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S1.7.1

Strengthening of iron aluminide alloys for high-temperature applications Martin Palm, André Schneider, Frank Stein and Gerhard Sauthoff Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, 40237 Düsseldorf, Germany ABSTRACT An overview is given on materials developments of ferritic and Fe3Al-based iron aluminium alloys with strengthening precipitate phases for high-temperature applications currently underway at the Max-Planck-Institut für Eisenforschung GmbH (MPIE). The development of high-temperature alloys for structural applications is to be focussed on optimisation of strength, creep and corrosion resistance at high temperatures and sufficient ductility at lower temperatures. This is discussed with respect to recent studies on Fe-Al-based alloys with strengthening precipitates, such as κ-phase Fe3AlCx, MC-carbides, Laves phase, and the B2-ordered intermetallic phase NiAl. The following alloy systems have been investigated: Fe-Al-X (X=C, Ti, Ta, Mo, Zr), Fe-Al-Ti-Nb, Fe-Al-Ni-Cr, and Fe-Al-M-C (M=Ti, V, Nb, Ta). The investigations have been focussed on the microstructure, constitution, mechanical properties, and high-temperature corrosion behaviour of Fe-Al-based alloys with Al contents ranging from 10 to 30 at. %.

INTRODUCTION Iron aluminide-based alloys have a considerable potential for structural applications at elevated temperatures as they show a good corrosion resistance at high temperatures, have a low density compared with other iron-based alloys and they may be manufactured and processed with existing equipment [1-5]. However, in order to qualify as structural materials, strength and creep resistance at high-temperatures have to be improved and appropriate strategies have been discussed [1, 4, 6-10]. These strategies include strengthening by solid-solution hardening, coherent or incoherent precipitates, dispersoids, and/or ordering. To what extent these measures are possible in certain Fe-Al-X(-Y) systems and how they affect other important properties such as ductility at low temperatures and corrosion resistance is not well understood. Therefore a variety of ternary and higher-order Fe-Al systems have been investigated at the MPIE with respect to basic mechanical properties and oxidation resistance.

SOLID-SOLUTION HARDENING Few data are available on the effect of solid-solution hardening in iron aluminium alloys at high temperatures [11]. This is at least partly due to the fact that even at high temperatures the solid solubility of many elements in the Fe-Al phases is restricted to a few atomic percent or less. In figure 1 the 0.2%-proof stress of some Fe-Al-X alloys at 800 °C is shown in dependence of the concentration of X. All alloys under consideration are single-phase with ordered B2 structure at this temperature (see Table I). A marked influence of the yield stress anomaly can be ruled out as this temperature is well above of that for the anomaly for the respective alloys.

S1.7.2

140

0.2%-proof stress (MPa)

120 100 80 Cr V Mo Ti

60 40 20 0 0

1

2

3

4

5

at.% X

Figur