Improved Scale Adhesion Research Articles

Effect of SiC, Si3N4 and TiB2 additions on the oxidation resistance of TiAl alloys

The oxidation behavior of TiAl alloys containing dispersed particles of (5, 10, 15 wt.%) SiC, (3,5 wt.%) Si3N4 or (3, 5, 10 wt.%) TiB2 was studied between 800 and 1200°C in atmospheric air. The TiAl−(SiC, Si3N4) alloys oxidized to TiO2, Al2O3, and SiO2. The TiAl−TiB2 alloys oxidized to TiO2, Al2O3, and B2O3 which evaporated during oxidation. Improvement in oxidation resistance accompanied by thin, dense scale formation due to the addition of dispersoids originated primarily from the enhanced alumina-forming tendency, improved scale adhesion by oxide grain refinement owing to the beneficial effect of dispersoids, and the incorporation of SiO2 within the oxide scale in the case of TiAl−(SiC, Si3N4) alloys.

The effect of H2-anneal on the adhesion of Al2O3 scales on a Fe3Al-based alloy

The cyclic (1,100°C, air) and isothermal (1,000°C, O2) oxidation behavior of a Fe-28Al-5Cr (at%) alloy, with and without a prior H2-anneal heat treatment at 1,200°C for 100 h, was studied. Changes in interfacial chemistry were evaluated using Scanning Auger Microscopy after removal of the oxide film in ultra high vacuum. This was achieved by making a scratch on the specimen surface, which caused spallation of the film at various locations along the scratch. The scale thickness and the temperature drop at which spallation took place during cooling were utilized to semi-quantitatively compare the adherence of the scales. Porosity at the scale–alloy interface and the scale microstructure were determined from scanning electron microscope observations. It was found that H2-anneal greatly improved scale adhesion and resulted in a pore-free and sulfur-free interface. The effects were similar to that of a 0.1 at% Zr-containing alloy, except that the improvement in scale adhesion was not as great as that from Zr doping. This implies that oxide/alloy interfaces are not intrinsically strong and the effect of reactive elements, such as Zr, is more than preventing impurity from segregating to the interface. Results are also compared with the effect of H2-anneal on other model alloys, such as NiCrAl, FeCrAl and NiAl, and on single crystal superalloys.

The Effect of Si3N4 Additions on the Oxidation Resistance of TiAl Alloys

The oxidation kinetics of TiAl alloys with and without 3 and 5 wt.%additions of Si3N4 particles were studied at 1173 and1273 K in 1 atm of air. The Si3N4 dispersions wereunstable in the matrix phase, so that some of them reacted with titaniumduring sintering to form Ti5Si3 and dissolvednitrogen. The oxide scale formed on TiAl–Si3N4alloys consisted of an outer TiO2, an intermediate(Al2O3+TiO2), and an inner(TiO2+Al2O3) mixed layers. The enhancedalumina-forming tendency, the presence of discrete SiO2 particlesbelow the outer TiO2 layer, and the improved scale adhesion bySi3N4 dispersions were attributable mainly to theincreased oxidation resistance compared to the Si3N4-freeTiAl alloys. Marker experiments showed that, for TiAl–Si3N4 alloys, the primary mode of scale growth was the outward diffusion oftitanium ions for the outer scale and the inward transport of oxygen ionsfor the inner scale.

On the formation of interfacial and internal voids inα-Al2O3 scales

Microstructural observations were used as the basis for a discussion of the formation and growth of voids in alumina scales. Reactive-element additions to alloys and alloy desulfurization appear to inhibit the growth of interfacial voids, thus improving scale adhesion. This phenomenon is analyzed in terms of surface energies. In addition, a model is proposed for the formation of large internal voids in α-Al2O3 scales. These voids appear to be too large to form as a result of vacancy coalescence and are more frequently observed in scales not doped with a reactive element. The model is based on a growth mechanism where inward and outward growing ridges at scale grain boundaries eventually seal off and form internal voids.

