Revealing the causes of the unusual mechanical behavior of B2 FeAl compounds

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The project addresses the contribution of dislocation-mediated plastic deformation to the brittle-to-ductile transition (BDTT) of B2 FeAl intermetallic compounds, which show an unusual dependence of mechanical properties on composition. Even though being single-phase B2-ordered over a wide range of Al contents, FeAl alloys exhibit an abrupt, strong increase of both, yield strength/hardness and BDTT between approx. 40 to 45 at.% Al. To this day, a variety of interdependent, difficult-to-control materials parameters do not permit an unequivocal understanding of this phenomenon. Hence, a novel experimental approach applying micromechanical testing of diffusion couples and a single-specimen macroscopic tensile test procedure is proposed. The diffusion couples cover the entire composition range of the B2 FeAl phase within one sample. This allows studying the mechanical properties in dependence on composition by micro-mechanical testing to 800 °C of one and the same specimen with identical low contamination level and identical thermal history for all compositions. Testing of diffusion couples after heat treatments to adjust different levels of vacancy concentrations will provide quantitative and semi-quantitative information about the dependence of hardness/strength as a function of Al content and vacancy concentration, at temperatures covering BDTT. The diffusion couples are produced from the same alloys, which are also used for macroscopic tensile tests. Since conventionally applied mechanical tests to obtain BDTT require large numbers of specimens, a recently developed testing strategy only requiring a single specimen is applied in the present project. The test strategy relies on total strain-controlled cyclic tests with small total strain amplitudes at steps of increasing temperatures. Monitoring plastic strain amplitude allows for determination of BDTT indicated by a strong increase of plastic strain amplitude, additionally allowing for the strain rate dependence of BDTT to be addressed with a single sample. The dependence of BDTT on Al content and different vacancy concentration levels can be tracked in detail, and rationalized in conjunction with the micromechanical tests. The combined application of the two unique strategies to alloys fabricated from the same well-defined material allows the identification of the relationship of external conditions, such as temperature and strain rate, and intrinsic material parameters, such as Al content and vacancy concentration, on the onset stress of dislocation motion and the BDTT. This should lead to an improved understanding of the unusual mechanical behavior of B2-ordered FeAl alloys.