293 research outputs found
Solid Solution Strengthening and Softening Due to Collective Nanocrystalline Deformation Physics
Solid solution effects on the strength of the finest nanocrystalline grain
sizes are studied with molecular dynamics simulations of different Cu-based
alloys. We find evidence of both solid solution strengthening and softening,
with trends in strength controlled by how alloying affects the elastic modulus
of the material. This behavior is consistent with a shift to collective grain
boundary deformation physics, and provides a link between the mechanical
behavior of very fine-grained nanocrystalline metals and metallic glasses.Comment: Published in Scripta Materiali
Dislocation-assisted linear complexion formation driven by segregation
Atomistic simulations are used to study linear complexion formation at
dislocations in a body-centered cubic Fe-Ni alloy. Driven by Ni segregation,
precipitation of the metastable B2-FeNi and stable L10-FeNi phases occurs along
the compression side of edge dislocations. If the Ni segregation is not intense
enough to ensure precipitate growth and coalescence along the dislocation
lines, linear complexions in the form of stable nanoscale precipitate arrays
are observed. Critical conditions such as global composition and temperature
are defined for both linear complexion formation and dislocation-assisted
precipitation.Comment: 4 figure
Effect of grain boundary character on segregation-induced structural transitions
Segregation-induced structural transitions in metallic grain boundaries are
studied with hybrid atomistic Monte Carlo/molecular dynamics simulations using
Cu-Zr as a model system, with a specific emphasis on understanding the effect
of grain boundary character. With increasing global composition, the six grain
boundary types chosen for this study first form ordered complexions, with the
local segregation pattern depending on the grain boundary core structure, then
transform into disordered complexions when the grain boundary composition
reaches a critical value that is temperature dependent. The tendency for this
transition to a disordered interfacial structure consistently depends on the
relative solute excess, instead of the grain boundary energy or misorientation
angle. Grain boundaries with high relative solute excess go through gradual
disordering transitions, whereas those with low relative solute excess remain
ordered until high global Zr concentrations but then abruptly transform into
thick disordered films. The results presented here provide a clear picture of
the effect of interface character on both dopant segregation patterns and
disordered intergranular film formation, showing that all grain boundaries are
not equal when discussing complexion transitions.Comment: 15 figure
High-temperature stability and grain boundary complexion formation in a nanocrystalline Cu-Zr alloy
Nanocrystalline Cu-3 at.% Zr powders with ~20 nm average grain size were
created with mechanical alloying and their thermal stability was studied from
550-950 {\deg}C. Annealing drove Zr segregation to the grain boundaries, which
led to the formation of amorphous intergranular complexions at higher
temperatures. Grain growth was retarded significantly, with 1 week of annealing
at 950 {\deg}C, or 98% of the solidus temperature, only leading to coarsening
of the average grain size to 54 nm. The enhanced thermal stability can be
connected to both a reduction in grain boundary energy with doping as well as
the precipitation of ZrC particles. High mechanical strength is retained even
after these aggressive heat treatments, showing that complexion engineering may
be a viable path toward the fabrication of bulk nanostructured materials with
excellent properties.Comment: 12 figure
Emergence of localized plasticity and failure through shear banding during microcompression of a nanocrystalline alloy
Microcompression testing is used to probe the uniaxial stress-strain response
of a nanocrystalline alloy, with an emphasis on exploring how grain size and
grain boundary relaxation state impact the complete flow curve and failure
behavior. The yield strength, strain hardening, strain-to-failure, and failure
mode of nanocrystalline Ni-W films with mean grain sizes of 5, 15, and 90 nm
are studied using taper-free micropillars that are large enough to avoid
extrinsic size effects. Strengthening is observed with grain refinement, but
catastrophic failure through strain localization is found as well. Shear
banding is found to cause failure, resembling the deformation of metallic
glasses. Finally, we study the influence of grain boundary state by employing
heat treatments that relax nonequilibrium boundary structure but leave grain
size unchanged. A pronounced strengthening effect and increased strain
localization is observed after relaxation in the finer grained samples.Comment: Published in Acta Materiali
Amorphous Intergranular Films Enable the Creation of Bulk Nanocrystalline Cu-Zr with Full Density
Nanocrystalline metal alloys show great potential as structural materials,
but are often only available in small volumes such as thin films or powders.
