Zr50Cu50-XAlX (X = 2 to 16) and Zr90-YCuYAl10 (Y = 30 to 45) bulk metallic glasses (BMGs) have been prepared by tilt casting techniques. The viscosity of the supercooled liquids has been measured by using a penetration viscometer with a column-shaped tungsten indenter with 0.5mm in diameter and an applied load of 0.020N at a high-speed heating at a rate of 400K/min in a highly-purified He-gas atmosphere. In the Zr50Cu50-XAlX (X = 2 to 16) system, the viscosity of the supercooled liquids exhibited almost the same values when the Al-content was increased from 2 to 10 at. %. When the Al-content increased to 12 at. %, the viscosity of the supercooled liquids increased largely. A large amount of the ZrCu (B2) phase after crystallization of this BMG system was detected when the Al-content was 10 at. % and below. While, when the Al-content was increased to 12 at. %, the main precipitation phase was changed to the ZrCu (B19') phase. Similar increase in the viscosity of the Zr90-YCuYAl10 (Y = 30 to 45) supercooled liquids was also observed when the Cu-content was increased from 30 to 45 at. %. This may be due to the strong relationships between the compositional dependence of structure and the viscosity of the supercooled liquids in these BMG systems.
