Direct nanoscale observation of melting and solute redistribution in a hypoeutectic Al-Cu alloy with $\it{in\ situ}$ STEM
Melting and solidification of eutectic systems are classical topics in physical metallurgy, yet the mechanisms at nanoscale are less investigated, due to experimental limitations in spatiotemporal resolution. The advent of $\it{in\ situ}$ STEM heating with MEMS technology has recently enabled investigation of eutectic behavior as a function of temperature, time and electrical resistivity. Using this methodology, we investigate the evolution of a nanocrystalline hypoeutectic Al-Cu alloy. Melting initiated in the hotter central region and propagated outward, with grain boundaries acting as preferred sites for eutectic liquid formation via Cu enrichment. The Al$_2$Cu phase melted prior to complete matrix melting. Liquid-state Cu redistribution over a distance of 258 $\mu$m - several orders of magnitude beyond solid-state diffusion limits - resulted in Al-rich rim accumulations and Cu enrichment at the outermost edge of the observed chip region. These observations are discussed in the context of classical predictions for melting of eutectic systems.