Roles of nanoclusters in shear banding and plastic deformation of bulk metallic glasses Page: 1 of 6
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DE-FG02-06ER46338 FINAL REPORT
During the course of this research we published 33 papers in various physics/material
journals. We select four representing papers in this report and their results are summarized as
follows.
To study shear banding process, it is pertinent to know the intrinsic shear strain rate within a
propagating shear band. To this aim, we used nanoindentation technique to probe the
mechanical response of a Au49Ags.5Pd2.3Cu6.9Si16.3 bulk metallic glass in locality and found
notable pop-in events associated with shear band emission. Using a free volume model and
under the situation when temperature and stress/hardness are fixed result in an equation:
[n( )- A]oc AH
where i is the strain rate, AH is the hardness serration due to pop-in, and A is a constant. This
equation predicts that hardness serration caused by pop-in is expected to decreases exponentially
with the strain rate. Our data are in good agreement with the prediction, as shown in the figure
below.
0.45
I' 0.40 RT
0.
C2 0.35
* 0.30
'ii 0.25
o 0.20
2 0.15
4) 0.10
(A 1700
= 0.05
0.00
0.001 0.1 10 1000 100000
Strain Rate (s1')
The result also predicts that, when strain rate is higher than a critical strain rate of i,=1700 s-1,
there will be no hardness serration, thereby no pop-in. In other words, multiple shear bandings
will take place and material will flow homogeneously. The critical strain rate of i,=1700 s-1 can
be treated as the intrinsic strain rate within a shear band. We subsequently carried out a
simulation study and showed that, if the imposed strain rate was over ge, the shear band spacing
would become so small that the entire sample would virtually behave like one major shear band.
Using the datum i,=1700 s-1 and based on a shear band nucleation model proposed by us, the
size of a shear-band nucleus in Au-BMG was estimated to be 3 x 106 atoms, or a sphere of -30
nm in diameter.
Inspired by the peculiar result published in a Science article "Super Plastic Bulk Metallic
Glasses at Room Temperature", we synthesized the Zr-based bulk metallic glass with a
composition identical to that in the paper (Zr64.13Cuis.75Niio.12Alio) and, subsequently, tested in1
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Nieh, T.G. Roles of nanoclusters in shear banding and plastic deformation of bulk metallic glasses, report, July 31, 2012; Knoxville, Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc828578/m1/1/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.