The doctoral thesis deals with controlled formation of quasicrystals by the addition of substrates for heterogeneous nucleation of the primary metastable icosahedral quasicrystalline (iQc) phase in Al-Mn-Si and Al-Mn-Si-Cu-Mg alloys. Stable iQc-AlCuFe phase and crystalline TiB2 and TiC particles were used as the substrates for heterogeneous nucleation. The influence of the amount of added substrates and holding time on the microstructure and mechanical properties during compression test were also examined. Characterization involved light, scanning electron and transmission electron microscopy. X-ray diffraction was also used for phase analysis. Electron diffraction and electron backscatter diffraction were used for determining of the orientation of phases. Compression strength, compressive yield strength and compressive fracture strain were determined by compression testing.
It was found that the stable iQc-AlCuFe phase, TiB2 and TiC particles can serve as a substrate for heterogeneous nucleation of metastable iQc phase. Four orientation relationships between TiB2 as a substrate and the primary metastable iQc phase and two between TiC as a substrate and the primary metastable iQc phase were found in addition to the epitaxy which was found between stable iQc-AlCuFe phase and metastable iQc phase. All orientation relationships found between a crystalline substrate and the metastable iQc phase can be explained by the heterogeneous nucleation of the metastable iQc phase on facets of substrates in such a way, that substrate facets are parallel with at least one of the metastable iQc phase plane with lowest surface energy.
Addition of substrates decreases the length of dendrite arms of the primary iQc phase and the thickness of the edge region while increases the content of the primary metastable iQc phase. Compression test reveals that addition of substrates increases the compressive fracture strain and compression strength while the compressive yield strength remains mainly unchanged. Optimal compression test results were achieved in the alloys with the addition of 0.04 wt. % Ti in the form of AlTi3B1 master alloy and 0.06 wt. % Ti, in the form of AlTi3C0.15 master alloy. An increase in the ductility of the alloys with the added substrates is associated with a decrease in the notch effect due to the shorter dendritic arms of the primary iQc phase.
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