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All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Development and Processing of Nickel Aluminide-Carbide Alloys

With the upper temperature limit of the Ni-based superalloys attained, a new class of materials is required. Intermetallics appear as likely candidates because of their attractive physical properties. With a relatively low density, high thermal conductivity, excellent oxidation resistance, high melting point, and simple crystal structure, nickel aluminide (NiAl) appears to be a potential candidate. However, NiAl is limited in structural applications due to its low room temperature fracture toughness and poor elevated temperature strength. One approach to improving these properties has been through the application of eutectic composites. Researchers have shown that containerless directional solidification of NiAl-based eutectic alloys can provide improvement in both the creep strength and fracture toughness. Although these systems have shown improvements in the mechanical properties, the presence of refractory metals increases the density significantly in some alloys. Lower density systems, such as the carbides, nitrides, and borides, may provide NiAl-based eutectic structure. With little or no information available on these systems, experimental investigation is required. The objective of this research was to locate and develop NiAl-carbide eutectic alloys. Exploratory arc-melts were performed in NiAl-refractory metal-C systems. Refractory metal systems investigated included Co, Cr, Fe, Hf, Mo, Nb, Ta, Ti, W, and Zr. Systems containing carbides with excellent stability (i.e.,HfC, NbC, TaC, TiC, and ZrC) produced large blocky cubic carbides in an NiAl matrix. The carbides appeared to have formed in the liquid state and were randomly distributed throughout the polycrystalline NiAl. The Co, Cr, Fe, Mo, and W systems contained NiAl dendrites with a two-phase interdendritic microconstituent present. Of these systems, the NiAl-Mo-C system had the most promising microstructure for in-situ composites. Three processing techniques were used to evaluate the NiAl-Mo-C system: arc-melting, slow cooling, and containerless directional solidification. Arc-melting provided a wide range of compositions in an economical and simple fashion. The slow cooled ingots provided larger ingots and slower cooling rates than arc-melting. Directional solidification was used to produce in-situ composites consisting of NiAl reinforced with molybdenum carbides.

Newport, Timothy Scott↗
NASA NTRS1994

Scanning and Transmission Electron Microscopy of High Temperature Materials

Software and hardware updates to further extend the capability of the electron microscope were carried out. A range of materials such as intermetallics, metal-matrix composites, ceramic-matrix composites, ceramics and intermetallic compounds, based on refractory elements were examined under this research. Crystal structure, size, shape and volume fraction distribution of various phases which constitute the microstructures were examined. Deformed materials were studied to understand the effect of interfacial microstructure on the deformation and fracture behavior of these materials. Specimens tested for a range of mechanical property requirements, such as stress rupture, creep, low cycle fatigue, high cycle fatigue, thermomechanical fatigue, etc. were examined. Microstructural and microchemical stability of these materials exposed to simulated operating environments were investigated. The EOIM Shuttle post-flight samples were also examined to understand the influence of low gravity processing on microstructure. In addition, fractographic analyses of Nb-Zr-W, titanium aluminide, molybdenum silicide and silicon carbide samples were carried out. Extensive characterization of sapphire fibers in the fiber-reinforced composites made by powder cloth processing was made. Finally, pressure infiltration casting of metal-matrix composites was carried out.

Source record↗
NASA NTRS1996

Mechanical and Thermal Properties of Two Cu-Cr-Nb Alloys and NARloy-Z

A series of creep tests were conducted on Cu-8 Cr-4 Nb (Cu-8 at.% Cr-4 at.% Nb), Cu-4 Cr-2 Nb (Cu-4 at.% Cr-2 at% Nb), and NARloy-Z (Cu-3 wt.% Ag-0.5 wt.% Zr) samples to determine their creep properties. In addition, a limited number of low cycle fatigue and thermal conductivity tests were conducted. The Cu-Cr-Nb alloys showed a clear advantage in creep life and sustainable load over the currently used NARloy-Z. Increases in life at a given stress were between 100% and 250% greater for the Cu-Cr-Nb alloys depending on the stress and temperature. For a given life, the Cu-Cr-Nb alloys could support a stress between 60% and 160% greater than NARloy-Z. Low cycle fatigue lives of the Cu-8 Cr-4 Nb alloy were equivalent to NARloy-Z at room temperature. At elevated temperatures (538 C and 650 C), the fatigue lives were 50% to 200% longer than NARloy-Z samples tested at 538 C. The thermal conductivities of the Cu-Cr-Nb alloys remained high, but were lower than NARloy-Z and pure Cu. The Cu-Cr-Nb thermal conductivities were between 72% and 96% that of pure Cu with the Cu-4 Cr-2 Nb alloy having a significant advantage in thermal conductivity over Cu-8 Cr4 Nb. In comparison, stainless steels with equivalent strengths would have thermal conductivities less than 25% the thermal conductivity of pure Cu. The combined results indicate that the Cu-Cr-Nb alloys offer an attractive alternative to current high temperature Cu-based alloys such as NARloy-Z.

