grain materials. Except for recent samplings of extraterrestrial particles in near-Earth orbit and in the stratosphere observations were the only method of deducing the properties of extraterrestrial particles. In order to elucidate the detailed characteristics of observed dust, the observations must be compared with theoretical studies, some of which are discussed in this volume, or compared with terrestrial laboratory experiments. The formation and optical characterization of simulated interstellar and interplanetary dust with particular emphasis on studying the properties on irregularly shaped particles were discussed. Efforts to develop the techniques to allow dust experiments to be carried out in low-Earth orbit were discussed, thus extending the conditions under which dust experiments may be performed.

Stephens, J. R.↗

NASA NTRS1985

Creep rupture behavior of Stirling engine materials

The automotive Stirling engine, being investigated jointly by the Department of Energy and NASA Lewis as an alternate to the internal combustion engine, uses high-pressure hydrogen as the working fluid. The long-term effects of hydrogen on the high temperature strength properties of materials is relatively unknown. This is especially true for the newly developed low-cost iron base alloy NASAUT 4G-A1. This iron-base alloy when tested in air has creep-rupture strengths in the directionally solidified condition comparable to the cobalt base alloy HS-31. The equiaxed (investment cast) NASAUT 4G-A1 has superior creep-rupture to the equiaxed iron-base alloy XF-818 both in air and 15 MPa hydrogen.

Titran, R. H.↗

NASA NTRS1985

The B2 aluminides as alternative materials

The potential of the B2 aluminides as structural material alternatives for the strategic element containing superalloys currently used in gas turbine engines is being explored with emphasis on the equiatomic Fe and Ni aluminides. Although Co is a strategic material, the equiatomic Co aluminide is also being studied to gain a more complete understanding of these fourth period intermetallics. Research focuses on initial processing techniques such as ingot melting, power metallurgy, and rapid solidification with and without additional thermomechanical processing; high temperature deformation primarily compressive creep; compositional effects within the binary B2 aluminides; third-element alloying addition effects on high temperature strength and oxidation resistance, and near room temperature ductility as influenced by processing, alloying, and grain size. Various programs now underway are reviewed and some highlights of research results are presented.

Stephens, J. R.↗

NASA NTRS1984

Oxidation and corrosion resistance of candidate Stirling engine heater-head-tube alloys

Sixteen candidate iron base Stirling engine heater head tube alloys are evaluated in a diesel fuel fired simulator materials test rig to determine their oxidation and corrosion resistance. Sheet specimens are tested at 820 C for 3500 hr in 5 hr heating cycles. Specific weight change data and an attack parameter are used to categorize the alloys into four groups; 10 alloys show excellent for good oxidation and corrosion resistance and six alloys exhibit poor or catastrophic resistance. Metallographic, X-ray, and electron microprobe analyses aid in further characterizing the oxidation and corrosion behavior of the alloys. Alloy compositions, expecially the reactive elements aluminum, titanium, and chromium, play a major role in the excellent oxidation and corrosion behavior of the alloys. The best oxidation resistance is associated with the formation of an iron nickel aluminum outer oxide scale, an intermediate oxide scale rich in chromium and titanium, and an aluminum outer oxide scale adjacent to the metallic substrate, which exhibits a zone of internal oxidation of aluminum and to some extent titanium.

Stephens, J. R.↗

NASA NTRS1984

Advanced high temperature materials for the energy efficient automotive Stirling engine

The Stirling Engine is under investigated jointly by the Department of Energy and NASA Lewis as an alternative to the internal combustion engine for automotive applications. The Stirling Engine is an external combustion engine that offers the advantage of high fuel economy, low emissions, low noise, and low vibrations compared to current internal combustion automotive engines. The most critical component from a materials viewpoint is the heater head consisting of the cylinders, heating tubes, and regenerator housing. Materials requirements for the heater head include compatibility with hydrogen, resistance to hydrogen permeation, high temperature oxidation/corrosion resistance and high temperature creep-rupture and fatigue properties. A continuing supporting materials research and technology program has identified the wrought alloys CG-27 and 12RN72 and the cast alloys XF-818 and NASAUT 4G-A1 as candidate replacements for the cobalt containing alloys used in current prototype engines. Based on the materials research program in support of the automotive Stirling engine it is concluded that manufacture of the engine is feasible from low cost iron-base alloys rather than the cobalt alloys rather than the cobalt alloys used in prototype engines. This paper will present results of research that led to this conclusion.

