Lamellar Fe-Al in situ composite materials: microstructure and mechanical properties

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Since decades Fe-Al alloys with up to 50 at.% Al are potentially interesting for the scientific and industrial community, as they possess a large solubility for Al in the disordered bcc crystal structure of iron as well as in the ordered intermetallic compounds D03 and B2. The interest stems from their intrinsically high corrosion and oxidation resistances as well as from the specific weight advantage as compared to e.g. steel. As a drawback, moderate high temperature strength and low room temperature ductility limit their application capabilities. At even higher Al concentrations beyond 50 at.% (and even lower advantageous densities) several intermetallic compounds exist with so far not precisely determined stability ranges. In particular, at concentration very close to 61 at.% an very rapid eutectoid reaction at 1095°C yields an extremely fine lamellar microstructure consisting of FeAl and FeAl2. This finding was one essential result of the first period of funding and already published within this project. The major goal of the submitted continuation proposal focuses on the detailed characterization and modelling of the creep behaviour of fully lamellar FeAl-FeAl2-based alloys which exhibit excellent creep resistances albeit being microstructural instable. Therefore, we will concentrate on the complex interaction of creep rate with parameters such as lamellar spacing, lamellar orientation with respect to the loading axis as well as size of lamellar colonies. In order to characterize the impact of these microstructural parameters, it is mandatory to understand the mechanisms acting within the material. Samples will thus be investigated with specially aligned lamellae within one colony using our knowledge about the nature of the eutectoid reaction which follows a strict, experimentally derived and crystallographically understandable orientation relation. This knowledge was also established and published within the frame of the predecessor of this proposal and makes this reaction attractive for the application of the method of directional (solidification and) transformation with a further potential benefit in creep resistance. This will eventually lead to being able to describe the orientation dependent creep behaviour of fully lamellar Fe-61Al alloy based on a single lamellar colony. To aid this goal, the modelling approach chosen will incorporate all relevant microstructural mechanisms and thus serve to determine not only the acting creep mechanisms based on existing experimental creep curves but also provide the capability to predict creep response at different temperature and stress regimes not investigated by experiment so far.