Ellis, David L.↗
NASA NTRS1996

The Unusual S Star Binary HD 191589

Recently, we discovered with International Ultraviolet Explorer (IUE) an F0-F2 IV-V companion to the T(sub c)-deficient S star HD 191589. If the magnitude difference is (delta)V=3.7, as indicated by several arguments, and E(B-V) = 0.0, we obtain a value of M(sub v)= - 1.5 +/- 0.4 for the Peculiar Red Giant (PRG), too faint for it to be a thermally-pulsing asymptotic giant branch star. According to the binary mass-transfer hypothesis for T(sub c)-deficient PRG's, a white dwarf must be the source of the s-process enhancement of the current primary star, but it cannot be seen because of the presence of the secondary. If such is the case, the F-star companion may also have been contaminated by s-process material. High-dispersion IUE observations indicate an enhancement of Zr II in the photosphere of the F-star as well. Thus, HD 191589 is likely a triple system, where what was once the most massive component of the system has polluted both of its companions with s-process material. One of these is the current S star, while the other is the companion still near the main sequence.

Ake, Thomas B.↗
NASA NTRS1996

Trace Element Zoning and Incipient Metamictization in a Lunar Zircon: Application of Three Microprobe Techniques

We have determined major (Si, Zr, Hf), minor (Al, Y, Fe, P), and trace element (Ca, Sc, Ti, Ba, REE, Th, U) concentrations and Raman spectra of a zoned, 200 microns zircon grain in lunar sample 14161,7069, a quartz monzodiorite breccia collected at the Apollo 14 site. Analyses were obtained on a thin section in situ with an ion microprobe, an electron microprobe, and a laser Raman microprobe. The zircon grain is optically zoned in birefringence, a reflection of variable (incomplete) metamictization resulting from zo- nation in U and Th concentrations. Variations in the concentrations of U and Th correlate strongly with those of other high-field-strength trace elements and with changes in Raman spectral parameters. Concentrations of U and Th range from 21 to 55 ppm and 6 to 31 ppm, respectively, and correlate with lower Raman peak intensities, wider Raman peaks, and shifted Si-O peak positions. Concentrations of heavy rare earth elements range over a factor of three to four and correlate with intensities of fluorescence peaks. Correlated variations in trace element concentrations reflect the original magmatic differentiation of the parental melt approx. 4 b.y. ago. Degradation of the zircon structure, as reflected by the observed Raman spectral parameters, has occurred in this sample over a range of alpha-decay event dose from approx. 5.2 x 10(exp 14) to 1.4 x 10(exp 15) decay events per milligram of zircon, as calculated from the U and Th concentrations. This dose is well below the approx. 10(exp 16) events per milligram cumulative dose that causes complete metamictization and indicates that laser Raman microprobe spectroscopy is an analytical technique that is very sensitive to the radiation-induced damage in zircon.

