Diffusional Decomposition and Glass Forming Ability of Solidifying Ternary Liquids
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Diffusional Decomposition and Glass Forming Ability of Solidifying Ternary Liquids E. Eshed1, M. Bamberger2, A. Katsman2 1 Israel Institute of Metals, Techion, 32000 Haifa, Israel 2 Department of Science and Materials Engineering, Technion, 32000 Haifa, Israel ABSTRACT An innovative model based on spinodal-like decomposition of supercooled multicomponent liquids was developed for prediction of the glass forming ability (GFA) of ternary alloys. The up-hill diffusion in the initially homogeneous freezing ternary liquid was considered as a necessary condition for solidification of crystalline phases. New generalized criteria of spinodal decomposition of ternary alloys for time/space correlated fluctuations were formulated. These criteria take into account both thermodynamic and kinetic properties of the system. The introduced criteria were found to provide adequate GFA evaluation of different compositions in the Mg-based ternary alloys. INTRODUCTION Finding new highly glass-formable alloys has gained much attention in recent decades since amorphous alloys can possess higher mechanical properties and higher corrosion resistance compared to their crystalline counterparts [1-5]. There were several attempts to describe crystallization/amorphization processes in a supercooled liquid by computer simulations [6-8]. However, in those calculations only density fluctuations in a simple liquid were considered. At the same time, in binary, and especially in ternary alloys, substantial compositional fluctuations in an initially homogeneous liquid are expected to occur before the solidification of the first crystalline phase. In such cases, the supercooled unstable liquid decomposes into crystalline phases by means of preliminary up-hill diffusion of alloy components. The up-hill diffusion is characteristic of spinodal decomposition. The model considering the initial stages of crystallization of a supercooled ternary liquid as a spinodallike decomposition, was recently suggested and applied to Mg-Y-La and Mg-Zn-Nd alloys [9]. Based on the developed model we offer further kinetic and thermodynamically-based criteria for the predictions of high GFA ternary compositions. MODEL Nucleation and growth of crystalline phases in a multi-component homogeneous liquid may require substantial compositional fluctuations appearing due to a prolonged up-hill diffusion process. The up-hill diffusion can be realized in a highly supercooled unstable system undergoing spinodal-like decomposition. The compositional fluctuations, driven by up-hill diffusion, are able to grow exponentially in time, culminating in the nucleation of different crystalline phases. The subsequent solidification may appear as homogeneous or heterogeneous nucleation of different crystalline phases. Let us consider a homogeneous ternary alloy M-A-B that is rapidly solidified from a liquid condition to a temperature T below the melting point. The equations describing the evolution in time of the concentrations of both alloying elements A and B in the matrix M can be written in the
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