However, recent research has suggested that dopant segregation and grain
boundary structural transitions between states known as complexions can
dramatically alter grain size stability and potentially enable activated
sintering. In this study, we explore strategic consolidation routes for
mechanically alloyed Cu-4 at.% Zr powders to capture the effects of amorphous
complexion formation on the densification of bulk nanostructured metals. We
observed an increase in density of the consolidated samples which coincides
with the formation of amorphous intergranular films. At the same time, the
grain size is reasonably stable after exposure to these temperatures. As a
result, we are able to produce a bulk nano-grained metal with a grain size of
57 nm and a density of 99.8%, which shows an impressive balance of small grain
size and high density using simple consolidation techniques.Comment: 4 Figure
Amorphous intergranular films act as ultra-efficient point defect sinks during collision cascades
Atomistic simulations are used to explore the effect of interfacial structure
on residual radiation damage. Specifically, an ordered grain boundary is
compared to a disordered amorphous intergranular film, to investigate how
interface thickness and free volume impacts point defect recombination. The
collision cascades induced by neutron bombardment are simulated and residual
point defect populations are analyzed as a function of boundary type and
primary knock on atom energy. While ordered grain boundaries easily absorb
interstitials, these interfaces are inefficient vacancy sinks. Alternatively,
amorphous intergranular films act as ultra-efficient, unbiased defect sinks,
providing a path for the creation of radiation-tolerant materials.Comment: 4 figure
Thick amorphous complexion formation and extreme thermal stability in ternary nanocrystalline Cu-Zr-Hf alloys
Building on the recent discovery of tough nanocrystalline Cu-Zr alloys with
amorphous intergranular films, this paper investigates ternary nanocrystalline
Cu-Zr-Hf alloys with a focus on understanding how alloy composition affects the
formation of disordered complexions. Binary Cu-Zr and Cu-Hf alloys with similar
initial grain sizes were also fabricated for comparison. The thermal stability
of the nanocrystalline alloys was evaluated by annealing at 950 {\deg}C (>95%
of the solidus temperatures), followed by detailed characterization of the
grain boundary structure. All of the ternary alloys exhibited exceptional
thermal stability comparable to that of the binary Cu-Zr alloy, and remained
nanocrystalline even after two weeks of annealing at this extremely high
temperature. Despite carbide formation and growth in these alloys during
milling and annealing, the thermal stability of the ternary alloys is mainly
attributed to the formation of thick amorphous intergranular films at high
temperatures. Our results show that ternary alloy compositions have thicker
boundary films compared to the binary alloys with similar global dopant
concentrations. While it is not required for amorphous complexion formation,
this work shows that having at least three elements present at the interface
can lead to thicker grain boundary films, which is expected to maximize the
previously reported toughening effect.Comment: 9 figure
Concurrent transitions in wear rate and surface microstructure in nanocrystalline Ni-W
Nanocrystalline metals are promising materials for wear-resistant
applications due to their superior strength and hardness, but prior work has
shown that cyclic loading can lead to coarsening. In this study, scratch wear
tests were carried out on nanocrystalline Ni-19 at.% W films with an
as-deposited grain size of 3 nm, with systematic characterization performed
after different wear cycles. A new gradient nanograined microstructure is
observed and a direct connection between wear rate and subsurface
microstructure is discovered. A second Ni-W specimen with the same composition
and a 45 nm average grain size is produced by annealing the original specimen.
Subsequent wear testing shows that an identical subsurface microstructure is
produced in this sample, emphasizing the importance of the cross-over in
deformation mechanisms for determining the steady-state grain size during wear.Comment: 8 figure
Strain localization in a nanocrystalline metal: Atomic mechanisms and the effect of testing conditions
Molecular dynamics simulations are used to investigate strain localization in
a model nanocrystalline metal. The atomic mechanisms of such catastrophic
failure are first studied for two grain sizes of interest. Detailed analysis
shows that the formation of a strain path across the sample width is crucial,
and can be achieved entirely through grain boundary deformation or through a
combination of grain boundary sliding and grain boundary dislocation emission.
Pronounced mechanically-induced grain growth is also found within the strain
localization region. The effects of testing conditions on strain localization
are also highlighted, to understand the conditions that promote shear banding
and compare these observations to metallic glass behavior. We observed that,
while strain localization occurs at low temperatures and slow strain rates, a
shift to more uniform plastic flow is observed when either strain rate or
temperature is increased. We also explore how external sample dimensions
influence strain localization, but find no size effect for the grain sizes and
samples sizes studied here.Comment: Published in Journal of Applied Physic
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