Unusual crystallization of Al₈₅Y₈Ni₅Co₂ metallic glass observed in situ in TEM at different heating rates

Crystallization of metallic glasses produced by rapid solidification and supercooled liquids on heating takes place at large undercooling which has some peculiarities compared to crystallization on casting. By using transmission electron microscopy we present a detailed in-situ study of an unusual isothermal crystallization reaction observed in the Al₈₅Y₈Ni₅Co₂ metallic glassy alloy below T_g. It started from the formation of a primary intermetallic compound (IMC) immediately followed by a pseudo-eutectic reaction consisting of IMC and nanoscale Al (solid solution) particles while α-Al particles are significantly smaller in size than the intermetallic phase. Moreover, a majority of these nanoscale α-Al particles do not have a common interface with the primary IMC particles but are separated by the residual glassy phase which remains present within these pseudo-eutectic colonies. On the other hand, as can be assumed from the Gaussian type of the heat flow plot at the isothermal conditions and reasonable linearity of the Avrami plot this reaction takes place like a single stage process. This residual glassy phase has a high stability versus crystallization and crystallizes only at a higher temperature. However, such a crystallization mechanism is observed only on slow heating or isothermal exposure at a temperature below T_g. On rapid heating or at isothermal annealing in the supercooled liquid, primary nanocrystallization of the aluminum solid solution is observed followed by the formation of intermetallic compounds at a higher temperature.