Charles A. Barrett

Effect of 0.1 at.% zirconium on the cyclic oxidation resistance of?-NiAl

The effect of 0.1 at.% Zr (0.2 wt.% Zr) on the cyclic oxidation of hipped β-NiAl was studied. Oxidation testing was performed in static air at 1100–1200 °C, using 1-hr exposure cycles for test times up to 3000 hr. The weight change versus time data were modeled with the COSP computer program to analyze and predict cyclic-oxidation behavior. Zr additions significantly change the nature of the scale-spalling process during cooling so that the oxide spalls near the oxide-air interface at a relatively low depth within the scale. Without Zr, the predominantly α-Al2O3 scale tends to spall randomly to bare metal at relatively high effective-scale-loss rates, particularly at 1150°C and 1200°C. This leads to higher rates of Al consumption for the Zr-free aluminide and much earlier depletion of Al, leading to eventual breakaway (i.e., failure).

COSP: A computer model of cyclic oxidation

A computer model useful in predicting the cyclic oxidation behavior of alloys is presented. The model considers the oxygen uptake due to scale formation during the heating cycle and the loss of oxide due to spalling during the cooling cycle. The balance between scale formation and scale loss is modeled and used to predict weight change and metal loss kinetics. A simple uniform spalling model is compared to a more complex random spall site model. In nearly all cases, the simpler uniform spall model gave predictions as accurate as the more complex model. The model has been applied to several nickel-base alloys which, depending upon composition, form Al2O3 or Cr2O3 during oxidation. The model has been validated by several experimental approaches. Versions of the model that run on a personal computer are available.

Resistance of Ni-Cr-Al alloys to cyclic oxidation at 1100 and 1200 C

A series of Ni-rich alloys in the Ni-Cr-Al system were cyclically oxidized in still air for 500 1 -hr heating cycles at 1100°C and 200 1 -hr heating cycles at 1200° C. The specific sample weight-change data for each sample were then used to determine both a scaling constant k1 and a spalling constant k2 for each alloy, using the regression equation Δw/A=k11/2t1/2 − k2t±σ.These in turn were combined to form an oxidation attack parameter Ka,where Ka= (k11/2 + 10 k2).Log Ka was then fitted to a fourth-order regression equation as a function of the Cr and Al content at the two test temperatures. The derived estimating equations for log Ka were presented graphically as iso-attack contour lines on ternary phase diagrams at each temperature. At 1100°C compositions estimated to have the best cyclic oxidation resistance were Ni-45 at. % Al and Ni-30 at. % Cr-20 at. % Al, while at 1200°C compositions estimated to have the best cyclic oxidation resistance were Ni-45 at. % Al and Ni-35 at. % Cr-15 at. % Al. In general, good cyclic oxidation resistance is associated with Al2O3 and/or NiAl2O4 formation. The analysis also indicated that alloys prepared by zirconia crucible melting, compared to other types of melting, had tramp Zr pickup, which significantly improved the cyclic oxidation resistance. The nature of the improvement in oxidation due to tramp Zr pickup, however, is not yet understood.

Comparison of isothermal and cyclic oxidation behavior of twenty-five commercial sheet alloys at 1150 C

Twenty-five commercial nickel-, iron-, and cobalt-base sheet alloys incorporating chromium or chromium and aluminum additions for oxidation resistance were tested at 1150°C in air for 100 hr in both isothermal and 1-hr cyclic furnace exposures. The alloys were evaluated by sample specific weight change, by type of scale formed, by amount and type of spall, and by sample thickness change and microstructure. In isothermal steady-state oxidation, four types of controlling oxides were observed depending on alloy composition: NiO, Cr2O3-chromite spinel, ThO2-blocked Cr2O3, and αAl2O3-aluminate spinel. The latter three types are considered protective. In the Cr2O3-forming alloys, however, scale vaporization is a critical factor in determining the parabolic scaling rate based on paralinear oxidation. In cyclic oxidation the alloys which form Cr2O3-chromite spinel scales were degraded severely when sufficient chromite spinel developed to trigger spalling. The cyclic behavior of the other three types of alloys does not differ greatly from their isothermal behavior. If chromite spinel formation is minimal, the thinner the oxide formed, the less the tendency to spall. Factors contributing to a thin scale are low isothermal scaling rates; reactive element additions, such as thorium, lanthanum, and silicon; and scale vaporization. Scale vaporization may, however, lead to catastrophic oxidation at high gas velocities or low pressures or both. A tentative mass-balance approach to scale buildup, scale vaporization, and scale spalling was used to calculate the critical oxidation parameter—the effective metal thickness change. In general, this calculated thickness change agrees with the measured change to within a factor of 3 if a correction is made for grain boundary oxidation. The calculated thickness change parameter was used to rate the oxidation resistance of the various alloys under isothermal or cyclic conditions. The best alloys in cyclic furnace oxidation tests were either αAl2O3-aluminate spinel formers or Cr2O3 formers with ThO2 blockage.

