Investigation of the nucleation delay time in Al-based metallic glasses by high rate calorimetry

Abstract During isothermal annealing at temperatures lower than the crystallization onset temperature, Tx, measured during continuous heating, the Al88Y7Fe5 metallic glass undergoes a period of delay time (τ) before the primary crystallization reaction. The delay time is an important parameter, which is closely related with the transport behavior and can be used to calculate the attachment frequency coefficient and steady state nucleation rate. Previously, τ was usually determined from the nanocrystal number density vs. annealing time plot through tedious TEM measurements. Now, a more effective approach to measure τ has been developed by analyzing the Tg shift resulting from annealing with high rate differential scanning calorimetry (FlashDSC). With the increase of annealing time (tannealing), Tg shifts to higher temperatures and in the Tg vs. tannealing plot, there is a break point in slope. Before this break point, no Al nanocrystals could be detected by transmission electron microscopy (TEM) but after this break point, Al nanocrystals were identified by TEM. Thus, the break point time corresponds to the onset of Al nanocrystal formation and the delay time. The underlying mechanism is unveiled through the matrix composition measurement by energy dispersive spectroscopy (EDS). The analysis reveals that before the break point the amorphous matrix composition is unchanged by annealing so that Tg shifts solely due to the increased glass stability through a relaxation process. After the break point, the precipitation of Al nanocrystals induces a composition change in the amorphous matrix so that Tg shifts to higher temperatures because of both the relaxation and the composition change effects.