The Oxidation behavior of ODS iron aluminides

AbstractOxide‐dispersed Fe‐28at.% Al‐2%Cr alloys were produced by a powder metallurgy technique followed by hot extrusion. A variety of stable oxides were added to the base alloy to assess the effect of these dopants on the oxidation behavior at 1200°C in air and O2. An Al2O3 dispersion flattened the α‐Al2O3 scale, but produced none of the other reactive element effects and had an adverse influence on the long‐term oxidation behavior. A Y2O3 dispersion improved the alumina scale adhesion relative to a Zr alloy addition at 1200 and 1300°C. However, the Y2O3 dispersion was not as effective in improving scale adhesion in Fe3Al as it is in FeCrAl. This inferior performance is attributed to a larger amount of interfacial void formation on ODS Fe3Al.

The effect of reactive element additions on the selective oxidation, growth and adhesion of chromia scales

The addition of minor amounts of reactive elements to Cr 2O 3 or Al 2O 3 forming alloys has been known to produce a number of beneficial effects in improving their oxidation resistance. The mechanism by which these elements influence oxidation is, however unclear. In this paper, the effect of reactive element addition, whether as surface or alloying additives, on the development, growth and adhesion of Cr 2O 3 scales is discussed. It is pointed out that the development of a continuous external Cr 2O 3 layer and the elimination of base metal oxidation are not coupled processes; reactive element additions can eliminate the latter, but not always affect the former. The incorporation of a reactive element in oxide scales is essential for modifying scale growth. However, a slower oxidation rate or a change in the overall oxide growth direction are not critical factors in improving scale adhesion. In light of these and other experimental results, current theories concerning the reactive element effect are critically evaluated.

On-line measurement of parameters in atomization process of metals : J. Guangming (Research Inst. of Electric Light Source Materials, Nanjing, China.) PM Technology, vol 12, no 1, 1994, 43–49. (In Chinese)

Isothermal oxidation kinetic studies on electrolytic grade iron as well as Fe-Cr alloys using different linear heating rates have been carried out. Mass changes of the samples were measured gravimetrically. Some of the iron and iron-chromium alloy samples were given a pretreatment with a solution that resulted in improved scale adherence. Fraction of reaction against temperature plots have been analysed in terms of available non-isothermal kinetic equations utilizing both the differential and integral forms and activation energies have been evaluated. Isothermal rate constant values evaluated from non-isothermal kinetic equations match fairly well with those obtained isothermally, especially for the iron system. The activation energy and the rate constants have been found to change with heating rates, indicating the influence of initial oxide grain structure and morphology on the kinetics of scale growth.

Atmosphere generation for sintering by dissociation of methanol : Y. Jialin. (Dongxin No 3 Plant for Electro-Carbon, Zigong City, Sichuan, China) PM Technology, vol 12, no 1, 1994, 23–25. (In Chinese)

Isothermal oxidation kinetic studies on electrolytic grade iron as well as Fe-Cr alloys using different linear heating rates have been carried out. Mass changes of the samples were measured gravimetrically. Some of the iron and iron-chromium alloy samples were given a pretreatment with a solution that resulted in improved scale adherence. Fraction of reaction against temperature plots have been analysed in terms of available non-isothermal kinetic equations utilizing both the differential and integral forms and activation energies have been evaluated. Isothermal rate constant values evaluated from non-isothermal kinetic equations match fairly well with those obtained isothermally, especially for the iron system. The activation energy and the rate constants have been found to change with heating rates, indicating the influence of initial oxide grain structure and morphology on the kinetics of scale growth.

