In-situ observation of eutectic growth in Al-based alloys by light microscopy

• Method for in-situ study of solidification phenomena in metallic alloys is developed.
• The approach has been successfully validated for several Al-based alloys.
• Lamellar spacing selections of the Al–Al2Cu eutectic are investigated in more detail.

An experimental method for the in-situ observation of solidification in eutectic metallic alloys growing at moderate temperatures, i.e. 500–600 °C was developed. It is based on unidirectional solidification of thin samples (200 to 700 μm thick) contained in rectangular quartz glass tubes allowing for in-situ observation of growth on the sample surface by means of light microscopy using a long-distance microscope. The method is illustrated for two-phase eutectic growth in Al–17.5Cu–1.0Ag, Al–11.8Si and Al–11.8Si–0.025Sr as well as for three phase growth in Al–13.3Cu–5.7Si, all at%.Coupled growth of lamellar Al–Al2Cu eutectic in Al–17.5Cu–1.0Ag was investigated in more detail with regard to spacing selection: experiments were performed in a temperature gradient of 38±1 K cm−1 using different, but constant withdrawal velocities ranging from 0.025 to 1.8 μm s−1. Measured spacing was compared with literature data and discussed with reference to the Jackson–Hunt theory and the recent amendment proposed by Akamatsu, Faivre, Karma and Plapp. In-situ observations were complemented by post-mortem metallographic analysis using SEM and EBSD in transverse sections.