Bulk glass formability for Cu-Hf-Zr-Ag and Cu-Zr-Ag-Si alloys

The effects of the gradual substitution of Zr by Hf on glass formability and thermal stability in the Cu45Zr45-xHfxAg10 alloys and the effects of small additions of Si on glass formability in the Cu45Zr45Ag10 alloy are reported and discussed. The samples were prepared as ribbons of thickness in the range 25-200 mu m by melt spinning and as conical bulk shapes, with a length of 50 mm and cone base diameters in the range 2-10 mm, by suction die casting. The alloy Cu45Zr45Ag10 had a critical cylindrical rod diameter for glass formation, d(c), of 3.5 mm but substitution of 1.5 and 3.5 at.% Zr by Hf resulted in substantial increases to 5.5 and 4.5 mm, respectively. However, for x in the range 5-40 at.%, d(c) was reduced to <1 mm. The small addition of Si proved to be beneficial to the glass forming ability (GFA), increasing d(c) up to 5.5 mm for 0.5 at.% Si. Thus the chemical and atomic size similarities of Hf and Zr do not guarantee that bulk glass formation will be maintained on substituting large proportions of Zr by Hf though small substitutions of Hf and of minor additions of Si were beneficial to the GFA. These effects are discussed in terms of the possible influence of the alloying additions on the liquid structure and on the number density of heterogeneous nucleants. (C) 2008 Elsevier B.V. All rights reserved.