Controlled formation of quasicrystals in Al-Mn alloys

Doktorsko delo obravnava nadzorovano tvorbo kvazikristalov z dodajanjem kristalizatorjev za heterogeno nukleacijo primarne faze iQc v zlitinah Al-Mn-Si in Al-Mn-Si-Cu-Mg. Kot kristalizatorji so bili uporabljeni delci stabilne iQc-AlCuFe faze ter delci TiB2 in TiC. Delo obravnava tudi vpliv količine dodanih kristalizatorjev in kontaktnega časa (čas zadrževanja v talini) na mikrostrukturo in mehanske lastnosti pri tlačnem preizkusu. V okviru karakterizacije so bile uporabljene svetlobna, vrstična elektronska in presevna elektronska mikroskopija ter rentgenska fazna analiza. Ugotavljane orientacije kristalov je potekalo z metodama uklona pri presevni elektronski mikroskopiji in uklona povratno sipanih elektronov, karakterizacija mehanskih lastnosti pa je obsegala ugotavljanje tlačne trdnosti, napetosti tečenja in skrčka ob porušitvi. Rezultati kažejo, da stabilna iQc-AlCuFe faza in delci TiB2 in TiC lahko služi kot kristalizator za heterogeno nukleacijo metastabilne faze iQc. Poleg epitaksije med stabilno in metastabilno fazo iQc smo odkrili štiri orientacijske zveze med TiB2 kot kristalizatorji in primarno metastabilno fazo iQc ter dve med TiC kot kristalizatorji in primarno metastabilno fazo iQc. Vse orientacijske zveze lahko pojasnimo s heterogeno nukleacijo na fasetah kristalizatorjev tako, da so te vzporedne z vsaj eno od ravnin faze iQc z najmanjšo površinsko energijo. Dodani kristalizatorji vplivajo na zmanjšanje dolžine dendritnih vej primarne faze iQc, debelino robnega območja ter na povečanje deleža primarne faze iQc. Rezultati tlačnih preizkusov so pokazali, da sta se z dodatkom kristalizatorjev v zlitino povečala skrček ob porušitvi in tlačna trdnost, napetost tečenja pa je ostala praktično nespremenjena. Optimalne kombinacije mehanskih lastnosti pri tlačnem preizkusu so bile dosežene v zlitinah pri dodatku 0,04 mas. % Ti v obliki predzlitine AlTi3B1 in pri dodatku 0,06 mas. % Ti, v obliki predzlitine AlTi3C0,15. Povečanje duktilnosti zlitin z dodanimi kristalizatorji povezujemo z zmanjševanjem zareznega učinka zaradi krajših dendritnih vej primarne faze iQc.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