David R. Sigler

Aluminum oxide adherence on Fe-Cr-Al alloys modified with group IIIB, IVB, VB, and VIB elements

A recent theory, explaining the effect of reactive elements on oxide adherence, states that sulfur adversely affects adherence and that reactive elements improve adherence by tying up sulfur as refractory sulfides. This theory is supported by work presented here, which correlates adherence behavior of Fe-Cr-Al alloys containing group IIIB through VIB elements with the stability of the sulfides that these elements form. Results show that poor adherence is produced by elements that form sulfides less stable than Al2S3 (VB and VIB elements), while good adherence is produced by elements which form sulfides more stable than Al2S3 (IIIB and IVB elements). In addition, the relative stability of sulfides, oxides, nitrides, and carbides must be considered. For example, group-IIIB elements are effective at much lower concentrations than group-IVB elements, because they react preferentially with S while group IVB elements react with C, N, and O before S.

The influence of sulfur on adherence of Al2O3 grown on Fe-Cr•Al alloys

Recently a new theory was proposed to explain the effect that reactive elements have on oxide adherence. Based on data obtained on Ni-Cr-Al-Y material, this theory stated that trace quantities of sulfur in the alloy degrade adherence by weakening the metal-Al2O3 bond. The work presented here extends this concept to Fe-Cr-Al alloys by examining Al2O3 adherence on foil samples with various bulk sulfur levels obtained using high-temperature vacuum anneals. Results show that long-time vacuum anneals dramatically increase the adherence of the subsequently grown aluminum oxide, concurrent with removal of sulfur from the matrix. This evidence shows that the Al2O3-metal bond is intrinsically strong without the presence of reactive elements such as Y or rare earths in the alloy. Sulfur in the alloy, and not void formation, was found responsible for oxide spalling. In addition, voids were eliminated by reducing the sulfur concentration near the oxide-metal interface.

Adherence behavior of oxide grown in air and synthetic exhaust gas on Fe-Cr-Al alloys containing strong sulfide-forming elements: Ca, Mg, Y, Ce, La, Ti, and Zr

This work evaluated the adherence of oxide grown in air and synthetic exhaust gas on Fe-20Cr-5Al alloys containing strong sulfide-forming elements: Ca, Mg, Y, Ce, La, Ti, and Zr. Results support the theory that reactive elements provide good oxide adherence on alumina-forming materials primarily by tying up sulfur as stable Sulfides; however, other influences on adherence were found. Highly volatile elements, such as Ca and Mg, lose their sulfur-controlling ability by diffusing out of the matrix and into the growing oxide scale. Zirconium results in the growth of an extensive network of oxide pegs into the substrate which improves adherence. Titanium segregates to the alumina scale and acts as a sink for S in the matrix. In synthetic exhaust gas (N2+CO2+H2O), local oxide spalling was observed and was shown to be caused by H2O in the atmosphere. The added benefits of Ti and Zr, i.e., forming oxide pegs and sinks for S, improve adherence in this environment.

Oxidation behavior of Fe-20Cr-5Al rare earth alloys in air and synthetic exhaust gas

Fe-20Cr-5Al alloy foils are used in automotive catalytic converters. This work examines oxidation behavior of four production-processed alloy foils in both air and synthetic exhaust gas environments. Oxidation tests were performed between 750° C and 1150° C for times to 96 hrs. Weight gain results in both atmospheres were similar, an indication that the same mechanism controls oxidation in both environments. At high temperatures (>-950° C) both atmospheres produce weight gains consistent with α-alumina growth. Activation energies of 323 kJ/gmole and 271 kJ/gmole were calculated for oxidation in air and synthetic exhaust gas, respectively. At lower temperatures (<-850° C), accelerated weight gains can occur from growth of transition alumina. Despite similar weight gain results, the two atmospheres produce different oxide morphologies: at 950° C and above, air produces a rounded, porous oxide while synthetic exhaust produces a more compact, angular oxide. Unexpectedly, oxide spalling occurred on foils oxidized in synthetic exhaust at 1050° C and above.

Metallography of fatigue crack initiation in an overaged high-strength aluminum alloy

Crack initiation was observed by optical microscopy using Nomarski interference contrast during fatigue cycling of an overaged 2024 aluminum alloy. The number of cracks more than five microns long at any given fraction of the fatigue life, and the distribution of cracks among various possible initiation sites, both depend on the applied stress amplitudeσa). The crack density at failure falls from approximately 300/mm2 when σa is 90 pct of the yield strength, to less than I/mm2 when σa is less than 60 pct of the yield strength. Cracks may begin in the matrix, in grain boundaries, or at constituent particles. At all stress amplitudes, however, the most common initiation sites areβ (Al7Cu2Fe) constituent particles. At low stress amplitudes in particular, fatigue cracks develop from the interface between closely-spaced fragments of β particles broken during prior processing (cluster sites). The stress-raising effect of voids which often occur at cluster sites may be responsible for their effectiveness in initiating fatigue cracks.

The oxidation behavior of Fe-20Cr alloy foils in a synthetic exhaust-gas atmosphere

Oxidation tests of rare-earth-modified and Ti-modified Fe−20Cr alloy foils, which are under consideration for catalytic converter supports, were performed in a synthetic exhaust-gas atmosphere (N2+H2O+CO2) between 900°C and 650°C. Between 900°C and 750°C, the rare earths had no effect on oxide growth rates while Ti increased growth rates. Oxide growth rates for the rareearth alloys at 800°C and 750°C are much lower than those found in the literature for oxidation of Fe−Cr alloys or pure Cr in O2-rich atmospheres. The slow growth rates for the rare-earth alloys agree with literature data for oxidation of stainless steels containing >20% Cr in wet atmospheres and are caused by growth of an oxide scale only one grain thick. At temperatures ≤700°C, Fe−20Cr alloys grow massive Fe oxides; however, this can be suppressed by adding rare earths or Ti. To ensure good oxide adherence, free sulfur must be eliminated in the alloy by tying it up with a reactive-element addition. Both Ti and the rare earths can be used to tie up S, but the rare earths are more effective. For converter applications, the optimum alloy composition may contain rare earths for good oxide adherence and a small amount of Ti to suppress growth of Fe-rich oxides.