High glass-forming ability correlated with fragility of Mg-Cu(Ag)-Gd alloys

We report bulk metallic glasses with critical diameter (Dc) in the 20-27 mm range over a relatively wide composition range in the Mg-Cu-Ag-Gd quaternary system. Such an extraordinary glass-forming ability is correlated with the relatively strong liquid behavior of these alloys in terms of Angell's [Science 267, 1924 (1995)] fragility concept. The relaxation time of the ternary Mg61 Cu28 Gd11 and quaternary Mg59.5 Cu22.9 Ag6.6 Gd11 alloys was measured. In terms of the fragility parameter D*, the Mg59.5 Cu22.9 Ag6.6 Gd11 alloy with a critical diameter of 27 mm under copper mold casting has a D* of 25, higher than all the bulk metallic glass-forming alloys reported so far. The implications of these findings are discussed.