Wopenka, Brigitte↗
NASA NTRS1960

Oxidation Behavior of Binary Niobium Alloys

This investigation concludes a study to determine the effects of up to 25 atomic percent of 55 alloying additions on the oxidation characteristics of niobium. The alloys were evaluated by oxidizing in an air atmosphere for 4 hours at 1000 C and 2 hours at 1200 C. Titanium and chromium improved oxidation resistance at both evaluation conditions. Vanadium and aluminum improved oxidation resistance at 1000 C, even though the V scale tended to liquefy and the Al specimens became brittle and the scale powdery. Copper, cobalt, iron, and iridium improved oxidation resistance at 1200 C. Other investigations report tungsten and molybdenum are protective up to about 1000 C, and tantalum at 1100 C. The most important factor influencing the rate of oxidation was the ion size of the alloy additions. Ions slightly smaller than the Nb(5+) ion are soluble in the oxide lattice and tend to lower the compressive stresses in the bulk scale that lead to cracking. The solubility of the alloying addition also depends on the valence to some extent. All of the elements mentioned that improve the oxidation resistance of Nb fit this size criterion with the possible exception of Al, whose extremely small size in large concentrations would probably lead to the formation of a powdery scale. Maintenance of a crack-free bulk scale for as long as possible may contribute to the formation of a dark subscale that ultimately is rate- controlling in the oxidation process. The platinum-group metals, especially Ir, appear to protect by entrapment of the finely dispersed alloying element by the incoming Nb2O5 metal-oxide interface. This inert metallic Ir when alloyed in a sufficient amount with Yb appears to give a ductile phase dispersed in the brittle oxide. This scale would then flow more easily to relieve the large compressive stresses to delay cracking. Complex oxide formation (which both Ti and Zr tend to initiate) and valence effects, which probably change the vacancy concentration in the scale, are masked by the overriding tendency for a porous scale.

Barrett, Charles A.↗
NASA NTRS1998

Design Considerations for Lightweight Space Radiators Based on Fabrication and Test Experience with a Carbon-Carbon Composite Prototype Heat Pipe

This report discusses the design implications for spacecraft radiators made possible by the successful fabrication and Proof-of-concept testing of a graphite-fiber-carbon-matrix composite (i.e., carbon-carbon (C-C)) heat pipe. The proto-type heat pipe, or space radiator element, consists of a C-C composite shell with integrally woven fins. It has a thin-walled furnace-brazed metallic (Nb-1%Zr) liner with end caps for containment of the potassium working fluid. A short extension of this liner, at increased wall thickness beyond the C-C shell, forms the heat pipe evaporator section which is in thermal contact with the radiator fluid that needs to be cooled. From geometric and thermal transport properties of the C-C composite heat pipe tested, a specific radiator mass of 1.45 kg/m2 can be derived. This is less than one-fourth the specific mass of present day satellite radiators. The report also discusses the advantage of segmented space radiator designs utilizing heat pipe elements, or segments, in their survivability to micro-meteoroid damage. This survivability is further raised by the use of condenser sections with attached fins, which also improve the radiation heat transfer rate. Since the problem of heat radiation from a fin does not lend itself to a closed analytical solution, a derivation of the governing differential equation and boundary conditions is given in appendix A, along with solutions for rectangular and parabolic fin profile geometries obtained by use of a finite difference computer code written by the author.

Juhasz, Albert J.↗
NASA NTRS1995

Constant-Pressure Specific Heat to Hemispherical Total Emissivity Ratio for Undercooled Liquid Nickel, Zirconium, and Silicon

Radiative cooling curves of nickel, zirconium, and silicon melts that were obtained using the high-temperature, high-vacuum electrostatic levitator (HTHVESL) have been analyzed to determine the ratio between the constant-pressure specific heat and the hemispherical total emissivity, c(sub p)(T)epsilon(sub T)(T). This ratio determined over a wide liquid temperature range for each material allows us to determine c(sub p)(T) if epsilon(sub T)(T) is known or vice versa. Following the recipe, the hemi-spherical total emissivities for each sample at its melting temperature, epsilon(sub T)(T(sub M)), have been determined using c(sub p)(T(sub m)) values available in the literature. They are 0.15, 0.29, and 0.17, for Ni, Zr, and Si, respectively.

Rulison, Aaron J.↗
NASA NTRS1998

Modeling of Substitutional Site Preference in Ordered Intermetallic Alloys

We investigate the site substitution scheme of specific alloying elements in ordered compounds and the dependence of site occupancy on compound stoichiometry, alloy concentration. This basic knowledge, and the interactions with other alloying additions are necessary in order to predict and understand the effect of various alloying schemes on the physical properties of a material, its response to various temperature treatments, and the resulting mechanical properties. Many theoretical methods can provide useful but limited insight in this area, since most techniques suffer from constraints in the type of elements and the crystallographic structures that can be modeled. With this in mind, the Bozzolo-Ferrante-Smith (BFS) method for alloys was designed to overcome these limitations, with the intent of providing an useful tool for the theoretical prediction of fundamental properties and structure of complex systems. After a brief description of the BFS method, its use for the determination of site substitution schemes for individual as well as collective alloying additions to intermetallic systems is described, including results for the concentration dependence of the lattice parameter. Focusing on B2 NiAl, FeAl and CoAl alloys, the energetics of Si, Ti, V, Cr, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Hf, Ta and W alloying additions are surveyed. The effect of single additions as well as the result of two simultaneous additions, discussing the interaction between additions and their influence on site preference schemes is considered. Finally, the BFS analysis is extended to ternary L1(sub 2) (Heusler phase) alloys. A comparison between experimental and theoretical results for the limited number of cases for which experimental data is available is also included.

