Unveiling the mechanism of phase and morphology selections during the devitrification of Al-Sm amorphous ribbon

The complex interplay between energetic and kinetic factors that governs the phase and morphology selections can originate at the earliest stage of crystallization in the amorphous parent phases. Because of the extreme difficulties in capturing the microscopic nucleation process, a detailed picture of how initial disordered structures affect the transformation pathway remains unclear. Here, we report the experimental observation of widely varying phase selection and grain size evolution during the devitrification of a homogeneous melt-spun glassy ribbon. Two different crystalline phases $\theta \text{−}{\mathrm{Al}}_5\mathrm{Sm}$ and $\varepsilon \text{−}{\mathrm{Al}}_{60}{\mathrm{Sm}}_{11}$ are found to form in the different regions of the same metallic glass (MG) ribbon during the devitrification. The grain size of the $\varepsilon \text{−}{\mathrm{Al}}_{60}{\mathrm{Sm}}_{11}$ phase shows a strong spatial heterogeneity. The coarse-grained $\varepsilon \text{−}{\mathrm{Al}}_{60}{\mathrm{Sm}}_{11}$ phase coupled with the small volume fraction of the $\theta \text{−}{\mathrm{Al}}_5\mathrm{Sm}$ phase is preferably formed close to the wheel side of the melt-spun ribbon. Combining experimental characterization and computational simulations, we show that phase selection and microstructure evolution can be traced back to different types and populations of atomic clusters that serve as precursors for the nucleation of different crystalline phases. Inhomogeneous cooling rates cause different structure orders across the glass sample during the quenching process. Our findings provide direct insight into the effect of structural order on the crystallization pathways during the devitrification of MG. It also opens an avenue to study the detailed nucleation process at the atomic level using the MG as a platform and suggests the opportunity of microstructure and property design via controlling the cooling process.

Figure 1

Phase transformation and microstructure of Al-Sm metallic glass (MG) upon devitrification. (a) Accumulated wide-angle x-ray scattering (WAXS) patterns as a function of temperature with a constant heating rate of 10 °C/min and corresponding dynamic scanning calorimetry (DSC) curve. (b) General Structure Analysis System Rietveld result of WAXS pattern at a temperature of 500 K. (c) Bright-field (BF) transmission electron microscopy (TEM) images of melt-spun ribbon after heating to 500 K in the DSC. The selected area diffraction (SAD) pattern (inset) is obtained from the amorphous region close to the wheel side. The arrow indicates the direction away from the wheel side. (d) High-resolution (HR)-TEM image of the interface between $\varepsilon \text{−}{\mathrm{Al}}_{60}{\mathrm{Sm}}_{11}$ and amorphous phases; fast Fourier transform (FFT) patterns (inset) from two different grains. (e) Energy-dispersive x-ray spectroscopy (EDS) element profiles along the dashed line marked in (c).

Figure 2

Dependence of phase selection on the distance from the wheel side of a fully crystallized sample. (a) Bright-field (BF) transmission electron microscopy (TEM) images of melt-spun ribbon (MSR) annealed at 508 K for 720 s. The arrow indicates the direction away from the wheel side. (b) High-angle annular dark-field (HAADF) scanning TEM (STEM) images of the region marked by the dashed line in (a). (c) and (d) High-resolution HAADF-STEM images of $\varepsilon \text{−}{\mathrm{Al}}_{60}{\mathrm{Sm}}_{11}$ and $\theta \text{−}{\mathrm{Al}}_5\mathrm{Sm}$ phases corresponding to A and B regions in (b), respectively. The insets are fast Fourier transform (FFT) pattern and superposed atomic structures with the patterns where red dots represent Sm, and blue dots are Al.

Figure 3

Dependence of grain size on the distance from wheel side to free side. (a) Bright-field (BF) transmission electron microscopy (TEM) images of melt-spun ribbon (MSR) sample annealed at 478 K for 660 s, in which minor amorphous phase is retained close to the wheel side. The arrow indicates the direction away from the wheel side. (b) Grain size as a function of distance to wheel side surface in three samples: two samples were partially crystallized at 478 K for 660 s, and one sample was fully crystallized at 508 K for 720 s; blue curve is a guide to the eye.

Figure 4

The population of crystalline clusters as a function of cooling rates. Crystal structures of (a) $\theta \text{−}{\mathrm{Al}}_5\mathrm{Sm}$ and (b) $\varepsilon \text{−}{\mathrm{Al}}_{60}{\mathrm{Sm}}_{11}$. The 3661 and 16661-type clusters are highlighted with yellow and green, respectively. Red is Sm, and blue is Al. The population of (c) 3661- and (d) 16661-type clusters as a function of cooling rates. The inserts in the upper and lower panel show the 3661 and 16661 clusters, respectively. The dashed line indicates the exponential fittings.

Figure 5

Grain size at $t=6\beta /{\alpha }^2$ as a function of $t_0$ according to the crystal growth model. The arrow indicates the direction of increasing distance ($d$) from the wheel side.