Fe-Al alloys are of engineering interest for structural applications at elevated temperatures due to their low density, excellent corrosion resistance, and low price when compared to the Ni- and Co-based superalloys [1]. An intriguing characteristic of B2-ordered FeAl alloys is that within their wide composition range (which extends from 23.5 to 53 at.% Al) abrupt changes in properties such as the brittle-ductile transition temperature occur [2] and there is a strong dependency of mechanical properties on the vacancy content [3]. Therefore, the thermal history and processing of the alloys are crucial in determining the final properties and have to be carefully monitored and controlled for any investigated sample. To avoid this problem, we employed the diffusion couple technique to vary the content of Al and obtain a concentration profile covering the entire B2 composition range within a single specimen. The application of micromechanical testing techniques to this specimen enabled us to measure and scrutinize the abrupt mechanical property changes as a function of Al content in just one sample with an identical thermal history and impurity content at all positions along the concentration gradient. The diffusion-bonded specimens were characterized for their composition profile and grain orientations using electron probe microanalyzer and electron backscatter diffraction, respectively. Subsequently, their nanohardness was measured using nanoindentation, and pop-in behavior was analyzed. Micropillar compression was also performed via an in situ micromechanical testing setup inside a scanning electron microscope. Our investigations aim at providing fundamental insights into the particular mechanical behavior of B2 FeAl alloys. This combination of the diffusion couple technique and micromechanical testing is a method that can also be implemented for other alloy systems.

References:

[1] M. Palm, F. Stein, G. Dehm, Iron aluminides, Annual Review of Materials Research 49 (2019) 297-326.

[2] D. Risanti, J. Deges, L. Falat, S. Kobayashi, J. Konrad, M. Palm, B. Pöter, A. Schneider, C. Stallybrass, F. Stein, Dependence of the brittle-to-ductile transition temperature (BDTT) on the Al content of Fe–Al alloys, Intermetallics 13(12) (2005) 1337-1342.

[3] M. Zamanzade, G. Hasemann, C. Motz, M. Krüger, A. Barnoush, Vacancy effects on the mechanical behavior of B2-FeAl intermetallics, Materials Science and Engineering: A 712 (2018) 88-96.