Understanding the formation mechanisms and stability of the Anti-Phase boundaries (APBs) in Al-Fe B2

Al-Fe B2 alloys (with ordered Body Centered Cubic lattice, BCC) are an example of high strength, brittle materials containing multiple crystal defects, such as dislocations, vacancies and Anti-Phase Boundaries (APB). Although latter were reported to play a major role in mechanisms influencing mechanical properties, reports on formation mechanisms of APBs are limited and contradictory. To close this gap in understanding, Fe_xAl_1−x alloys were studied in Transmission Electron Microscope (TEM). Changing the Long-Range Order, η, parameter (calculated using statistical thermodynamics according to the composition of the samples) from 1 (50 at% Fe, i.e., perfect order) to η = 0.53 (68 at% Fe) caused an increase in density of the <111> type dislocations. At η = 0.53 – APBs have started to form. These experimental results were in-line with Density Functional Theory calculations. Since above 68 at% Fe, D0₃-type structure has also formed in the B2 grains, its role in formation of the APBs was assessed. It was determined that these two phenomena are not related. Therefore, outcomes of current research are of general applicability to other B2 structures regardless existence of D0₃ or lack of such additional ordering. Furthermore, driven conclusions have a potential to impact the BCC-based High Entropy Alloys, where B2 is the matrix.