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

该项目研究了位错介导的塑性变形对B2FeAl金属间化合物脆性-韧性转变(BDTT)的贡献，这种转变表现出力学性能对成分的异常依赖。尽管FeAl合金在很大的Al含量范围内都是单相B2有序的，但两者的屈服强度/硬度和BDTT都有很大的突然增加。40%至45at.%Al。直到今天，各种相互依赖、难以控制的材料参数还不能让人们对这一现象有明确的理解。为此，提出了一种新的实验方法，将扩散偶的细观力学测试和单试件宏观拉伸测试相结合。扩散偶覆盖了一个样品中B2FeAl相的整个成分范围。这使得可以通过对同一样品进行800°C的微机械测试来研究依赖于成分的机械性能，该样品具有相同的低污染水平和所有成分的相同的热历史。对热处理后的扩散偶进行测试，以调整不同水平的空位浓度，将提供关于在超过BDTT的温度下，硬度/强度随Al含量和空位浓度变化的定量和半定量信息。扩散偶由相同的合金制成，也用于宏观拉伸试验。由于获得BDTT的常规机械试验需要大量试件，因此在本项目中采用了最近开发的只需要单个试件的测试策略。测试策略依赖于总应变控制循环测试，在温度升高的步骤中具有小的总应变幅度。通过监测塑性应变幅度，可以通过塑性应变幅度的显著增加来确定BDTT，此外，还允许使用单个样本来解决BDTT的应变率相关性。BDTT对Al含量和不同空位浓度水平的依赖关系可以被详细地跟踪，并结合细观力学测试进行合理化。将这两种独特的策略组合应用于由同一材料制成的合金，可以识别外部条件(如温度和应变速率)和内在材料参数(如Al含量和空位浓度)对位错运动和BDTT起始应力的关系。这将有助于更好地理解B2有序FeAl合金的异常力学行为。

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.