Computer simulation of diffusion during liquid phase sintering : R.M. Spriggs, Z.S. Nokolic (Alfred University, New York, USA) Science of Sintering, vol 26, no 1, 1994, 1–9

Isothermal oxidation kinetic studies on electrolytic grade iron as well as Fe-Cr alloys using different linear heating rates have been carried out. Mass changes of the samples were measured gravimetrically. Some of the iron and iron-chromium alloy samples were given a pretreatment with a solution that resulted in improved scale adherence. Fraction of reaction against temperature plots have been analysed in terms of available non-isothermal kinetic equations utilizing both the differential and integral forms and activation energies have been evaluated. Isothermal rate constant values evaluated from non-isothermal kinetic equations match fairly well with those obtained isothermally, especially for the iron system. The activation energy and the rate constants have been found to change with heating rates, indicating the influence of initial oxide grain structure and morphology on the kinetics of scale growth.

Improved High Temperature Oxidation Behaviour of AISI 347 Grade Stainless Steel by Superficial Coating of CeO2.

Influence of superficially applied CeO2 coating on the high temperature oxidation behaviour of AISI 347 grade stainless steel under different rates of heating followed by isothermal holding at 1273 K in dry air is reported. The reactive oxide coating not only reduced the oxidation rate but also facilitated imporved oxide scale adherence to the alloy substrate. The coated steel could withstand a number of thermal cyclings without scale rupture and exhibited further reduction in rate in sequential exposures. Reduced reaction rate has been attributed mainly to a change over in the rate controlling process from outward migration of cations to the ingress of oxidant species. Improved scale adhesion is due to change in chemical composition and grain size of the resultant oxide scale promoted by the formation of silicon-oxide stringers into the base alloy favouring pegging mechanism. Scale growth mechanism has been substantiated by post oxidation analyses using SEM, EDS, EPMA and XRD which could illustrate the role of reactive oxide coating and the formation of various cmplex oxide phases along with NiNb2O6.

Improvement in the oxidation behavior of austenitic stainless steels by superficially applied, cerium oxide coatings

The influence of superficially applied CeO2 coatings on the isothermal-oxidation behavior, preceded by nonisothermal heating, as well as cyclic-oxidation behavior of three grades of austenitic stainless steel (AISI-316, −321, and −304), in dry air is reported. The superficial coating had a thickness of 2.1 μm. The linear heating rate employed was 6 K min−1 up to a maximum temperature of 1423 K, and the isothermal holding temperature was 1273 K. The results clearly depict that CeO2 coatings not only reduced the rates of scale growth for all three varieties of steel but also imparted improved scale adhesion to the respective alloy substrates, as evident from the fact that the coated steels could withstand a number of thermal cycles without scale rupture. In the bare condition, 321-grade steel exhibited the best performance. However, in the presence of the coating, the improved performances of 316 and 321 grades were almost identical, whereas the 304 variety showed improvement only in the first cycle of exposure. The kinetics results have been substantiated by postoxidation analyses of the alloy/scale combinations by SEM, EDS, EPMA, and XRD techniques to reveal the role of rare-earthoxide coatings on the observed behavior.

Effect of Reactive Element Oxide Coatings on the High Temperature Oxidation Behavior of a FeCrAl Alloy

Thin coatings of nitrate‐converted oxides of Y, Zr, Hf, Ce, La, Ca, and Al have been applied to the surface of a Fe‐18 weight percent (w/o) Cr‐5 w/o Al alloy by hot‐dipping. The influence of these coatings on the oxidation behavior of the alloy was studied at 1100 and 1200°C. Results were also compared with the oxidation of a 1 w/o Hf‐containing alloy of the same base composition, with or without the application of selected coatings. It was found that all of the coated samples, apart from that coated with aluminum oxide, developed extremely convoluted scales. Oxide ridges several times larger than those formed on the untreated alloy developed at the oxidation temperature. These convoluted scales showed good spallation resistance after isothermal testing, but failed under thermal cyclic conditions. A thin layer of sulfur was found everywhere at the scale/alloy interface. The primary effect of the surface coatings was to increase the number of buckles in the oxide at the early stage of oxidation. Growth of the buckled oxide into large ridges took place by way of aluminum vapor phase transport and oxide lateral growth. The results are discussed in relation to the effect of reactive elements in improving scale adhesion, and it is suggested that the mechanisms governing this effect may be different for and for alloys.

