Mechanical Behavior at the Nanoscale: What’s in your Toolbox?
Mechanical Behavior at the Nanoscale: What's in your Toolbox?
MEGAN J. CORDILL 0
CHRISTOPHER R. WEINBERGER 0
0 1.-Erich Schmid Institute for Materials Science, Austrian Academy of Sciences , 8700 Leoben , Austria. 2.-Department of Material Physics, Montanuniversita ̈t Leoben , 8700 Leoben , Austria. 3.-Department of Mechanical Engineering, Colorado State University , Fort Collins, CO 80523 , USA. 4.-School of Advanced Materials Discovery, Colorado State University , Fort Collins, CO 80523, USA. 5.-
For materials scientists studying mechanical
behavior at the nanoscale, there are several tools
available for use, the most notable being a
nanoindenter. Since the introduction of depth-sensing
nanoindentation1 in the 1980s material scientists
working the field of mechanical behavior have, at
some point, used a nanoindenter to measure
hardness and modulus of single grains or thin films. A
nanoindenter can be compared to a hammer in the
toolbox of a carpenter. It is used not only to drive in
nails, but has other uses as well (to remove nails).
However, like material scientists, carpenters need
more than a hammer. Materials scientists want to
see how their nails are reacting to the wood or how
the grain of the wood changes how far the nail goes.
Because we are always searching for more, a
nanoindenter was installed inside a transmission
electron microscope (TEM).2 With this simple
addition, the indenter’s reaction to dislocations, grain
boundaries, and precipitates could be observed,
quantified, and simulated since now we could see
and thus believe what the nail was doing.
Initially, the mechanical behavior of thin films
and bulk materials was evaluated with
nanoindentation. When smaller samples could be achieved,
such as single crystalline nanoparticles, then the
superhard material behavior could also be
determined.3 Additionally, with the widespread
availability of focused ion beam (FIB) tools, researchers
could shape their own nails, or micropillars,4 of any
material to evaluate the stress–strain behavior at
the microscale, and eventually, the nanoscale.5
From the stress–strain curve, yield and work
hardening could be determined from within single grains
or of thin films, something that was lacking at the
nanoscale. There have been over 20,100 papers with
Megan J. Cordill and Christopher R. Weinberger are the JOM advisors for
the Nanomechanical Materials Behavior Committee of the TMS Materials
Processing & Manufacturing Division (MPMD), and guest editors for the
topic Mechanical Behavior at the Nanoscale in this issue.
the word ‘‘micropillar’’ in the title since this
technique was introduced. However, researchers
wanted nanoindenters to provide them with more
information, more data, more insight into the
behavior at the nanoscale. For that reason,
indenters were additionally installed inside scanning
electron microscopes, with x-ray beam lines and
equipped with high- and low-temperature
capabilities. New sample geometries were created to
remove the testing artefacts of the pillars, such as
tensile bars and various cantilevers. From all of
these innovative experiments came the ‘‘smaller is
stronger’’ theory,6 dislocation starvation,
sourcelimited deformation, exhaustion hardening, and
discussions on FIB damage and tip degradation.
While there is no doubt that the nanoindenter is
the tool of choice for material scientists studying the
mechanical behavior at the nanoscale
experimentally, modelers tend to use tools that are
appropriate to the specific phenomenon in which they are
interested, which is often a compromise between
domain size, time, and accuracy. For example,
discrete dislocation dynamics (DDD) has provided
dramatic insight into the motion of a large number
dislocations in microscale samples, and nanoscale
mechanics has provided the fuel to further develop
this tool. Specifically, DDD has demonstrated how
dislocation source truncation can give rise to the
smaller-is-stronger behavior as well as how
dislocation starvation can give rise to exhaustion
hardening.7 However, to understand fundamental defect
mechanisms, such as dislocation nucleation,
classical atomistic simulations are more often used.
Atomistic simulation methods have further clarified
the role dislocation nucleation plays in contributing
to mechanical response at these scales.8
The inequity of length scales associated with the
fundamentals of mechanical behavior, notably
defects, has been a great challenge and often
prevents direct comparisons between experiments
and modeling. This problem has been partially
alleviated at the nanoscale since the sample sizes
are reduced below 1 lm, allowing a direct size-scale
comparison. This has led to more in-depth
discussions of the mismatch in time scales. However,
necessity is the mother of invention, and
advancements in both experiments and modeling in efforts
to bridge this gap have come about. This includes
the development of high-speed in situ TEM to study
high strain rate behavior,9 as well as accelerated
time scale sampling methods for both molecular
dynamics simulations10 and discrete dislocation
In the first paper of this issue focusing on
mechanical behavior at the nanoscale,
nanoindentation and ultrasonic velocity measurements were
used to independently confirm that hydrogen
exposure can reduce the shear modulus and the c44
stiffness constant by approximately 20%. This
reduction of the elastic properties of
hydrogencharged nickel was also determined to reduce the
necessary stress needed for dislocation nucleation
and motion compared to uncharged nickel. The
details of these findings can be found in ‘‘Probing
the Effect of Hydrogen on Elastic Properties and
Plastic Deformation in Nickel Using
Nanoindentation and Ultrasonic Methods’’ by Samantha
Lawrence and co-workers.
