M. A. Montealegre

The high temperature oxidation behaviour of an ODS FeAl alloy

The oxidation behaviour of an oxide dispersion-strengthened (ODS) FeAl intermetallic, microalloyed with Zr and B and strengthened by a fine dispersion of Y2O3, is investigated at 1100°C for exposures of up to 200 h. The results show that a pure alumina scale is formed irrespective of the exposure time. The oxidation rate is far inferior to that found on PM 2000, a commercial alumina forming ODS ferritic superalloy. Limited scale spallation is observed in the intermetallic alloy from the early stages of oxidation. Scale failure, which is shown to occur during the cooling stage after oxidation and not at the high temperature of oxidation itself, results from the high compressive residual stresses in the scale induced by the misfit in the thermal expansion coefficients of the scale and the substrate. Failure of the scale may be supressed by using a very low cooling rate after oxidation.

Oxidation behaviour of Fe40Al alloy strip

Abstract The oxidation behaviour of FeAl foils of 200 μm thickness was investigated in air at 900 and 1100°C for up to 1000 h exposure. Microstructural examination reveals the formation of an even alumina scale with small islands of (Fe,Al)-oxides which may be associated with chemical heterogeneities reaching the surface. No cracking or spallation was found. At the lowest temperature the scale is decorated with small platelets associated with metastable θ-alumina which transforms to the more stable α-alumina with progressing oxidation. At the highest temperature, metastable phases were not observed. For the longest exposure, however, whiskers of alumina on the top of the scale were found to develop with progressing oxidation. The observation of significant differences between the oxidation behaviour of thin foils and thick materials regarding scale spallation is related to the relaxation of growth stresses in the scale by spontaneous deformation of the substrate by creep.

Evaluation of magnetic behaviour and in vitro biocompatibility of ferritic PM2000 alloy

PM2000 is a ferritic alloy obtained by powder metallurgy and is being investigated for potential applications as a biomaterial. This work aimed to assess the biological compatibility and to determine the influence of the processing route and further recrystallisation treatment on the magnetic behaviour. The magnetic behaviour has been analysed as a function of the hysteresis loop obtained by using an inductive method. The biocompatibility has been tested using human osteoblast-like cells seeded onto discs of PM2000. The ability of cells, on its surface, to attach, grow, and produce alkaline phosphatase (ALP) was determined. It is shown that PM2000 is a soft magnetic material irrespective of its material condition, its remanent magnetisation being very low (up to about 3% for the recrystallised swaged material). Fields close to 200 Oe are required to saturate the material. The saturation magnetisation is about 135 emu g−1. In vitro tests indicate that cells are able to attach and grow onto its surface, and produce ALP, a specific marker of cells with bone-forming activity. In this respect, PM2000 holds promise as a suitable substrate for bone integration. These properties could make PM2000 a useful candidate for the preparation of medical devices where biocompatible and soft magnetic materials are sought. Applications for dental magnetic attachments could be envisaged.

Comparative study of the alumina-scale integrity on MA 956 and PM 2000 alloys

The present work analyzes the oxidation kinetics of MA 956 and PM 2000 alloys at 900 and 1100°C for exposure times up to 1000 hr. Special emphasis was placed on a comparison of the alumina-scale integrity formed at 1100°C by means of electrochemical tests at room temperature, which have been shown to be very reliable methods to detect the presence of microdefects within oxide scales. To check whether a preoxidation treatment makes these materials corrosion resistant against aggressive fluids, an electrolyte containing chloride ions was chosen. The mass gain of MA 956 was found to be slightly lower than that of PM 2000 up to 200 hr exposure at 1100°C and for the whole exposure range at 900°C. A subparabolic time dependence (n=0.3) of the oxide growth rate was determined for both alloys at both temperatures. On the other hand, the electrochemical-impedance spectroscopy (EIS) and anodic-polarization tests performed on preoxidized alloys (1100°C/100 hr) revealed good room-temperature corrosion behavior for both alloys, the corrosion resistance and polarization values being somewhat higher for preoxidized PM 2000. Consideration of these results and those of both surface and cross-section examinations of the scale, the better room-temperature corrosion behavior of preoxidized PM 2000 denotes the formation of a denser and mechanically more stable alumina scale containing a lower number of microdefects. This could result from the higher aluminum content of this alloy and the lower density of chemical heterogeneities within the scale. The higher mass gain of PM 2000 could be related to the higher concentration of oxide nodules on top of the alumina scale, as deduced from SEM examination.

Possibilities for improving the corrosion resistance of Fe–40Al intermetallic strip by prior oxide protection

Abstract Iron aluminides do not generally show good aqueous corrosion resistance. The present study attempts to improve their corrosion resistance by prior surface oxidation. While good corrosion resistance can indeed be obtained, it is found to be difficult to ensure that the oxide layer is continuous and defect free.

Influence of grain size on the oxidation behaviour of MA 956

The influence of grain size of alloy MA 956 on the oxidation kinetics and scale morphology is investigated at 900 and 1100°C for up to 1000 h exposure. Specimens with grain sizes of about 3, 150, and 300 μm mean diameter were studied. Grain growth during oxidation was not observed. The results of this work at 1100°C show that grain size of the alloy does not significantly influence the oxidation kinetics. At 900°C, however, mass gain values for the small- and medium-grain size material are somewhat lower than those for the large-grain size material, especially for exposures below 100 h. This feature indicates that grain boundaries could have played a role for the supply of Al to the scale. Einfluss der Korngrose auf das Oxidationsverhalten von MA 956 Der Einfluss der Korngrose der Legierung MA 956 auf die Oxidationskinetik und die Zundermorphologie ist bei 900 und 1100°C und bei bis zu 1000-stundiger Auslagerung untersucht worden. Proben mit Korngrosen von etwa 3, 150 und 300 μm mittleren Durchmessers wurden verwendet. Kornwachstum wahrend der Oxidation wurde nicht festgestellt. Die Ergebnisse dieser Arbeit zeigen, dass bei 1100°C die Korngrose der Legierung die Oxidationskinetik nicht nennenswert beeinflusst. Bei 900°C sind die Werte der Gewichtszunahme fur das Material mit kleiner und mittlerer Korngrose allerdings etwas niedriger als die Werte fur das Material mit groser Korngrose, insbesondere bei Auslagerungen unter 100 Stunden. Diese Erscheinung deutet an, dass die Korngrenzen eine Rolle bei der Zufuhr von Al zum Zunder gespielt haben konnen.