Development of high temperature creep resistance in Fe-Al alloys
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Development of high temperature creep resistance in Fe-Al alloys D.G. Morris1, M.A. Muñoz-Morris1 and C. Baudin2 Department of Physical Metallurgy, CENIM, CSIC, Avenida Gregorio del Amo 8, E-28040 Madrid, Spain. 2 Instituto de Ceramica y Vidrio, CSIC, Campus de Cantoblanco, Camino de Valdelatas, E-28049, Madrid, Spain 1
ABSTRACT Most of the studies aimed at the development of creep-resisting Fe-Al intermetallics have been oriented at application temperatures of the order of 500-650ºC, where these materials may compete with conventional stainless steels. The Fe-Al intermetallics are, however, particularly excellent in their oxidation and corrosion resistances at temperatures of the order of 1000ºC, where Chromium-Nickel steels are no longer able to withstand the aggressive environments. This presentation is part of a study aimed at the development of good creep resistance at such high temperatures. Studies of a variety of cast Fe3Al-base alloys, strengthened by solution or precipitate/dispersoid-forming alloying additions, are reported. The alloys show good strength from room temperature to about 500ºC, but thereafter strength falls rapidly as thermally-activated deformation processes become operative. Solution additions are capable of producing good low temperature strength, but do not contribute significantly to creep strength at very high temperatures (above 700ºC). Precipitation hardening has been examined in Nb-containing alloys, where Fe2Nb Laves precipitates form at intermediate temperatures. These materials show good strength up to about 700ºC, but at higher temperatures the fine precipitates coarsen excessively. Strengthening in the intermediate temperature range varies depending on whether the solute is precipitated prior to high temperature testing or concurrent with this. INTRODUCTION Most of the work carried out to date on the high temperature or creep strength of iron aluminides has examined strengthening at temperatures of the order of 500-650ºC [1-8]. While the corrosion and oxidation resistances of iron aluminides remain excellent up to much higher temperatures, considerable improvement of flow and creep strength at such temperatures is needed [9]. The present study examines the high temperature flow behaviour of several Fe3Al-based intermetallics, and analyses their behaviour in terms of the strengthening mechanisms operating. EXPERIMENTAL DETAILS The alloys reported here are a solution-hardened alloy designated 28CrSi [composition (atomic percentage throughout) Fe-28%Al-5%Cr-1%Si] and a Fe-25%Al-2%Nb alloy examined in two heat-treated states – solutionised by a 1h anneal at 1300ºC and water quench [designated 25-2-I], and precipitation hardened by a subsequent anneal of 1h at 900ºC [designated 25-2-II]. Compression testing was carried out on cylindrical specimens of these alloys at temperatures from room temperature to 900 C at a standard strain rate of 2 x 10-4/s, with several additional tests carried out at 750ºC at different strain rates (from 10-3/s to 10-6/s). Initial microstructures,
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