Critical-point wetting at the metastable chemical binodal in undercooled Fe-Cu alloys

Complementary results of differential thermal analysis and microstructural examination on Fe-Cu alloys provide the first evidence for critical-point w etting occurring at a completely metastable miscibility gap. The perfect we tting conditions hold for a composition range of 50-65 at.% Fe in the vicin ity of the critical concentration. For samples encased with a glass slag, t he Cu-rich liquid completely wets the glass upon undercooling to the metast able miscibility gap. In the perfect-wetting range, the metastable homogene ous liquid phase exhibited phase separation without undercooling below the chemical binodal. At deep undercooling, solidification of alloys with phase separated liquids results in a coarse scaled two-phase microstructure. In contrast, the homogeneous liquid phase of samples with compositions outside the perfect wetting range did undercool below the equilibrium onset of the metastable phase separation reaction. The phase separation in these sample s occurred on a much finer scale. For samples without a glass encasement an d thus in the presence of the Al2O3 crucible and an iron oxide layer, perfe ct wetting occurred near the consolute point on both sides of the metastabl e miscibility gap. This demonstrates that critical-point wetting is indepen dent of the surface environment, but the wetting phase selected is surface sensitive. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Lt d. All rights reserved.