In this paper, phase transformation in alloys of the Al-Fe and Al-Fe-Cr systems was investigated. The alloys were prepared by melting and gravity casting. The studies of phase transformation were carried out on samples after casting and annealing, using the differential thermal analysis (DTA) and dilatometric method. The knowledge on the phase transformations in these alloys including the information about order-disorder transition is very important from the point of view of obtained mechanical and physical properties of alloys of the Al-Fe system. These results are an important contribution in development of knowledge on iron aluminides. In the article, temperatures of phase transformations connected with a change in order type and transition into disordered solid solution were defined. Conformity of the recorded DTA results and dilatometric analysis was achieved.

The defect structure of Fe28Al samples is examined with the Positron Annihilation Lifetime Spectroscopy. The studies are carried out for samples in as-cast state and after heat treatments: annealing for 24 hours at 900°C (or 1050°C) and either slow cooling with furnace or quenching to oil. The PALS spectra are analyzed using two-state trapping model. Only one type of defects is detected. The positron lifetime in these defects (V) suggests that they are quenched-in Fe-monovacancies (VFe). The vacancy concentration strongly depends on the rate of cooling. Besides, V also depends slightly on the rate of cooling of the material. This fact suggests, according to the predictions of latest theoretical calculations, that V is sensitive to the atomic configuration in the nearest neighborhood of VFe, which give hope to estimate the degree of atomic ordering in alloys by the PALS technique.

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Alloys of the Fe-Al system are interesting due to occurrence of long-range order and many thermal vacancies at high temperature, which lead to not only significant hardening, but also cause changes of physical properties. High temperature diffusion is conditioned by structural defects in solids, such as vacancies, foreign atoms and dislocations influencing thermal characteristics of a solid solution, among others the thermal diffusivity coefficient. Measurement of thermal diffusivity was performed at room temperature using the laser flash method. For characterization of the defect structure, positron annihilation lifetime spectroscopy was used. The data were presented for alloys with 28 and 38 at.% aluminium without chromium and containing 5 at. % Cr addition. The results showed that thermal diffusivity decreased with aluminium content and deviation from stoichiometry. In the studies, different structural defects in the alloys were observed.

In the literature it is proven that thermal vacancies have a great influence on the mechanism of hardening of Fe-Al alloys. Moreover, in these alloys, we observed a long-range ordering, which can significantly affect the mechanical and physical properties and their stability. In this paper, influence of low-temperature annealing on elimination of excess vacancies was investigated. TEM observation of annealed specimens for the alloys with 28 and 38 at.% aluminum have helped elucidate the phenomena responsible for vacancies elimination due to the occurrence of particular interactions between point and linear structure defects. It was shown that the aluminum content influences significantly changes in defects structure. The alloy with 28 at.% aluminum has mainly superdislocations in the structure, while in alloy with 38 at.% aluminum, mainly unit dislocations and high-energy dislocation configurations, like dislocation loops, dislocation dipoles, and dislocation jogs, were observed. The results suggest that different defect types may control the diffusion process during low-temperature annealing and that it is affected by alloy composition.