Growth mechanisms of oxide scales on ODS alloys in the temperature range 1000–1100°C

AbstractAfter a short overview of the production, microstructure and mechanical properties of nickel‐ and iron‐based Oxide Dispersion Strengthened (ODS) alloys, the oxidation properties of this class of materials is extensively discussed. The excellent oxidation resistance of ODS alloys is illustrated by comparing their behaviour with conventional chromia and alumina forming wrought alloys of the same base composition.ODS alloys exhibit improved scale adherence, decreased oxide growth rates, enhanced selective oxidation and decreased oxide grain size compared to corresponding non‐ODS alloys. It is shown, that these experimental observations can be explained by a change in oxide growth mechanism. The presence of the oxide dispersion reduces cation diffusion in the scale, causing the oxides on the ODS alloys to grow mainly by oxygen grain boundary transport. As oxide grain size increases with time, the oxide growth kinetics obey a sub‐parabolic time dependence especially in the case of the alumina forming iron‐based ODS alloy.

Improvements of scale adherence on heat-resisting alloys and coatings by rare earth additions

Addition of rare earth elements to heat-resisting alloys and coatings to prevent the spallation of corrosion-resistant Cr 2O 3 and Al 2O 3 scales has been known for some time. However, the exact mechanism by which the rare earths improve scale adherence is still a matter of controversy. In this paper, several of the mechanisms proposed to explain the “rare earth effect” are discussed. It is concluded that, depending on the alloy composition, the rare earths improve scale adherence by several different mechanisms. These include the elimination of void formation at the alloy-scale interface and decreasing the scale thickness of Cr 2O 3 scales, and the changes in scale microstructure and morphology for Al 2O 3 scales. Development of an irregular alloy-scale interface and the gettering of the indigenous alloy impurities by the rare earth additions are also important for the adherence of both types of scales. A uniform distribution of the oxide dispersions in or over the alloy is recognized as an important factor which affects their effectiveness.

Differences in growth mechanisms of oxide scales formed on ODS and conventional wrought alloys

The oxidation behavior in air of two ODS alloys, MA 754, a chromia former, and MA 956 an alumina-scale former, has been compared with that of conventional wrought model alloys with similar compositions. The main effects on scale properties of both oxide types due to oxide dispersions were found to be improved adherence, decreased growth rates, and enhanced selective oxidation. In addition to metallography, X-ray diffraction, energy dispersive X-ray analysis of the scales, and studies of scale morphology, the detailed growth mechanisms of the oxide layers were studied using an18O tracer technique. The results show that the oxides on the conventional alloys grow by both metal and oxygen transport, and that the addition of oxide dispersions suppresses the outward scale growth. This change in growth mechanism is a possible explanation for the observed improved scale adherence, decreased growth, and enhanced selective oxidation in the yttria-containing alloys.

Non-isothermal oxidation kinetics of iron and iron-chromium alloys

Isothermal oxidation kinetic studies on electrolytic grade iron as well as Fe-Cr alloys using different linear heating rates have been carried out. Mass changes of the samples were measured gravimetrically. Some of the iron and iron-chromium alloy samples were given a pretreatment with a solution that resulted in improved scale adherence. Fraction of reaction against temperature plots have been analysed in terms of available non-isothermal kinetic equations utilizing both the differential and integral forms and activation energies have been evaluated. Isothermal rate constant values evaluated from non-isothermal kinetic equations match fairly well with those obtained isothermally, especially for the iron system. The activation energy and the rate constants have been found to change with heating rates, indicating the influence of initial oxide grain structure and morphology on the kinetics of scale growth.

