Crystallization microstructure in transparent monotectic alloys

Abstract

Transparent binary metallic alloy models are important in material and metallurgical science, as phase transformations and processes can be observed during solidification. The best transparent models for metallic growth and solidification morphology studies are members of the ‘plastic’ crystals group. These compounds have low entropies of fusion and cubic crystal structures, resulting in nonfaceted solid/liquid interfaces^1,2. We describe here the surprising morphology which occurs at the Hquid₁/liquid₂/solid (L₁/L₂/S) triple junction in directionally solidifying miscibility gap systems at the monotectic temperature. The monotectic temperature in a binary mixture is the temperature at which two immiscible solutions of the same two components form phases in equilibrium with the solid phase of one of the components. These observations were made on succinonitrile (sn)-based systems (C₄H₄N₂, the ‘plastic’ crystal component, with a second component) on a thermal gradient microscope apparatus^3–5. Figure 1 shows two succinonitrile-based monotectic phase diagrams used in this study. The specimen cells contained sample gaps ranging from 50 to 150 μm, temperature gradients from 5 to 70° Ccm⁻¹, and growth rates from 0.75 to 10.0 μm s⁻¹.