Linkage Between Atomic Structure of Liquids and Meta-stable
Phase Formation in Amorphous
Metallic Systems
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Personnel: D. J. Sordelet (PI), M. F. Besser (Assistant Scientist), E. Rozhkova (Post Doc)
Abstract:
Various amorphous metallic alloys exhibit extended non-equilibrium behavior
by transforming from the amorphous state into a non-equilibrium phase(s)
upon heating before ultimately transforming into a stable assembly of crystalline
phases. The objective of this project is to examine the formation, stability
and transformation of meta-stable devitrification phases in amorphous structures
formed by various routes. In particular, special emphasis will be directed
toward searching for a linkage between the atomic structure of the liquid
and amorphous states and the ensuing devitrification phases.
Recent Results:
Numerous Zr- and Hf-based metallic glasses have been found to form non-equilibrium
icosahedral quasicrystalline (QC) structures as intermediate devitrification
phases. Koester1 was the first to report
devitrification phases having a QC structure in bulk metallic glasses.
The existence of icosahedral short-range order (SRO) in metallic glasses
has often been speculated to be associated with meta-stable QC formation.
Supportive evidence for icosahederal order in undercooled liquids was observed
by Holland-Moritz2 based on the argument
of decreased interfacial energy between icosahedral solidification phases
and the melt in comparison to other polytetrhedral configurations and the
melt. Recent reports by Saida et al. using TEM3 and
DSC4 analyses to investigate Zr-base metallic
glasses strongly infer that icosahedral SRO is present in the amorphous structure
and evolves to a meta-stable QC phase. However, no reports to date have unequivocally
shown that icosahedral SRO clusters in the liquid or amorphous states are
indeed precursors to QC formation.
A long-range goal of this project is to gain a deeper understanding of the
linkage between SRO in the liquid, amorphous and meta-stable devitrified
states. Currently, we are examining the role of SRO in amorphous Zr70Pd20Cu10 alloys
by employing contrasting synthesis routes: melt spinning and mechanical milling.
As illustrated in the DSC results to the right, amorphous melt spun ribbons
(MSR) devitrify initially to a meta-stable QC phase. As discussed above,
this may be associated with icosahedral SRO in the melt that is quenched-in
during melt spinning. Conversely, mechanically milled powders (MMP) produced
from a fully crystallized Zr70Pd20Cu10 ingot
devitrify through a single transformation to the equilibrium Zr2Pd
structure.
High energy X-ray diffraction data obtained at the Advanced Photon Source do not show any evidence of crystallization in either material; however some structural differences are visible in the patterns on the facing page. Some Fe and O contamination occurs during mechanical milling. To address the potential effects of such contaminants, additional Zr-Pd-Fe-O ribbons were prepared with amounts of Fe and O comparable to the milled powder; however, QC formation was still observed upon heating. Isothermal DSC is a sensitive probe to discern between nucleation and growth from an amorphous phase and simple growth of existing nuclei or grains. The parabolic exothermic profiles below of the mechanically milled Zr70Pd20Cu10 powders are strong evidence that the solid-state derived material is predominantly amorphous and devitrifies to the final equilibrium structure via nucleation and growth transformation.
Significance:
The arguments favoring icosahedral SRO in the MSR are linked to the direct
formation of the amorphous structure from a liquid. The route from a fully
crystallized Zr2Pd structure to an amorphous
powder by mechanical disordering does not offer the opportunity for a similar
SRO. These preliminary results suggest that without icosahedral SRO in the
amorphous structure, transformation to the meta-stable QC phase transformation
is not energetically favored. As such, the formation of a meta-stable QC
phase from an amorphous structure is not dominated directly by composition.
The idea of alternate synthesis routes for forming amorphous structures opens
the opportunity to examine a wide variety of alloys having contrasting devitrification
behaviors.
Future Work:
High energy X-ray scattering studies will be extended to incorporate analysis
of pair distribution functions analysis to understand the subtle structural
differences currently observed between the Zr70Pd20Cu10 MSR
and MMP in both as-prepared states and throughout devitrification. Similar
studies of glass-forming alloys in their liquid state using X-rays and neutrons
will be performed. Alternate solid-state synthesis routes, such a rolling
of elemental foils, are desirable for mitigating some of the possible artifacts
introduced using the current mechanical milling approach. This will enable
a more precise determination of chemical and SRO affects among the prime
constituents of specific alloys. Determining such effects will provide a
more fundamental understanding of the linkage between atomic SRO and devitrification
in metallic glass-forming systems.
Interactions:
- M. J. Kramer (M&C) and A. I. Goldman (CMP) together with the Advanced Photon Source (MU-CAT and SRI-CAT) at Argonne National Laboratory for performing dynamic structural investigations with high energy X-ray scattering in real time isothermal modes
- U. and M. Calvo-Dahlborg at the CNRS Laboratoire LSG2M
at the Ecole des Mines de Nancy to analyze the structure of liquid and
amorphous metallic alloys using neutrons
1 Köster, U., Meinhardt, J., Roos,
S. and Liebertz, H., Appl. Phys. Lett. 1996; 69; 179.
2 Holland-Moritz, D., Mat.
Sci. Eng. 2001; A304-306; 108.
3 Saida, J., Matsushita,
M. and Inoue, A. Appl. Phys. Lett. 2001; 79; 412.
4 Saida, J., Matsushita,
M. and Inoue, A. J. Appl. Phys. 2000; 88; 6081.
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Last update: February 19, 2008