Amorphous and Aperiodic Highlights:
|
Molecular Dynamics
Simulation of A 20,000-Atom Model Binary Glass System Showing Evolution
of Shear Bands Under a Cylindrical Indentor
S.B. Biner |
||
|
|
Using MD simulations to constrain deformation under a localized load having a nanometer length scale, the embryonic stage of shear band formation in a model binary amorphous system was illuminated. Moreover, subsequent growth and transport of the shear bands can be observed throughout the simulation. In addition, parallel MD simulations reveal dynamic evolution and annihilation of free volume during deformation and also suggest that local temperature rises do not approach Tg for the model binary amorphous system.
|
|
|
The images are high resolution TEM (HRTEM) micrographs along the 5f direction of a large single grain Cd84Yb16 icosahedral quasicrystal (QC) grain grown. The ‘pin wheel’ contrast shows the local cluster structure of concentric polyhedra consisting of Cd (red) tetrahedron (1/3 occupied icosahedron), Cd dodecahedron, Yb (green) icosahedron, and a Cd icosidodecahedron, which are the building blocks of this structure described by Tsai et al. in Nature 408 (537) 2000. The unique cluster structure in this class of QCs is unlike previous structures, which are based either on Mackay or Bergman clusters. This CdYb structure is more like Rare Earth-Mg-Zn QCs where the neighboring clusters appear share edges. The light blue lines in the enlarged HRTEM image to the right show the overlapping cluster structure. The CdYb system show some very unusual physical properties compared to other QCs, such as very low hardness, decreased electronic resistivity and a low Debye temperature. |
||
|
|
||
|
Simulation of short-range
order in amorphous solid-state synthesized Zr70Pd30
M.J. Kramer and D.J. Sordelet |
||
|
|
High-energy X-ray diffraction (HEXRD) coupled with Reverse Monte Carlo (RMC) simulations can now show the difference in short-range order (SRO) between amorphous Zr70Pd30 alloys synthesized by solid-state (mechanically alloying) and vitrification (melt spinning) techniques. The partial pair distributions based on RMC simulations suggest that in both alloys chemical SRO exists with short Pd-Pd, intermediate Zr-Pd and longer Zr-Zr bonds in the first shell. Previous time-resolved HEXRD studies of vitrified Zr70Pd30 clearly show that the Al2Cu structure exists as a meta-stable phase that precedes formation of the stable Zr2Pd (MoSi2-type structure). Using the Al2Cu structure to initiate the RMC simulations reveals that the solid-state synthesized Zr70Pd30 amorphous structure shows an affinity for this meta-stable structure, in contrast to the vitrified material. The upper pair distribution function exhibits the short Zr-Pd and longer Zr-Zr bonds that exist in the Al2Cu structure. These simulations help to explain why the vitrified Zr70Pd30 devitrifies to meta-stable phases prior to forming the stable Zr2Pd phase, while the amorphous solid-state Zr70Pd30 exhibits a single transformation to the same stable phase, as illustrated in the DSC data below. |
|
 |
||
|
Critical oxygen impurity
level for glass formation in melt spun Zr80Pt20
ribbons
D.J. Sordelet and M.J. Kramer |
||
 |
Ultra-high purity synthesis techniques were used to obtain Zr80Pt20 melt spun ribbons with oxygen contents below 200 ppmw. In contrast to normal melt spun Zr ribbons, which typically contain ~ 1000 ppmw oxygen and are amorphous, the ultra-high purity ribbons are fully crystalline and have a distorted b-Zr(Pt) superstructure. At higher oxygen contents a mixture of a big cube Zr6Pt3O phase within an amorphous matrix is formed. These results suggests that oxygen plays a role in stabilizing a particular atomic coordination (e.g., icosahedral) that promotes glass formation during rapid solidification. Without sufficient oxygen at a particular quench rate, a fully crystalline solidified structure can be achieved. |
|
|
