# Analysis of Nucleation and Glass Formation by Chip Calorimetry
## 3 Results and Discussions
### Nucleation and Primary Crystallization
#### 3.1.2 Delay Time Determination in Various Amorphous Materials
The signal from the primary nucleation of the precipitation of FCC Al nanocrystals is so weak that the crystallization peak is difficult to detect in the isothermal DSC tests [21]. Therefore, one precise and convenient method would be valuable for the determination of delay time for crystal nucleation in various amorphous materials. Based on the Flash DSC with ultrafast heating and cooling rates, a new method is demonstrated to decide the delay time for primary crystallization. Under isothermal treatment, the evolution of glass transition temperature with annealing time for amorphous materials should follow two different paths: structural relaxation before the onset of crystal nucleation, and the composition or ordering change after the onset of crystallization. Thus, the break time point in the slope of the plot of the glass transition temperature change with annealing time should correspond to the delay time for the primary crystallization [22; 23]. Three different kinds of amorphous systems were selected to verify the effectiveness and universality of the method, including one typical marginal Al-based MG with poor glass formation ability (Al\({}_{92}\)Sm\({}_{8}\)), two stable MG systems with good glass formation  
Figure 2: Schemes of traditional experimental methods to determine the delay time \(\tau\) for primary crystallization in MGs: (**a**) TEM method (I); (**b**) isothermal method based on the thermal tests (II). The inserted graph in (**b**) gives the confirmation details of delay time by method II. ability (La\({}_{60}\)Ni\({}_{15}\)Al\({}_{25}\) and Mg\({}_{65}\)Cu\({}_{25}\)Gd\({}_{10}\)), and one amorphous polymer (polybutylene terephthalate, PBT). (1) Marginal Al-Based MG  
For the conventional DSC with heating rate of several K/s, the thermal signal of the glass transition for marginal Al-based MGs cannot be detected due to the close overlap of the strong crystallization exotherm. In contrast, the Flash DSC with the large heating rate range from several K/s to 10\({}^{4}\) K/s allows for the separation of the \(T_{g}\) and \(T_{x}\) signals, as described in part Section 3.1.1. Thus, the ultrafast chip calorimetry instrument provides a good opportunity to study the delay time for marginal Al-based MGs. For an Al\({}_{25}\)Sm\({}_{8}\) MG, a series of Flash DSC heat flow curves with the same heating rate of 1000 K/s was measured after annealing at 150 \({}^{\circ}\)C for different times ranging from 20 s to 10,000 s, as shown in Figure 3a. It is obvious that all of Flash DSC heat flow curves display the obvious endothermic reaction signals corresponding to glass transition and the \(T_{g}\) shifts to the higher temperature with the increase of annealing time \(t_{a}\). These results are very close to the previous research on the structural relaxation effect on MGs [24; 25]. To study the evolution of the glass transition temperature \(T_{g}\) with annealing time \(t_{a}\), \(T_{g}\) is plotted against \(t_{a}\) at 150 \({}^{\circ}\)C, as shown in the Figure 3b. From Figure 3b, it is evident that with the increase of \(t_{a}\), there appears one break point at 930 s. Before and after 930 s, the evolution paths of \(T_{g}\) with \(t_{a}\) can be roughly fitted by two lines with different slopes. These two different evolution paths correspond to the structural relaxation effect and the composition effect, and the samples before and after the break points should be amorphous and contain nanocrystals, respectively [22; 23]. Figure 3: (**a**) A series of Flash DSC heat flow curves corresponding to different annealing times ranging \(t_{a}\) from 20 to 10,000 s at 150 \({}^{\circ}\)C for Al\({}_{25}\)Sm\({}_{8}\) MG; (**b**) plot of \(T_{g}\) and \(t_{a}\) at 150 \({}^{\circ}\)C (\(t_{1}\) and \(t_{2}\) are two selected annealing times before and after the break point of 930 s); (**c**) TEM image for annealed sample corresponding to the annealing time of \(t_{1}\); and (**d**) TEM image for annealed sample corresponding to the annealing time of \(t_{2}\).