Nanoindentation is also an ideal technique to
study super-elastic and shape memory effects in
materials. In ‘‘A Nanoindentation Study of the
Plastic Deformation and Fracture Mechanisms in
Single-Crystal CaFe2As2’’ by Seok-Woo Lee and
coworkers, it was found that indenting along the 
generated strain bursts (pop-ins), radial cracking,
and lateral cracking. Examination of the indent
imprints correlated to the first pop-ins being related
to dislocation nucleation in the single crystal and
the radial crack formation to the second pop-ins in
the load–displacement curves. The lateral cracking
under the indent was examined with density
functional theory calculations to reveal that the atomic
layered structure of the CaFe2As2 has a weak (001)
plane compared to the (100) plane.
Since an indenter was incorporated into a TEM,
our understanding of dislocation nucleation and
plastic deformation of single crystals has improved.
However, our experimental understanding about
how grain boundaries interact is still in its early
stages. Qianying Guo and Gregory Thompson
demonstrate how nanocrystalline grains plastically
deform in their paper ‘‘In-situ Indentation and
Correlated Precession Electron Diffraction Analysis
of Polycrystalline Cu Thin Films.’’ With precession
electron diffraction, the diffraction contrast and
microstructure phenomena can be better deciphered
to provide more insight into grain boundary
preservation, strain distribution and dislocation
nucleation at large-angle grain boundaries.
Dislocation velocities in single-crystal
body-centered cubic (BCC) Nb, face-centered cubic (FCC) Au,
and an Al0.3CoCrFeNi high-entropy alloy (HEA),
also FCC, were evaluated in the paper ‘‘Fast Slip
Velocity in a High-Entropy Alloy’’ by Rizzardi,
Sparks, and Maaß. Here, clear statistically
significant differences were observed between the BCC
and FCC materials, but not much difference
between the Au and HEA. The results indicate that
the slip in the FCC examples proceeds statistically
with the same peak velocity and could suggest that
the mobility of dislocations is dominated by a
In order to provide models of mechanical
behavior, the behavior of the fundamental carriers of
plastic deformation, i.e. dislocations, must not only
be characterized but this information must also be
suitably transferred to larger scales. For example,
the work by Khanh Dang and Douglas Spearot
titled ‘‘Pressure Dependence of the Peierls Stress in
Aluminum’’ provides an atomic level study of how
stress states, which can be quite large in small
volumes, alter the properties of dislocations in
aluminum. This type of atomic-level
characterization is important for developing physically informed
DDD simulations. Similarly, it is important to
develop methods that are able to simulate
dislocations without interatomic potentials such that more
rigorous data, such as density functional theory, can
be used as inputs. In the paper ‘‘Comparing
Modeling Predictions of Aluminum Edge Dislocations:
Semidiscrete Variational Peierls–Nabarro Versus
Atomistics,’’ Lucas Hale examines the robustness of
Peierls–Nabarro models in representing dislocation
This collection of papers is a well-balanced
representation of ongoing work in the area of nanoscale
mechanics. In all of the experimental papers, the
nanoindenter is the main tool of choice and acts as
the primary method of investigation, supplemented
by additional experimental techniques providing
important advancements in our field. The modeling
papers show a balance of both explaining
experimental observations as well as further efforts in
developing comprehensive physically-based models.
However, we do not expect that these methods
signify the pinnacle of nanoscale mechanics, as new
methods, enhancements, and models are sure to be
developed to further our understanding of
mechanics at the smallest and most fundamental level.
Access the papers published under the topic
‘‘Mechanical Behavior at the Nanoscale’’ in the July 2018
issue (vol. 70, no. 7) of JOM via the JOM page at
‘‘Probing the Effect of Hydrogen on Elastic
Properties in Nickel Using Nanoindentation
and Ultrasonic Methods’’ by S.K. Lawrence,
B.P. Somerday, M.D. Ingraham, and D.F. Bahr.
‘‘A Nanoindentation Study of the Plastic
Deformation and Fracture Mechanisms in
SingleCrystalline CaFe2As2’’ by Keara G. Frawley,
Ian Bakst, John T. Sypek, Sriram Vijayan,
Christopher R. Weinberger, Paul C. Canfield,
Mark Aindow, and Seok-Woo Lee.
‘‘In-situ Indentation and Correlated Precession
Electron Diffraction Analysis of Polycrystalline
Cu Thin Films’’ by Qianying Guo and Gregory B.
‘‘Fast Slip Velocity in a High-Entropy Alloy’’ by
Q. Rizzardi, G. Sparks, and R. Maaß.
‘‘Pressure Dependence of the Peierls Stress in
Aluminum’’ by Khanh Dang and Douglas
‘‘Comparing Modeling Predictions of Aluminum
Edge Dislocations: Semidiscrete Variational
Peierls–Nabarro versus Atomistics’’ by Lucas
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