During isothermal annealing at temperatures lower than the crystallization onset temperature, Tx, that is measured during continuous heating, metallic glasses (MGs) undergo a period of delay time (τ) before the crystallization reaction. The delay time plays an important role in the understanding of the transport and nucleation behavior [1-2]. Previously, x 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 via high rate differential scanning calorimetry (Flash DSC) [3-4]. For Al-based MGs, with the increase of annealing time (ta), Tg shifts to higher temperatures in the Tg vs. ta plot and there appears a break point in slope. Before this break point, no Al nanocrystals could be detected by TEM, but after this break point, Al nanocrystals were identified by TEM. Thus, the break point time corresponds to the delay time for primary crystallization. The underlying mechanism is unveiled through the crystallization enthalpy analysis by Flash DSC and matrix composition measurement by energy dispersive spectroscopy. The analysis reveals that before the break point, Tg shifts solely due to 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. A similar behavior was observed for the shift of Tx following annealing and has a similar origin in both structural relaxation and composition change. As a further application of the new method, the effect of a 1 at.% substitution by Cu on the delay time of A188Y7Fe5 was investigated to determine the effect of minor alloying on the glass formation ability. The significant changes in Tx were demonstrated to be directly related to the corresponding changes in the delay time. The results indicate that minor alloying that is designed to increase the delay time is an effective strategy to enhance the glass formation ability [5].
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