Microstructure evolution during melting and resolidification in a temperature gradient

Microstructure evolution in binary Al-Cu alloys in a steep temperature gradient (12K/mm) has been investigated. Three different initial microstructures (fine grained equiaxed, fine grained columnar and coarse grained columnar) with three different Cu concentrations (6.0, 8.5 and 2.0 wt%, respectively) have been used. After different holding times between 5 min and I h in the temperature gradient, the samples were water quenched. As compared to the initial state, the microstructure changes in several respects: the grain size increases towards higher temperature regions, the grain aspect ratio in the equiaxed fine grained sample increases along the former mushy zone, and a gradient of solute concentration in the solid evolves. The solute concentration in the solid, measured by electron probe micro analysis, corresponds to the solidus concentration defined by the phase diagram. The grain coarsening Occurs on time scales that are much shorter than those expected from isothermal solid/liquid grain coarsening processes. This indicates a mechanism with a higher driving force than the reduction of interfacial area. A melting/resolidification mechanism is suggested that causes the evolution of the gradients in grain size, grain aspect ratio and solute concentration in the former mushy zone. (c) 2005 Elsevier B.V. All rights reserved.