Distribution of yttrium in aluminium containing stainless steels and its effect on structure of oxide layers

AbstractThe oxidation behaviour of two Fecralloy type steels with compositions Fe–16Cr–4Al–0·4Y and Fe–20Cr–4Al–0·04Y has been studied at 1300 and 1250°C, and the distribution of Y has been investigated. A possible mechanism for the effect of Y in these steels is suggested which accounts for the observed differences in scale morphology and may help explain the improved scale adherence seen in these alloys.MST/853

Ion-Implantation of Reactive Elements in Improving the Adhesion of Thermally Grown Cr2O3 Sales

AbstractThe high temperature oxidation behavior of Al, Y and Hf ion-implantation in Ni-25wtCr alloy with dosages ranging from 1015-1017 ions/cm2 has been studied. Results are compared with the unimplanted alloy, a Xe-implanted alloy and alloys with 0.2wt% Y or lwt% Al additions. Oxidation tests were carried out at 1000°C and 1100°C in 1 atm. O2. It is found that the ion-implantation of Y and Hf greatly reduced oxidation rate and improved scale adherence after a critical implantation dosage, ∼ l×1016 ions/cm2, has been reached. The improved scale adhesion is attributed to both a reduction in growth stress and a strengthened scale/alloy interface, which all developed as a result of a modified scale growth process.

Isothermal oxidation behavior of Ni3Al-0.1B base alloys containing Ti, Zr, or Hf additions

The isothermal oxidation behavior of Ni3Al-0.1B and of this alloy containing additions of approximately 2% Ti, Zr, or Hf was studied in purified oxygen at atmospheric pressure over the temperature range 1300 to 1500 K and for periods up to 400 ks. Ni3Al was also studied similarly for comparison. The oxidation of Ni3Al and Ni3Al-0.1B resulted in an extensive formation of geometric voids on the substrate surface leading to poorly adherent scales. The addition of B to Ni3Al did not change the oxidation behavior much, however, both the activation energy of oxidation and the relative thickness of the Al2O3 layer in the scale were increased. The addition of Ti led to the formation of adherent scales at 1300 K, but the oxidation was accelerated after an initial period at 1300 and 1400 K. At higher temperatures the scale was protective, and the parabolic rate constant, kp, decreased, however, the scale adherence was impaired by the formation of interfacial voids. The addition of Zr resulted in very adherent scales at all temperatures, but at the same time it increased kp considerably. The addition of Hf also resulted in very adherent scales at all temperatures and decreased kp except at 1300 K. The improved scale adherence can be accounted for by the keying effect, since the additives resulted in roughened scale/alloy interfaces. A complex of oxide particle and an associated void was found on the exposed surface of the Hf-containing alloy. This supports the vacancy-sink mechanism.

Oxidation behavior of Fe-Si-Al alloys at 1173?1373 K

The oxidation behavior of Fe-Al-Si alloys prepared by ingot metallurgy (IM) and powder metallurgy (PM) in air and oxygen was investigated in the temperature range 1173–1373 K. A fine dispersion of TiB2 in the PM alloys stabilized a fine grain size (<1 Μ) up to 1273 K. Isothermal oxidation tests in air and oxygen at 1173, 1273, and 1373 K were carried out in a thermobalance system and cyclic oxidation tests were carried out in air at 1273 K. The exceptional scale adherence and the extremely low oxidation rates, in both the isothermal and the cyclic tests, of the Al2O3-scale-forming Fe-Al-Si PM alloys makes them strong candidates for use below 1273 K; the IM alloys, in comparison, showed very poor scale adherence. The results suggest that the critical Al content for continuous Al2O3 scale formation at 1373 K is increased by the addition of TiB2. The Fe-8Si PM alloy which forms an SiO2 scale showed improved scale adherence but enhanced scale growth rate compared to the Fe-8Si IM alloy. The grain-growth inhibiting effect of TiB2 was lost in the Fe-8Si PM alloy in oxidizing environments because of Ti transport across the SiO2 scale. All the IM and PM alloys tested showed superior oxidation resistance compared to the 304 stainless steel. The relative rating in terms of decreasing oxidation resistance of the alloys tested is Fe-3Al-5Si PM, Fe-8Al PM, Fe-5Al-3Si PM, Fe-3Al-5Si IM, Fe-8Al IM, Fe-8Si IM, Fe-8Si PM, and 304 stainless steel.