Bozzolo, Guillermo↗
NASA NTRS1996

Diogenites: Cumulates from Asteroid 4 Vesta: Insights from Orthopyroxene and Spinel Chemistry

Cumulate rocks are important planetary lithologies, but they can be difficult to interpret. Important clues to the nature of their parental melts may still be present in the interiors of cumulus phases. However, in some cases, even the cores of the cumulus grains may have been modified by postcrystallization reactions with trapped melt and other cumulus phases. We have previously studied the major-, minor-, and trace-element chemistry of orthopyroxene from a suite of diogenites and concluded that their chemical attributes can best be explained by crystallization from parental melts that were derived from a depleted mantle source that had already experienced eucrite removal. However, we and others have had difficulty explaining the great range in concentration of minor elements (Al, Ti) and trace elements (REE, Y, Zr) if all diogenites were derived from a single magmatic system. Therefore, we have investigated the chemistry of diogenitic spinels to see if they still held clues to the diogenite parental melt compositions. Although spinel is low in abundance in diogenites (<5 vol%) it still may hold clues to the maomatic and metamorphic history of these rocks.

Papike, James J.↗
NASA NTRS1999

Time Temperature-Precipitation Behavior in An Al-Cu-Li Alloy 2195

Al-Cu-Li alloy 2195, with its combination of good cryogenic properties, low density, and high modulus, has been selected by NASA to be the main structural alloy of the Super Light Weight Tank (SLWT) for the Space Shuttle. Alloy 2195 is strengthened by an aging treatment that precipitates a particular precipitate, labeled as T1(Al2CuLi). Other phases, such as GP zone, (theta)', (theta)", theta, (delta)', S' are also present in this alloy when artificially aged. Cryogenic strength and fracture toughness are critical to the -SLWT application, since the SLWT will house liquid oxygen and hydrogen. Motivation for the Time-Temperature-Precipitation (TTP) study at lower temperature (lower than 350 F) comes in part from a recent study by Chen, The study found that the cryogenic fracture toughness of alloy 2195 is greatly influenced by the phases present in the matrix and subgrain boundaries. Therefore, the understanding of TTP behavior can help develop a guideline to select appropriate heat treatment conditions for the desirable applications. The study of TTP behavior at higher temperature (400 to 1000 F) was prompted by the fact that the SLWT requires a welded construction. Heat conduction from the weld pool affects the microstructure in the heat-affected zone (HAZ), which leads to changes in the mechanical properties. Furthermore, the SLWT may need repair welding for more than one time and any additional thermal cycles will increase precipitate instability and promote phase transformation. As a result considerable changes in HAZ microstructure and mechanical properties are expected during the construction of the SLWT. Therefore, the TTP diagrams can serve to understand the thermal history of the alloy by analyzing the welded microstructure. In the case welding, the effects of thermal cycles on the microstructure and mechanical properties can be predicted with the aid of the TTP diagrams. The 2195 alloy (nominally Al + 4 pct Cu + 1 pct Li + 0.3 pct Ag + 0.3 pct Mg + 0.1 pct Zr) used in this study was received in the form of 1.7 inch thick rolled plates. In brief goal of this study is to develop TTR diagram for the solutiontreated and stretched alloy 2195 from which the precipitation sequence at any service or aging conditions can be preducted.A subgrain boundary TTR diagram is also presented since precipitations at subgrain boundary can control and/or modify a wide veriety of material properties, such as cryogenic fracture toughness.