The incipient stage of shear band formation followed by continued deformation was observed while loading a bulk Zr-based metallic glass under a cylindrical indentor. These experiments are providing the necessary data to develop computer simulations at the micro and meso scales. Atomic force microscopy (AFM) was successfully used to fingerprint the deformed regions between shear bands, and clearly illustrates localized regions of inhomogeneous deformation, contrary to many earlier speculations. |
||
|
To view movie, click on desired file size. |
||
 |
Cd84Yb16 is a congruently melting quasicrystalline phase that is ideal for probing liquid-solid transformations of aperiodic structures because there is no modulation in composition during phase transformations. Moreover, having a binary, thermodynamically stable quasicrystal phase offers strong potential for obtaining full pair correlation analyses needed for complete structural determination, which to date is elusive in quasicrystalline alloys. Using a novel containment arrangement, we obtainment for the first time ever high-energy X-ray diffraction data from liquid Cd5.7Yb. These experiments were performed at MUCAT at the Advanced Photon Source within Argonne National Laboratory. |
|
|
Structural determination
of meta-stable crystalline devitrification phase from amorphous
Zr70Pd20Cu10
M.J. Kramer and D.J. Sordelet |
||
 |
Using the time-resolved synchrotron X-ray capabilities developed
in cooperation between the Ames Laboratory and the Midwestern
Universities Collaborative Access Team at the Advanced Photon
Source in Argonne National Laboratory, the frequently observed,
but until now unknown meta-stable phase that exists during a brief
transient period during the devitrfication of amorphous Zr70Pd20Cu10
alloys has been identified as the Zr2Si-type
structure using Rietveld refinement techniques. The image plate
to the side illustrates the sequential series of diffraction patterns
obtained while heating the Zr70Pd20Cu10
melt spun ribbon at 40 K/min during exposure to the high energy
X-rays (124 keV). A DSC trace obtain by heating a piece of the
sample at the same heating rate is overlayed to reveal the coordination
of the exothermic events with the structural phase transformations.
By selecting individual scans at various temperatures, the devitrification
sequence can be identified as: |
|
|
First report of synthesis-dependent
formation of quasicrystals in amorphous Zr70Pd30
and Zr70Pd20Cu10
D.J. Sordelet and M.J. Kramer |
||
 |
|
|
|
|
||
|
Synthesis
of largest (~0.75cm3)
Cd84Yb16
phase-pure quasicrystalline single grain
T.A. Lograsso |
||
 |
The largest single-grain of a Cd84Yb16 quasicrystal with high phase purity has been synthesized at Ames Laboratory using the Bridgman technique coupled with specially selected containment materials. As shown in the photograph, single grains approaching 1 cm3 can be obtained by maintaining the melt during growth in a chemically benign Ta crucible. Samples like these are being used for structural studies as well as for high-energy X-ray investigations to follow the formation of a quasicrystalline phase during congruent solidification. |
|
|
|
||
|
Determination of basic atomic
cluster structure in rare earth-Mg-Zn face-centered icosahedral
quasicrystals
M.J. Kramer |
||
 |
The large atomic number difference between yttrium and heavy rare-earth
elements is being exploited to utilize the large differences in
scattering cross-sections for electrons, X-rays and neutrons. High
resolution TEM and single grain XRD have been used to determine
the cluster structures and suggests that there are local clusters
in this face-centered icosahedral (fci) compound consisting of 5
concentric polyhedra containing 124 atoms. The HRTEM results indicate
that the local structure consists only of these 5 shells, which
may edge share with the next cluster. Based on this atomic arrangement,
the pair distribution analysis results are consistent with rare
eartch atoms partially occupying the vertices of the outer icosahedron
and dodecahedron. |
|
Phone: +1 (515)294-4446 | Fax +1(515)294-8727 | Security & Privacy Notice | Disclaimer
Ames
Laboratory is a member of the Institute
for Physical Research and Technology,
a consortium of research and technology-transfer centers at Iowa
State University.
Webmaster: bcarsten@ameslab.gov
Copyright©2006
Last update: February 19, 2008