Microstructural changes during quenching

Abstract

Al–6.8 wt% Cu samples were held at a known fraction of solid in the mushy zone for varied times and then cooled/quenched with cooling rates between 0.02 and 160 K/s. The holding period allows for a reduction of the interfacial energy of the primary α-Al phase by a decay of the initially dendritic microstructure and subsequent coarsening. During quenching, the melt fraction solidifies with a much finer microstructure. The fraction of solid of the primary α-phase at the holding temperature is accessible to measurements after cooling by separating the coarse and fine length scales of the microstructure by image analysis. In order to freeze the initial microstructure (volume fraction, size, shape and interface density) by quenching, cooling rates in the order of 150 K/s or higher are necessary for this type of alloy, depending on interface density and initial solid fraction. For low and moderate cooling rates (up to 80 K/s) the microstructural parameters change significantly.