Aim of the present work was the investigation of the Si-poisoning effect and the influence of grain refinement of titanium (Ti), boron (B) and tantalum (Ta), as well as the proof of t-phases in Ti alloys. Basis of the work was a detailed literature research which suggests effects of these elements on the grain refinement of AlSi-alloy. Theoretical calculations of growth restriction factor (Q) and undercooling factor (P) were performed. Samples were cast via TP1-test and subsequently microstructures were analysed by means of light microscopy and scanning electron microscopy for phases and grain dimensions. The Ti-, B- and Ta content was varied between 0,05 and 0,3 Gew.-%. The effects of different additions were compared and analysed with addition of pure 0.05% Ti and 0,05% Ti+TiB2 particle, as well as 0,05% of pure B with variation of the Si content between 1 and 10%. A grain refinement effect of Ti together with B was observed for off peritectic concentrations of Ti. The effect of only 0,05% B was the most significant. For peritectic Ti concentrations of 0,3%Ti+TiB2 similar grain size to B additons were found. Plate-shaped Al3Ti-phases were observed at a high content of Ti with 0,3%. The expected t-phase could not be detected with scanning electron microscopy. The interaction of the growth restriction factor Q and grain size showed an unexpected behavior of increased grain size with increasing Q. Ta caused significant difficulties during melting, because of low dissolution of Ta in the melt and prefer oxidation. Only the alloy with AlSi7+Ta0,1+TiB2+Ti0,05 showed the most efficient effect on grain refinement