The discovery of numerous bulk glass forming alloys over the past two decades and the possibility of exploiting their high intrinsic mechanical strengths has stimulated a renewed interest in the glass forming ability (GFA) of metallic systems.

The critical cooling rate for complete vitrification of an alloy, R_c , was shown earlier⁽¹⁾, for high and intermediate values of R_c,  to correlate satisfactorily with the reduced glass temperature T_g/T_l , where T_g and T_l are the glass transition and liquidus temperatures, respectively. On the other hand, for easy glass formers having low values of R_c <~ 1K/s, T_g/T_l is, in practice, no longer a useful ‘predictive’ parameter⁽²⁾. The reasons for this are discussed. In particular, the presence of heterogeneous nucleants is shown to have a much greater effect on the predicted R_c for bulk glass forming alloys than for compositions having high and intermediate values of R_c . In other words, the relative importance of homogeneous and heterogeneous nucleation rates is dependent on the magnitude of T_g/T_l ⁽³⁾. Recent experimental data, indicating large effects on the GFA of small concentrations of specific solutes⁽⁴⁾, are cited in support of the arguments presented.

(1) H A Davies, "The Formation of Metallic Glasses", Diamond Jubilee Edition of Phys Chem Glasses, 17, 159-173 (1976).

(2) H A Davies, "Metallic Glass Formation Re-Visited" in "Nanostructured and Non-Crystalline Materials", eds. M Vazquez, A Hernando, pp. 3-14, 1995: Singapore, World Scientific.

(3) B G Lewis, H A Davies, "The Formation of Amorphous Metallic Phases by Continuous Cooling from the Melt", in 'Liquid Metals', eds R Evans, D A Greenwood, pp. 274-82, 1977: London, Inst of Physics,.

(4) I A Figueroa, H A Davies, I Todd, “High Glass Formability for Cu-Hf-Ti Alloys with Small Additions of Y and Si” Phil Mag A, in the press.

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