The effect of phase composition and morphology on high-temperature strength in the compression of Fe-Al-Si-based iron aluminides manufactured by casting was investigated. The structure and high-temperature strength in the compression of three alloys—Fe28Al5Si, Fe28Al5Si2Mo, and Fe28Al5Si2Ti—were studied. Long-term (at 800 °C for 100 h) annealing was performed for the achievement of structural stability. The phase composition and grain size of alloys were primarily described by means of scanning electron microscopy equipped with energy dispersive analysis and Electron Backscatter Diffraction (EBSD). The phase composition was verified by X-ray diffraction (XRD) analysis. The effect of Mo and Ti addition as well as the effect of long-term annealing on high-temperature yield stress in compression were investigated. Both additives—Mo and Ti—affected the yield stress values positively. Long-term annealing of Fe28Al5Si-X iron aluminide alloyed with Mo and Ti deteriorates yield stress values slightly due to grain coarsening.

A structural studies have been performed on Fe-Al-Si iron aluminides with or without addition of chromium prepared by standard cast process. The effect of chromium addition and subsequent annealing on the phase composition was studied. Very fine precipitates were observed on the grain boundaries in the alloy without chromium addition. On the other hand, complex chromium carbides formed in the structure of chromium doped alloy. The annealing of alloy without Cr addition leads to partial dissolution of secondary phase particles. Unlike the chromium-doped alloy, where the secondary phase coarsed during the annealing process. The phase composition of the alloys was studied by means of scanning electron microscopy (SEM) equipped by energy dispersive X-ray spectrometer (EDX). The electron backscattered diffraction (EBSD) was used for phase verification.

A structural study as well as the determination of σ0.2 compression yield stress at high temperatures have been performed on the Fe3Al-based iron aluminide doped by Nb (5 at. %) and C (1.5 at. %). The carbon occurrence leads to the formation of three-phase structure. Fe3Al matrix, niobium carbides and (Fe, Al)2Nb Laves phase were observed in the structure of investigated alloys. Precipitates were inhomogeneously distributed in as cast state alloy. The annealing at 1000 °C for 50 hrs. leads to more homogenously distribution of secondary phase particles and also to refinement of grain size. The strengthening mechanism of material at high temperatures was detected as a combination of strengthening by incoherent precipitates (NbC and Laves phase) and a solid solution hardening by Nb atoms. Effect of two reinforcing mechanisms together with homogeneously precipitate’s distribution increase the values of σ0.2 compression yield stress about 50 MPa in whole measured temperature range (600 – 800 °C) in comparison to similar intermetallic’s system Fe-Al-Zr-C. Phase composition of the alloy was studied by means of scanning electron microscopy (SEM) equipped by energy dispersive X-ray spectrometer (EDX). The electron backscattered diffraction (EBSD) was used for phase verification and grain size visualization.

The structures of Fe-28Al-15Si-0.2Zr iron aluminide in the as cast state and in three states after heat-treatments (at 800 °C for 100 hours, at 1000 °C for 24 hours and at 1200 °C for 2 hours) were investigated for verification of secondary phases stability. The type and distribution of precipitates were described by means of light optical microscopy and scanning electron microscopy equipped with an energy dispersive analysis. The presence of complex carbides based on Fe-Si-Zr was shown. The bulk hardness and image analysis of samples was measured for verification of dissolution of secondary phase particles to the matrix. Short-term annealing did not influence distribution and dissolution of secondary particles significantly, while long-term annealing (at 800 °C for 100 hours) leads to the sporadic formation of fine eutectic areas.

The microstructure and fracture surfaces were investigated for five Fe3Al – based iron aluminides doped by different alloying elements (Nb, Zr + C, Cr) or without addition. Generally, iron aluminides are considered as brittle material at room temperature, therefore the type and distribution of secondary phases affect the fracture behaviour. The influence of present secondary phase particles on impact toughness at room temperature was evaluated in comparison to binary alloy. The type and the volume fraction of particles affect the value of impact toughness significantly – these values decrease with increasing volume fraction of precipitates. On the other hand, the solid solution strengthening improves impact toughness.