Like other amorphous materials, the nanoscale atomic structure of amorphous metal alloys is difficult to determine with conventional structural analysis techniques like x-ray diffraction. We apply fluctuation electron microscopy to measure nanoscale structure in various alloys which controls the recrystallization and mechanical properties of these materials. In the long term, these results may have broader implications for nucleation and crystallization of crystals from liquids and for the general understanding of plastic deformation of non-crystalline materials.

Crystallization of Metallic Marginal Glass Formers

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 10⁵-10⁶ K/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.

Figure 1: V(k) measured at 1.5 nm spatial resolution for amorphous Al₉₂Sm₈, as-quenched, quenched then annealed at 140 deg C, and mechanically amorphized. The as-quenched V(k) has peaks at the Al (200) and (220) reflections at 0.49 A^-1 and 0.69 A^-1 respectively. The mechanically amorphized V(k) shows no crystalline Al-like peaks.

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.

This work is supported by the US National Science Foundation under contract DMR-0347746.

Plastic Deformation of Bulk Metallic Glasses

The discovery of bulk metallic glass (BMG) alloys in the mid-1980s⁶ raised the possibility of structural applications of this class of materials. Since then, multi-component alloy systems based on Mg,  La, Zr, Fe, Pd, Cu, Pd-Fe, Ti, and Ni have been disocvered.^7,8  The critical cooling rate for modern BMGs has been reduced as low as 0.1 K/s, and maximum forming thickness has been increased up to 100mm.  Various alloys show greatly enhanced glass forming ability, high strength at low weight, extremely high elastic modulus, and excellent corrosion resistance. For a good recent review, see Ref. 7.

One obstacle to structural applications of BMGs is their low overall ductility. Plastic deformation in metallic glasses is concentrated into narrow regions called shear bands. While the deformation inside the band may be significant, the volume involved is small, so failure occurs at a macroscopic strain of only a few percent. Various strategies have been employed to deflect or blunt the shear bands, but a general atomistic understanding of how and why they form and how they change the atomic structure of the material is lacking.

The most successful theory so far, proposed originally by Argon⁹ and recently elaborated by Falk¹⁰ and Schuh,¹¹ is based on shear transformation zones (STZ). An STZ is analogous to a nanoscopic dislocation loop, as suggested in Figure 2. Under stress, some small group of atoms shifts with respect to a neighboring loop, creating plastic deformation. In a crystal, this deformation would be able to propagate in the form of a dislocation. A glass lacks the long-range structure to support that kind of propagation, so the deformation remains confined to the to a small volume, probably on the order of a nanometer or two in diameter.

Figure 2: A shear transformation zone visualized as a nanoscopic dislocation loop. Under stress, the top half of the blue atoms shift with respect to the bottom half, but the surrounding disordered yellow atoms prevent the displacement from propagating beyond a distance of 1-2 nm. Adapted from Ref. 9.

Structures and deformation that might serve as STZs have been observed in molecular dynamics models of simple metal alloys with, for example, a Lennard-Jones potential.¹⁰ Our goal is to use fluctuation electron microscopy, electron diffraction, and electron spectroscopy to find evidence for STZs in real metal alloys. This could involve finding nanometer scale atomic structures that might support some local deformation, or finding direct evidence for structural changes caused by STZs. A secondary goal of this project is to characterize the structural differences of material inside shear bands.

In parallel with our structural characterization, Don Stone's group is measuring the mechanical properties of the same BMGs using variable temperature nanoindentation.

This work is support by the National Science Foundation under contract CMS-0528073.

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