Thermal stability and viscosity of Mg-based glasses and composites

Mechanical alloying was used to prepare bulk glass forming Mg-Y-Cu alloys and to investigate the effect of different alloying elements (C, Ce, Ca and Si) and nanoscale oxide particles (Y2O3) on the glass transition, the crystallization and the viscosity of the supercooled liquid of the different alloys and metallic glass matrix composites. Whereas C, Ce and Ca dissolve upon milling, Si does not alloy with the elemental Mg, Y and Cu powders. No glass transition is found for the alloys containing C or Ca. Addition of up to 5 at.% Ce does not change the glass transition temperature significantly but shifts the onset of crystallization to higher temperatures, thus causing an increase in thermal stability compared to the Mg55Y15Cu30 alloy. For Y2O3 particle addition, an extended supercooled liquid region is observed for up to 30 vol.% of particles, suggesting that the oxide dispersoids embedded in the Mg55Y15Cu30 matrix alloy do not act as preferred heterogeneous nucleation sites for massive crystallization. The glass transition and crystallization temperatures of the composites are comparable to the data for the dispersoid-free material. The presence of alloying additions or nanoscale particles increases the viscosity of the supercooled liquid compared to the Mg55Y15Cu30 alloy.