On the glass forming ability of liquid alloys

By using the concepts of the short-range order (SRO) and middle-range order (MRO) characterizing structures, an attempt has been made to describe the glass forming ability (GFA) of liquid alloys. This includes the effect of more than two kinds of SRO in the liquid caused by the addition of second and third elements to a metallic solvent. The minimum solute concentration is related to the atomic volume mismatch estimated from the cube of the atomic radius. The optimum solute concentration for good glass formability in several binary and some ternary alloys is discussed on the basis of the empirical guideline. A new approach to obtaining good GFA of liquid alloys is based on four main factors: (i) formation of new SRO and coexistence of two or more kinds of SRO, (ii) stabilization of dense random packing structure by restraining the atomic redistribution for initiating the nucleation and growth process, (iii) formation of a stable cluster (SC) or the MRO by the harmonious coupling of SROs, and (iv) difference between SRO characterizing the liquid structure and the near-neighbor environment in the corresponding equilibrium crystalline phases. The use of interaction parameters, widely used in the thermodynamics of multicomponent metallic solutions, is proposed for effectively selecting the third solute element (X₃) for enhancing the GFA of a metallic liquid (M) containing the second solute (X₂). Fe₇₀-B₂₀-(X₃)₁₀ alloys (X₃=Cr, W, Nb, Zr and Hf) are used for illustration. Two typical model structures denoted by the Bernal and chemical-order types are used in describing the new glass structure as a function of solute concentration.