A marginal glass former is an alloy that requires a very high cooling rate to avoid crystallization. Amorphous metals consisting primarily of aluminum fall into this category. They generally require cooling rates of 105-106K/s to quench a high temperature liquid into the amorphous state. Al-based glasses have unusual crystallization behavior as well. Instead of crystallizing to a mix of intermetallic phases determined by the composition, the first phase that forms is pure Al in the form of nanocrystals surrounded by material that remains amorphous. These nanocrystals grow rapidly to about 10 nm in diameter, then are fairly stable over a range of temperature. Crystallization to an intermetallic occurs only at still higher temperature.
There are competing explanations for this phenomena. One, proposed first by Blanks-Bewersdorf,1 then advanced significantly by the Perepezko group at UW,2 is that the quenched samples contained quenched-in nuclei of Al that drive crystallization. These tiny “proto-crystals” are formed in the high-temperature liquid or during the rapid quench, but the thermal energy is removed from the system so quickly that there is not enough time for them to grow into recognizable crystals. The resulting material is a growth-controlled glass, since some nucleation occurs, but growth of the low-energy crystal phase is prevented.
The other hypothesis is that that there is a phase separation step preceding crystallization in which two phases, which are both structurally amorphous but have different composition form. The crystallization is then driven by the interface or composition gradient between these regions. This hypothesis was proposed by Gangopadhyay et al.3 and has been recently supported by others.4
Our FEM results on amorphous Al92Sm8 show that rapid quenching of this alloy from the liquid leads to the formation of small ordered clusters similar in structure to crystalline Al.5 Figure 1 shows FEM data V(k) for Al92Sm8 amorphized by rapid quenching and by intense mechanical deformation. For the as-quenched glass, the peaks in V(k) correspond to the Al (200) and (220) reflections. Simulations confirm that these peaks arise from Al-like clusters. These measurements were performed at 1.5 nm spatial resolution, which limits the cluster size to a range of approximately 1-3 nm.
The mechanically amorphized sample, on the other hand, shows no peaks in any Al reflection position, indicating a completely different form of nanoscale order. Mechanically amorphized Al92Sm8 also does not crystallize to Al nanocrystals, strongly suggesting that the Al-like clusters in the as-quenched samples drive that crystallization reaction.
Annealing the quenched samples causes a reduction in V(k) at both peak positions, indicating a reduction in nanoscale order. The reduced order could come from a reduction in the size of the clusters, their density, or both. (It is not possible to separate the effects of size and density from this data.) This suggests that at least some of the clusters we are measuring are subcritical in size and therefore unstable at this annealing temperature. Future experiments will further probe the relaxation of these subcritical clusters.
All of these structural results are consistent with the quenched-in nuclei hypothesis: we see small clusters with the same internal structure as the eventual crystallization products. Some of the clusters are subcritical, and the clusters are not present in a sample with different crystallization. Similar FEM results indicating nanoscale Al-like clusters have also been observed in rapidly quenched Al88 Y7 Fe5.
M. Blank-Bewersdorff, Crystallization behavior of Al86Ni10Zr4 and Al86Fe10Zr4 metallic glasses. J. Mat. Sci. Lett. 10, 1225-1227 (1991).
J. C. Foley, D. R. Allen, and J. H. Perepezko, Analysis of nanocrystal development in Al-Y-Fe and Al-Sm glasses. Scripta Mat. 35, 655-660 (1996); J. H. Perepezko and R. J. Hebert, Amorphous Aluminum Alloys – Synthesis and Stability. JOM 54, 34-39 (2002).
A. K. Gangopadhyay, T. K. Croat, and K. F. Kelton, The effect of phase separation on the subsequent crystallization in Al88Gd6La2Ni4. Acta Mater. 48, 4035-4043 (2000); K. F. Kelton, T. K. Croat, A. K. Gangopadhyay, L.-Q. Xing, A. L. Greer, M. Weyland, X. Li, and K. Rajan, Mechanisms for nanocrystal formation in metallic glass. J. Non-Cryst. Sol. 317, 71-77 (2003).
Y. B. Wang, H. W. Yang, B. B. Sun, B. Wu, J. Q. Wang, M. L. Sui, and E. Ma, Evidence of phase separation correlated with nanocrystallization in Al85Ni5Y6Fe2Co2 metallic glass, Scipta Mat. 55, 469-472 (2006).