Chen, P. S.↗
NASA NTRS1997

Microstructure and Mechanical Properties of Vacuum Plasma Sprayed Cu-8Cr-4Nb

This paper compares the tensile properties of Cu-8Cr-4Nb material produced by VPS to material previously produced by extrusion. The microstructure of the VPS material is also presented. The combustion chamber liner of rocket motors represents an extreme materials application. The liner hot wall is exposed to a 2760 C (5000 F) flame while the cold side is exposed to cryogenic hydrogen liquid. Materials for use in the combustion chamber liner require a combination of high temperature strength, creep resistance, and low cycle fatigue resistance along with high thermal conductivity. The hot side is also subject to localized cycles between reducing and oxidizing environments that degrade the liner by a process called blanching. A new Cu-8 at.% Cr-4 at% Nb (Cu-8Cr-4Nb) alloy has been developed at NASA Lewis Research Center as a replacement for the currently used alloy, NARloy-z (Cu-3 wt.% Ag-0.5 wt.% Zr). The alloy is strengthened by a fine dispersion of Cr2Nb particles. The alloy has better mechanical properties than NARloy-Z while retaining most of the thermal conductivity of pure copper. The alloy has been successfully consolidated by extrusion and hot isostatic pressing (HIPing). However, vacuum plasma spraying (VPS) offers several advantages over prior consolidation methods. VPS can produce a near net shape piece with the profile of the liner. In addition, oxidation resistant and thermal barrier coatings can be incorporated as an integral part of the liner hot wall during the VPS deposition. The low oxygen VPS Cu-8Cr-4Nb exhibits a higher strength than Cu-8Cr-4Nb produced by extrusion at elevated temperatures and a comparable strength at room temperature. Moduli and ductility were not significantly different. However, the ability to produce parts to near-net shape and maintain the good elevated temperature tensile properties of the extruded Cu-8Cr-4Nb makes VPS an attractive processing method for fabricating rocket engine combustion liners.

Holmes, Richard↗
NASA NTRS2000

Reduction of Sample Rotation in Electrostatic Levitation

In many containerless processing systems, control of sample rotation is an important issue. Sample rotation is even more important for microgravity containerless processing systems, where the centrifugal acceleration can approach 1 g for even a small rotation rate. Prior work on rotation control by Rhim focused on driving the sample rotation at a controlled rate for droplet dynamics experiments and measurement of electrical conductivity. His technique allows controlled, fast rotation, but for many microgravity experiments the goal is zero rotation. To minimize sample rotation, two approaches are apparent: first, to identify and balance or eliminate the driving forces for undesired sample rotation, or second, implement a feedback-based rotation control loop in parallel with the position control loop. In this work, we have taken the first approach. To minimize sample rotation, the simplest approach is to identify and balance or eliminate the driving forces for undesired sample rotation. Our experiments show that the dominant driving force for rotation of machined Zr spheres in the MSFC ESL is photon pressure from the heating laser. Experimental results showing the correlation between heating power and torque are compared to theoretical predictions, and a strategy for minimizing the torque due to photon pressure is presented.

Hyers, R. W↗
NASA NTRS1998

Raman and Photoluminescence Spectroscopy of Nano-crystalline PbTiO3 Sensor Materials with Different Doping Ions

Raman spectra & photoluminescence studies in PbTiO3, have been carried out, as a function of particle size, temperature, pressure and dopants. There appears respectively a distinct temperature-induced soft mode phase transition in each sample whose Curie temperature can be determined from the mean-field theory. The detailed Curie temperature shift in the modified PbTiO3 ceramics by Ba, Sr, La and Zr, has been investigated as a function of particle size. Pressure-induced phase transitions display an obvious diffuse behavior. Room temperature photoluminescence for nanocrystalline Ba(1-x)Pb(x)TiO3 have been observed. These studies favor preparations of high efficiency PbTiO3 sensors.

Katiyar, R. S.↗
NASA NTRS2000

Rotating Molten Metallic Drops and Related Phenomena: A New Approach to the Surface Tension Measurement

Molten aluminum and tin drops were levitated in a high vacuum by controlled electric fields, and they were systematically rotated by applying by a rotating magnetic field. When the evolution of the drop shape was measured as a function of rotation frequency, it agreed quantitatively well with the Brown and Scriven's theoretical prediction. The normalized rotation frequencies at the bifurcation point agreed with the predicted value 0.559, within 2%. An anomalous phenomenon which totally deviated from the prediction was observed in rotating molten tin drops when they were kept in a high rotation rate for several hours. No anomaly was observed in aluminum drops when they underwent similar condition. It was speculated that under the strong centrifugal force in the drop the tin isotopes must be separating. Since Al-27 is essentially the only naturally abundant isotope in the aluminum drops, the same anomaly is not expected. Based on the shape deformation of a rotating drop, an alternate approach to the surface tension measurement was verified. This new surface tension measurement technique was applied to a glassforming alloy, Zr(41.2)Ti(13.8)Cu(12.5)Ni(10.0)Be(22.5) in its highly viscous states. Also demonstrated in the paper was a use of a molten aluminum drop to verify the Busse's prediction of the influence of the drop rotation on the drop oscillation frequency.

