Optimisation of Precipitation for the Development of Improved Wrought Fe 3 Al-based Alloys
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Optimisation of Precipitation for the Development of Improved Wrought Fe3Al-based Alloys Satoru Kobayashi a, b, Stefan Zaefferer a, André Schneider b a Dept. of Microstructure Physics and Metal Forming, b Dept. of Materials Technology, Max-Planck-Institute for Eisenforschung, Max-Planck-Str. 1, D-40237 Düsseldorf, GERMANY. ABSTRACT Effect of TiC precipitates on the kinetics of static recrystallisation has been studied by using a Fe-26Al-5Cr (at%) single-phase (α:A2/B2/D03) alloy and two-phase (α+TiC) alloys with different amounts of TiC precipitates. Based on the results, a desirable thermo-mechanical processing is proposed for the development of wrought Fe3Al-based alloys with strengthening MC carbides. In the alloys with a high amount of TiC, needle-like TiC precipitates with 1-10 µm in length formed during air-cooling after homogenisation. Hot deformations with such large precipitates cause inhomogeneous deformation around the particles, leading to particle stimulated nucleation (PSN) and hence accelerate recrystallisation. The occurrence of PSN is harmful for the embrittlement problem, i.e. ductility drastically decreases when recrystallisation occurs, but useful for grain refinement. The following process is proposed to accomplish grain refinement, strengthening by precipitates and avoidance of the embrittlement: hot deformation with a large amount of precipitates to make grain refinement possible by using PSN, followed by hot deformation with a small amount of precipitates near α single-phase region and a subsequent heat treatment to obtain fine precipitates. The fine particles would also act to pin the boundaries of growing grains, thus leading to extended recovery rather than recrystallisation. This process is difficult to carry out in the (Fe-26Al-5Cr)-TiC system because the temperature necessary to enable precipitation is very high and the kinetics is quick. The precipitation temperature is significantly decreased by replacing TiC by VC or MoC. INTRODUCTION Fe3Al-based alloys with bcc structures (α: disordered A2 and B2- or D03 ordered) have been considered as a structural material for high-temperature applications due to their resistance to high-temperature oxidation and sulphidation [1, 2]. Serious unsolved problems are the poor high-temperature strength, creep resistance and the limited low-temperature ductility. This paper proposes a thermo-mechanical processing to develop wrought Fe3Al-based alloys with improved high-temperature strength as well as low-temperature ductility. An effective method to improve high-temperature strength is to introduce fine precipitates. Recent investigations demonstrated that precipitates such as carbides, borides and Laves phase are effective for strengthening even at very high temperatures around 800°C [3-6]. The limited ductility at low temperatures can be improved by careful thermo-mechanical treatments to achieve deformed and well-recovered states. The occurrence of recrystallisation, however, drastically reduces ductility [7]. Consequently, in order to accom
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