Rate dependence of serrated flow during nanoindentation of
a bulk metallic glass
C.A. Schuh and T.G. Nieh
Materials Science and Technology Division, Lawrence Livermore National Laboratory,
Livermore, California 94550
Y. Kawamura
Department of Mechanical Engineering and Materials Science, Faculty of Engineering, Kumamoto
University, Kumamoto, 860, Japan
(Received 8 January 2002; accepted 8 April 2002)
Plastic deformation of Pd–40Ni–20P bulk metallic glass (BMG) was investigated by
instrumented nanoindentation experiments over a broad range of indentation strain
rates. At low rates, the load–displacement curves during indentation exhibited
numerous serrations or pop-ins, but these serrations became less prominent as the
indentation rate was increased. Using the tip velocity during pop-in as a gauge of
serration activity, we found that serrated flow is only significant at indentation strain
rates below about 1–10/s. This result suggests a transition in plastic flow behavior at
high strain rates, in agreement with prior studies of BMGs under different modes of
loading.
separate work, these authors found similar results during
I. INTRODUCTION
a mode-III tear test of thin amorphous metallic rib-
bons.12,13 Kimura and Masumoto4,10–13 also demon-
strated that plastic deformation by shear band
propagation follows an Arrhenius-type kinetics and that
flow serrations disappear at high rates of displacement.
This suggests that shear band nucleation/propagation is a
rate-dependent process.
Although bulk metallic glasses (BMGs) exhibit desir-
able room-temperature strength, typically near 2 GPa,
plastic strain in these materials is carried by narrow
bands of shear localization. In tension, this shear banding
leads to low ductility and rapid shear-off failure along a
single plane inclined to the stress axis.1 In other modes of
loading, shear bands appear to nucleate and propagate in
discrete bursts, giving rise to rapid plastic strain devel-
opment at a critical stress level. For example, after the
yield stress has been reached during quasi-static com-
pression of Pd- or Zr-based BMGs, serrated plastic flow
is observed, where the stress-strain curve is not smooth,
but punctuated by many small load drops.2–6 These ser-
rations have been correlated with the motion of indi-
vidual shear bands through the specimen, where each
contributes a small increment of plastic strain to the mac-
roscopic stress–strain curve.2 The importance of shear
bands in plastic deformation of BMGs is emphasized by
recent results on BMG composite materials, where the
interception of shear bands by a dispersed second phase
impacts the global deformation response and can sub-
stantially increase tensile ductility.7–9
Recent investigations by Wright et al.2 and Golovin
et al.14 have pointed out that nanoindentation may be
used to probe details of serrated flow behavior in BMGs.
Working with a Zr-based BMG, Wright et al.2 observed
the onset of plastic flow during nanoindentation as a
discontinuity in the load–displacement curve, and attrib-
uted this “pop-in” to the nucleation of a shear band be-
neath the indenter tip. Aside from this isolated pop-in,
there was no other clear evidence of shear-band activity
in the load–displacement curve, a result consistent with
the recent study by Vaidyanathan et al.,15 who measured
a smooth load–displacement curve to depths as large as
10 m on a similar alloy. Although Vaidyanathan et al.15
did not study pop-in events, they did observe shear bands
ex situ, both in and around their Berkovich indentations.
In contrast with these studies on Zr-based BMGs, the
nanoindentation results of Golovin et al.14 exhibit pro-
nounced serrations in the load–displacement curves of a
Pd–30Cu–10Ni–20P alloy, not only at the initial onset of
plastic deformation (as in the work of Wright et al.2) but
at multiple loads during a single test. In this article, we
extend the use of nanoindentation for the study of ser-
rated flow, by examining the influence of indentation
strain rate in a Pd-based BMG.
Although serrated flow of BMGs is most commonly
observed under uniaxial compression,2–6 it has also
been observed under more complex stress states. For
example, Kimura and Masumoto10,11 have measured
displacement bursts during mode-I crack opening ex-
periments on a Pd–6Cu–16Si amorphous metal, and
observed the development of shear bands on the
exposed surfaces of specimens during testing. In a
J. Mater. Res., Vol. 17, No. 7, Jul 2002
© 2002 Materials Research Society
1651
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