Rhim, Won-Kyu↗
NASA NTRS2000

Thermophysical Properties of GRCop-84

The thermophysical properties and electrical resistivity of GRCop-84 (Cu - 8 at.% Cr-4 at.% Nb) were measured from cryogenic temperatures to near its melting point. The data were analyzed using weighted regression to determine the properties as a function of temperature and assign appropriate confidence intervals. The results showed that the thermal expansion of GRCop-84 was significantly lower than NARloy-Z (Cu-3 wt. % Ag-0.5 wt. % Zr), the currently used thrust cell liner material. The lower thermal expansion is expected to translate into lower thermally induced stresses and increases in thrust cell liner lives between 2X and 41X over NARloy-Z. The somewhat lower thermal conductivity of GRCop-84 can be offset by redesigning the liners to utilize its much greater mechanical properties. Optimized designs are not expected to suffer from the lower thermal conductivity. Electrical resistivity data, while not central to the primary application, show that GRCop-84 has potential for applications where a combination of good electrical conductivity and strength is required.

Ellis, David L.↗
NASA NTRS1999

Tests of Radar Rainfall Retrieval Algorithms

The NASA/JPL Airborne Rain Mapping Radar (ARMAR) operates at 14 GHz. ARMAR flew on the NASA DC-8 aircraft during Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean Atmosphere Response Experiment (COARE), collecting data in oceanic mesoscale convective systems, similar to those now being observed by the Tropical Rainfall Measuring Mission (TRMM) satellite, which includes a 14-GHz precipitation radar. Several algorithms for retrieving rain rate from downward looking radars are in existence. These can be categorized as deterministic and stochastic. Deterministic algorithms use the path integrated attenuation (PIA), measured by the surface reference technique, as a constraint. One deterministic algorithm corrects the attenuation-rainfall (k-R) relation, while another corrects the reflectivity rainfall (ZR) relation. Stochastic algorithms apply an Extended Kalman Filter to the reflectivity profile. One employs radar reflectivity only; the other additionally uses the PIA. We find that the stochastic algorithm with PIA is the most robust algorithm with regard to incorrect assumptions about the drop-size distribution (DSD). The deterministic algorithm that uses the PIA to adjust the Z-R relation is also fairly robust and produces rain rates similar to the stochastic algorithm that uses the PIA. The deterministic algorithm that adjusts only the k-R relation and the stochastic radar-only algorithm are more sensitive to assumptions about the DSD. It is likely that they underestimate convective rainfall, especially if the DSD is erroneously assumed to be appropriate for stratiform rain conditions. The underestimation is illustrated in the diagram. The algorithm labeled D IS initially assumes a DSD that is appropriate for stratiform. rain, while the rain is most likely convective. The PIA constraint causes the k-R relation to be adjusted, resulting in a much lower rain rate than the other algorithms. Additional information is contained in the original.

Durden, Stephen L.↗
NASA NTRS2000

Oxidation Behavior of GRCop-84 (Cu-8Cr-4Nb) at Intermediate and High Temperatures

The oxidation behavior of GRCop-84 (Cu-8 at %Cr-4 at %Nb) has been investigated in air and in oxygen, for durations of 0.5 to 50 hours and temperatures ranging from 500 to 900 C. For comparison, data was also obtained for the oxidation of Cu and NARloy-Z (Cu-3 wt% Ag-0.5 wt% Zr) under the same conditions. Arrhenius plots of those data showed that all three materials had similar oxidation rates at high temperatures (> 750 C). However, at intermediate temperatures (500 to 750 C) GRCop exhibited significantly higher oxidation resistance than Cu and NARloy-Z. The oxidation kinetics of GRCop-84 exhibited a sharp and discontinuous jump between the two regimes. Also, in the high temperature regime GRCop-84 oxidation rate was found to change from a high initial value to a significantly smaller terminal value at each temperature, with progress of oxidation; the two different oxidation rates were found to correlate with a porous intial oxide and a dense final oxide, respectively.

Thomas-Ogbuji, Linus U.↗