Analytical approach to glass formation in casting, thermoplastic forming and additive manufacturing of metal alloys

Developing metallic glasses (MGs) with high glass-forming ability (GFA) and a broad range of well-targeted properties is critical for their widespread industrial use. Until recently, thick sections of MG could be formed only by casting and only for a limited number of compositions. Advances in thermoplastic forming and additive manufacturing of MGs offer the prospect that bulk components can be made from compositions that on simple casting would form a glass only at high cooling rates and be impossible to realise in bulk. It is therefore no longer adequate to consider GFA only in terms of the critical cooling rate to avoid crystallization in simple castings. To redefine high GFA for each processing route, we develop an analytical model to predict the C-curve on a time-temperature-transformation (TTT) diagram for the crystallization of a glass-forming liquid. We show that the dominant parameters determining GFA are the reduced glass-transition temperature T_rg and the liquid fragility m. Using the model, we derive and validate a series of merit indices (combinations of T_rg and m) to guide the optimization of glass-forming compositions for casting, thermoplastic forming and additive manufacturing.