| dc.description.abstract | Metallic glasses (MGs), which are metals solidified in an amorphous state, have shown attractive
mechanical properties such as high strength (up to 5 GPa), yield strain (around
2%) and good corrosion resistance. They exhibit heterogeneous plastic flow by formation
of shear bands (SBs) at temperatures well below the glass transition temperature.
However, they can be very brittle with KIc ∼ 1 - 15 MPa
√
m or very tough (KIc ∼ 80
MPa
√
m). Experiments and MD simulations suggest that failure in the brittle MGs occurs
by cavitation with little shear banding and can be traced to nanoscale fluctuations in
atomic density. Also, notwithstanding their high KIc, MG samples lack tensile ductility
and fail catastrophically by crack propagation in a dominant SB. However, nano-sized
MG samples and a novel architecture called as nanoglass (NG) composed of nano-grains
of MGs separated by fine free volume rich interfaces do exhibit tensile ductility. Relatively
few continuum simulations have been undertaken to understand the deformation
and fracture behavior of MGs and NGs from a mechanics standpoint. Therefore, continuum
finite element analysis of cavitation and cavitation induced fracture in brittle
MGs are performed in this work. In addition, tensile deformation behavior of nano-scale
notched MG and NG samples are analyzed.
Brittle MGs are modeled as heterogeneous elastic-plastic solid containing doubly
periodic distribution of weak zones with lower yield strength. The presence of the weak
zones mimics the density/strength fluctuation in brittle MGs as observed in experiments
and atomistic simulations. Finite element simulations are performed by subjecting a
square unit cell containing a circular weak zone to different (biaxiality) stress ratios
under 2D plane strain conditions. A tiny void is introduced in the weak zone to trigger
cavitation. The results show that the critical hydrostatic stress at cavitation is reduced
due to the presence of the weak zones and is governed by yield properties of the weak
zone and the prevailing stress state. Moreover, unlike in a homogeneous plastic solid,
the cavitation stress of the heterogeneous aggregate does not reduce appreciably as the
stress ratio decreases from unity when the yield strength of the weak zone is low. The
volume fraction of the weak zones and stress ratio influence the nature of cavitation
bifurcation. This includes the possibility of snap cavitation wherein a void of finite size
suddenly forms in the intact material which does not happen in a homogeneous plastic
solid.
Further, continuum simulations of crack initiation under mode-I plane strain, small
scale yielding conditions in a heterogeneous elastic-plastic solid having a distribution
of weak zones are performed. The results show that a three-step process is involved
in the catastrophic fracture observed in brittle MGs. First, cavities nucleate in weak
zones ahead of the crack tip and start growing rapidly. Secondly, curved shear bands
form linking the current crack tip with the nearby cavity. Thirdly, as plastic strain
and free volume accumulate within these shear bands, failure takes place facilitating
further extension of the crack. The proposed fracture mechanism explains the formation
of nano-corrugations in brittle MGs. The results also predict a correlation between
notched fracture toughness and Poisson’s ratio and brittle-ductile transition which is
qualitatively similar to that observed in experiments.
the deformation behavior of nano-sized notched MG samples subjected to plane strain
tensile loading is modeled through finite element simulations using a non-local plasticity
theory for MGs. The results show that a plastic zone first develops around the notch
root and grows to a critical size before a dominant shear band emanates from this zone
that would lead to failure. The SB width and the saturation notch root plastic zone size
scales with an intrinsic material length lc associated with interaction stress between flow
defecrs. Also, the ratio of the ligament length to saturation plastic zone size governs the
transition from shear banding to necking.
The deformation behaviour of NGs subjected to plane strain tensile loading is investigated
through finite element simulations using the above non-local plasticity theory. It
is found that the ratio of the material length lc to nano-grain size governs the deformation
behavior of NGs. Also, SB width scales in same manner with lc both in MG and
NG specimens and moderate changes in specimen size have little effect on mechanical
response of NGs. | en_US |
