Viscosity and specific volume of bulk metallic glass-forming alloys and their correlation with glass forming ability

The trends in glass formation among bulk metallic glass-forming alloys are investigated within the framework of viscosity and specific volume measurements. This investigation was carried out using four alloys (Zr41.2Ti13.8Cu12.5Ni10Be22.5, Zr57Cu15.4Ni12.6Al10Nb5, Zr52.5Cu17.9Ni14.6Al10Ti5 and Ni59.5Nb40.5) that have widely different glass forming abilities (GFAs). This study shows that the viscosity at the melting temperature is correlated with volume change upon crystallization in accordance with Cohen–Grest free-volume theory. The viscosity of the best glass former (Zr41.2Ti13.8Cu12.5Ni10Be22.5) is found to be an order of magnitude larger compared to the worst glass former (Ni59.5Nb40.5) at their respective liquidus temperatures. The other two alloys have intermediate values. The specific volume results also support the trend in GFA with the best glass former showing the smallest volume change upon crystallization and worst glass former showing the largest change. This study suggests that high viscosity and correspondingly small free volume of the liquid at the melting temperature contribute significantly to improve the GFA.