Titran, R. H.↗

NASA NTRS1984

Stirling material technology

The Stirling engine is an external combustion engine that offers the advantage of high fuel economy, low emissions, low noise, and low vibrations compared to current internal combustion automotive engines. The most critical component from a materials viewpoint is the heater head consisting of the cylinders, heating tubes, and regenerator housing. Materials requirements for the heater head include compatibility with hydrogen, resistance to hydrogen permeation, high temperature oxidation/corrosion resistance, and high temperature creep-rupture and fatigue properties. A materials research and technology program identified the wrought alloys CG-27 and 12RN72 and the cast alloys XF-818, NASAUT 4G-A1, and NASACC-1 as candidate replacements for the cobalt containing alloys used in current prototype engines. It is concluded that manufacture of the engine is feasible from low cost iron-base alloys rather than the cobalt alloys used in prototype engines. Results of research that lead to this conclusion are presented.

Titran, R. H.↗

NASA NTRS1984

Homogeneous condensation of gaseous mixtures of Si, Fe, O, N, and C in relative cosmic abundance and implications for astronomical condensation

The pressure versus temperature curves for homogeneous nucleation and condensation of two gaseous mixtures with nearly relative solar abundance of Si, Fe, O, N, and C in an excess of H were determined experimentally. Mixtures of CO, Fe(CO)5, H2, SiH4, and N2O in Ar were heated behind reflected shocks in a shock tube. The nucleation and condensation, which took place in the subsequent gas-dynamic expansion (cooling phase), was monitored by light scattering and turbidity. Grain morphologies and crystalline phases present in the condensates were determined by electron microscopy. These data cast doubt on the validity of both equilibrium and classical nucleation theoretical approaches to predict homogeneous condensation in a solar nebula or stellar atmosphere.

Stephens, J. R.↗

NASA NTRS1984

The B2 aluminides as alternative materials

The potential of the B2 aluminides as structural material alternatives for the strategic element containing superalloys currently used in gas turbine engines is being explored with emphasis on the equiatomic Fe and Ni aluminides. Although Co is a strategic material, the equiatomic Co aluminide is also being studied to gain a more complete understanding of these fourth period intermetallics. Research focuses on initial processing techniques such as ingot melting, power metallurgy, and rapid solidification with and without additional thermomechanical processing; high temperature deformation - primarily compressive creep; compositional effects within the binary B2 aluminides; third-element alloying addition effects on high temperature strength and oxidation resistance, and near room temperature ductility as influenced by processing, alloying, and grain size. Various programs now underway are reviewed and some highlights of research results are presented.

Stephens, J. R.↗

NASA NTRS1983

Replacing critical and strategic refractory metal elements in nickel-base superalloys

Because of the import status and essential nature of their use, cobalt, chromium, tantalum, and niobium were identified as strategic and critical in the aerospace industry. NASA's Conservation of Strategic Aerospace Materials (COSAM) program aims to reduce the need for strategic materials used in gas turbine engines. Technological thrusts in two major areas are under way to meet the primary objective of conserving the use of strategic materials in nickelbase superalloys. These thrusts consist of strategic element substitution and alternative material identification. The program emphasizes cooperative research teams involving NASA Lewis Research Center, universities, and industry. The adoption of refractory metals in nickel-base superalloys is summarized including their roles in mechanical strengthening and environmental resistance; current research activities under way in the COSAM Program are presented as well as research findings to date.

Stephens, J. R.↗

NASA NTRS1983

Development of tough, strong, iron-base alloy for cryogenic applications

The development of an iron-base alloy that combines the normally divergent properties of high toughness and high strength at cryogenic temperatures is discussed. Specifically, alloy properties were sought which at -196 C would exhibit a fracture toughness of 220 MPa-m(1/2) with a corresponding yield strength of 1.4 GPa (200 ksi). Early work showed that high toughness could be achieved in Fe-12Ni alloys containing reactive metal additions such as Al, Nb, Ti, and V. Further research emphasized strengthening of these tough alloys by thermomechanical processing and the addition of Cu. Results showed that high strength and high toughness could be achieved in a single alloy at temperatures as low as -196 C. An alloy with composition Fe-12Ni-9.5Al-2Cu exhibited a yield strength of 1.65 GPa with a corresponding fracture toughness of 220 MPa-m(1/2) at -196 C. Strengthening due to Cu additions to the Fe-12Ni base alloys results primarily from precipitation of Cu-rich epsilon particles approximately 20 nm in diameter. Strengthening mechanisms are discussed in terms of an elastic modulus hardening model and are supported by transimission electron microscopy examinations of selected test specimens.

Stephens, J. R.↗

NASA NTRS1983

Considerations of technology transfer barriers in the modification of strategic superalloys for aircraft turbine engines

A typical innovation-to-commercialization process for the development of a new hot section gas turbine material requires one to two decades with attendant costs in the tens of millions of dollars. This transfer process is examined to determine the potential rate-controlling steps for introduction of future low strategic metal content alloys or processes. Case studies are used to highlight the barriers to commercialization as well as to identify the means by which these barriers can be surmounted. The opportunities for continuing joint government-university-industry partnerships in planning and conducting strategic materials R&D programs are also discussed.

Stephens, J. R.↗