Relaxation Behavior of Ca-Based Bulk Metallic Glasses

The relaxation behavior of Ca₆₀Mg₂₀Zn₂₀, Ca₆₀Mg₂₀Cu₂₀, Ca₆₅Mg₁₅Zn₂₀, Ca₅₀Mg₂₀Cu₃₀, and Ca₅₅Mg₁₈Zn₁₁Cu₁₆ bulk metallic glasses was determined in the glass transition region using differential scanning calorimetry (DSC) with heating rates from 1 to 160 K/min. The activation enthalpy of structural relaxation and the fragility index m were found to be smaller in the glassy state (onset of the glass transition) than in the supercooled liquid state (end of glass transition). The Ca-based glass-forming liquids showed strong behavior of the relaxation time, with the fragility indexes m in the range of 33 to 40. The strong liquid behavior implies sluggish kinetics of crystallization in the supercooled liquid region and explains the very good glass-forming ability (GFA) of these alloys. The critical cooling rate for amorphization R _c of the Ca-based bulk metallic glasses was estimated to be in the range of 0.3 to 10 K/s, which is similar to R _c values for the best Pd- and Zr-based metallic glass-forming alloys discovered so far.