Behaviour of the solid-liquid interface at the moment of quenching during solidification of aluminum alloys

The fraction of solid phase f(S) is a key parameter studied in order to understand how the microstructure evolves during solidification of aluminum alloys. In the present study aluminum alloys (i.e. Al-3 wt% Si and Al-7 wt% Cu) were solidified and quenched from different temperatures within the solid-liquid region. The microstructure is analyzed by optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Overestimation of solid fraction was found in samples quenched at high temperature, while those quenched close to the eutectic reaction did not show overestimation of solid fraction. The existence of interface instabilities was found in those samples that show overestimation of solid fraction. The formation of instabilities and its role in overestimation of solid fraction is discussed based on EPMA and metallography. The present research provides an insight in understanding the overestimation of solid fraction in order to overcome the problem when reconstructing the real microstructure that was present before quenching.