Monitoring eutectoid decomposition process of metastable phases in Al-Sm alloy via in-situ X-ray scattering and ex-situ transmission electron microscopy

Metallic glasses, formed from highly undercooled liquids, are ideal systems for fine tuning microstructures. The structure of the glass and the slow kinetics of devitrification provide many possible reaction pathways to form both stable and metastable phases. The Al-RE system is a prime example with many metastable-to-metastable phase transitions. Here we report observation of the transition from epsilon-Al60Sm11 to pi-Al5Sm and fcc-Al using multiple ex-situ and in-situ methods. Ex-situ transmission electron microscope (TEM) observation demonstrates that the decomposition of epsilon-Al60Sm11 propagates with nearly identical spacing between fcc-Al and pi-Al5Sm during isothermal annealing. In-situ high energy wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) methods were employed along with the DSC demonstrate the multiple phase evolution with temperature and time. A novel SAXS analysis approach is presented and validated by ex-situ TEM analysis. These results suggest that nearly monodispersed Al-nanocrystals forms at a constant rate during an isothermal annealing. Both in-situ and ex-situ observations elucidate the transition from epsilon-Al60Sm11 to pi-Al5Sm and fcc-Al as an eutectoid reaction.