Static and dynamic cyclic oxidation of 12 nickel-, cobalt-, and iron-base high-temperature alloys

Twelve typical high-temperature nickel-, cobalt-, and iron-base alloys were tested by 1 hr cyclic exposures at 1038, 1093, and 1149°C and 0.05 hr exposures at 1093°C. The alloys were tested in both a dynamic burner rig at Mach 0.3 gas flow and in static air furnace for times up to 100 hr. The alloys were evaluated in terms of specific weight loss as a function of time, and X-ray diffraction analysis and metallographic examination of the posttest specimens. A method previously developed was used to estimate specific metal weight loss from the specific weight change of the sample. The alloys were then ranked on this basis. In general the burner-rig test was more severe than a comparable furnace test and resulted in an increased tendency for oxide spalling due to volatility of Cr in the protective scale and the more draştic cooling due to the air-blast quench of the samples. Increased cycle frequency also increased the tendency to spall for a given test exposure. The behavior of the alloys in both types of tests was related to their composition, particularly their Cr and Al contents and their tendency to form four types of scales: NiO or CoO, Cr2O3-chromite spinel, α-Al2O3-aluminate spinel, or ThO2-blocked Cr2O3. The alloys with the best overall behavior formed α-Al2O3-aluminate spinels.

High temperature cyclic oxidation furnace testing at NASA Lewis Research Center

A standardized method of testing the cyclic oxidation resistance of various alloys in static air up to 1200°C has been developed and routinely used at the National Aeronautics and Space Administration (NASA) Lewis Research Center. Test specimens are automatically raised and lowered into a resistance wound furnace for a series of fixed-interval heating and cooling cycles. Spall catchers collect the accumulated spall from each specimen. The specimens are weighed intermittently to generate specific weight change with time data. At various test times the specimens and the accumulated spall are analyzed by X-ray diffraction. A computer program is used to print out the specific weight change versus time data and the X-ray data in tabular form and to plot the specific weight change versus time data in a publishable format. The data are also organized and indexed. So far several hundred iron-, nickel-, and cobalt-base alloys have been tested using this basic procedure and will form the basis of a series of cyclic oxidation handbooks to be published by NASA. Such specific weight change/time data have been used to estimate the oxidative metal consumption by several computer modeling techniques both to rank alloys and to estimate life.

The cyclic oxidation resistance of cobalt-chromium-aluminum alloys at 1100 and 1200° C and a comparison with the nickel-chromium-aluminum alloy system

A series of Co-rich alloys in the Co-Cr-Al system were cyclically oxidized in still air for 500 1 -hr heating cycles at 1100° C and 200 1 -hr heating cycles at 1200°C, The specific weight-change data for each sample were then used to determine both an oxide growth constant, k1, and a spoiling constant, k2, for each alloy, using the regression equation ΔW/A=k11/2t1/2— k2t±σ. These in turn were combined to form an oxidation attack parameter, Ka, where Ka=(k11/2+10k2). These attack parameters, along with X-ray diffraction results, were then compared with Ka values determined for comparable Al and Cr compositions in the Ni-rich Ni-Cr-Al system. The Ka and X-ray diffraction results indicated that initially, if the Cr and Al contents in both systems are high enough, protective α-Al2O3 and aluminate spinel(s) are formed. However, in the long run, when the scales eventually start to fail, mainly CoO is formed in the Co-Cr-Al system and mainly NiO in the Ni-Cr-Al system. The Ni-Cr-Al system is considered more oxidation resistant since CoO leads to massive spalling and sudden drastic weight loss, while NiO fails in a more gradual, predictive manner. Both sets of alloys were melted in zirconia crucibles and the resultant Zr pickup increased the cyclic oxidation resistance of the alloys in both systems.

Cyclic oxidation resistance of a reaction milled NiAl-AlN composite

Based upon recent mechanical property tests a NiAl-AlN composite produced by cryomilling has very attractive high temperature strength. This paper focuses on the oxidation resistance of the NiAl-AlN composite at 1473 and 1573 K as compared to that of Ni-47Al-0.15Zr, one of the most oxidation resistant intermetallics. The results of cyclic oxidation tests show that the NiAl-AlN composite has excellent properties although not quite as good as those of Ni-47Al-0.15Zr. The onset of failure of the NiAl-AlN was unique in that it was not accompanied by a change in scale composition from alumina to less protective oxides. Failure in the composite appears to be related to the entrapment of AlN particles within the alumina scale.