Compositional Dependence of Viscosity of Supercooled Liquids and Thermal Properties in Zr-Cu-Ni-Al Bulk Metallic Glasses

Viscosity of supercooled liquids and thermal properties in Zr55+xCu3o-x-yNi5+yAl10 (x= 0, 5, 10, y = 0, 2, 5, 10) (at%) bulk metallic glasses (BMGs) have been examined. Viscosity has been measured by using a penetration viscometer at a high speed heating rate of 6.67 K/s in a high purity He-gas atmosphere. With the increase of the Zr-content from 55 to 60 at% and above, the viscosity of the supercooled liquids was drastically decreased from roughly eta = 10(8) to 10(7) Pa.s at 750 K. In this case, the glass transition temperature (T-g) was slightly decreased and the crystallization temperature (T-x) was largely increased due to the change of the main crystallization phase from metastable fcc-Zr2Ni to stable bct-Zr2Cu, resulting in a large increase of the Delta T-X (= T-x-T-g) value. Maximum Delta T-x values of 128 K and 172 K were observed in the Zr65Cu18Ni7Al10 BMG at heating rates of 0.67 and 6.67 K/s, respectively. It may be suggested that the drastic change of the viscosity and the main crystallization phase may be due to the structure change of the supercooled liquids in the compositional border between 55 and 60 at% Zr.