Interfacial criterion of spontaneous and forced engulfment of reinforcing particles by an advancing solid/liquid interface

Author： Kaptay G.

Publisher： Springer Publishing Company

ISSN： 1543-1940

Source： Metallurgical and Materials Transactions A, Vol.32, Iss.4, 2001-04, pp. : 993-1005

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Abstract

The sign of the interfacial force acting between a ceramic particle and a solidification front through the thin layer of a liquid metal is determined by the sign of the quantity ∆σ _cls. A new, generally valid equation has been developed for this parameter: ∆σ _cls = 2σ _cs− σ _cl− σ _sl(where σ _cs, σ _cl, and σ _slare the interfacial energies in the ceramic/solid metal, in the ceramic/liquid metal, and in the solid metal/liquid metal systems, respectively). The interfacial force is attractive, i.e., spontaneous engulfment of reinforcing particles by the front is expected, if ∆σ _cls < 0. A new estimation method has also been developed for the quantity σ _cs. Combining this equation with the new equation for ∆σ _cls, the approximated expressions with easily available parameters for the parameter ∆σ _cls have been obtained for normal metals (∆σ _cls = σ _cv− σ _lv· (0.08 + 1.22 · cos Θ_clv)) and for Si and Ge (∆σ _cls = σ _cv− σ _lv· (0.57 + 1.66 · cos Θ_clv), where σ _cvand σ _lvare the surface energy of the ceramic and the surface tension of the liquid metal, respectively, while Θ_clv is the contact angle of the liquid metal on the ceramics). Calculations performed with these equations are in good qualitative agreement with all known pushing/engulfment experiments for metal/ceramic systems. Particularly, it has been theoretically predicted that, while in the majority of normal metal/ceramic and Ge/ceramic systems pushing (and, at higher solidification rates, forced engulfment) is expected, primary Si crystals (crystallizing from hypereutectic Al-Si and other alloys) will spontaneously engulf the majority of ceramic phases. The so-called “pushing/spontaneous engulfment” (PSE) diagrams have been constructed to help make a quick judgement as to whether spontaneous engulfment or pushing is expected in a given metalceramic system. For systems with ∆σ _cls > 0, a new equation (similar to that derived earlier by Chernov et al.) has been derived to estimate the critical velocity of the pushing-engulfment transition (PET). Calculations with this equation show excellent quantitative agreement with the critical interface velocity of the PET in the Al/ZrO₂ (250 µm) system, measured recently under microgravity conditions by Stefanescu et al.