525 research outputs found
Shear banding in metallic glasses described by alignments of Eshelby quadrupoles
Plastic deformation of metallic glasses performed well below the glass
transition temperature leads to the formation of shear bands as a result of
shear localization. It is believed that shear banding originates from
individual stress concentrators having quadrupolar symmetry. To elucidate the
underlying mechanisms of shear band formation, microstructural investigations
were carried out on sheared zones using transmission electron microscopy. Here
we show evidence of a characteristic signature present in shear bands
manifested in the form of sinusoidal density variations. We present an
analytical solution for the observed post-deformation state derived from
continuum mechanics using an alignment of quadrupolar stress field
perturbations for the plastic events. Since we observe qualitatively similar
features for three different types of metallic glasses that span the entire
range of characteristic properties of metallic glasses, we conclude that the
reported deformation behavior is generic for all metallic glasses, and thus has
far-reaching consequences for the deformation behavior of amorphous solids in
general
Influence of Crystalline Nanoprecipitates on Shear-Band Propagation in Cu-Zr Based Metallic Glasses
The interaction of shear bands with crystalline nanoprecipitates in
Cu-Zr-based metallic glasses is investigated by a combination of
high-resolution TEM imaging and molecular-dynamics computer simulations. Our
results reveal different interaction mechanisms: Shear bands can dissolve
precipitates, can wrap around crystalline obstacles, or can be blocked
depending on size and density of the precipitates. If the crystalline phase has
a low yield strength, we also observe slip transfer through the precipitate.
Based on the computational results and experimental findings, a qualitative
mechanism map is proposed that categorizes the various processes as a function
of the critical stress for dislocation nucleation, precipitate size, and
distance.Comment: 16 pages, 15 figure
Low temperature heat capacity of severely deformed metallic glass
The low temperature heat capacity of amorphous materials reveals a
low-frequency enhancement (boson peak) of the vibrational density of states, as
compared with the Debye law. By measuring the low-temperature heat capacity of
a Zr-based bulk metallic glass relative to a crystalline reference state, we
show that the heat capacity of the glass is strongly enhanced after severe
plastic deformation by high-pressure torsion, while subsequent thermal
annealing at elevated temperatures leads to a significant reduction. The
detailed analysis of corresponding molecular dynamics simulations of an
amorphous Zr-Cu glass shows that the change in heat capacity is primarily due
to enhanced low-frequency modes within the shear band region.Comment: 5 pages, 2 figure
Non-equilibrium intermixing and phase transformation in severely deformed Al/Ni multilayers
Al/Ni multilayers have been prepared by repeated folding and cold rolling
(F&R) of elemental foils. The thickness of Al and Ni foils is reduced down to
less than 50 nm after fifty F&R cycles. Three-dimensional atom probe analyses
clearly reveal the presence of supersaturated solid solutions and give the
evidence of deformation-induced intermixing. The formation of the solid
solutions and their transformation into the Al3Ni phase upon annealing is
discussed.Comment: 4 page
A comparison of in- and ex-situ shear bands in metallic glass by transmission electron microsopy
Plastic deformation of metallic glasses performed at temperatures well below
the glass transition proceeds via the formation of shear bands. In this
contribution shear bands originating from in situ tensile tests of
AlYFe melt-spun ribbons conducted in a transmission electron
microscope are compared with ones which had formed ex situ during cold rolling.
The observed contrasts for shear bands generated in situ at the edges of thin
electron-transparent foils are shown to be related to a meniscus-like foil
thickness reduction. In comparison with ex situ samples, alternating contrast
changes due to volume changes are missing and the shear band width is a factor
of 15 larger. The meniscus-like shear band profile is accounted for by the thin
foil allowing volume increase due to shear deformation to annihilate via the
free surfaces of the thin foil and the sheared zones to widen.Comment: submitted to Scripta Materiali
In Situ Generated Shear Bands in Metallic Glass Investigated by Atomic Force and Analytical Transmission Electron Microscopy
Plastic deformation of metallic glasses performed at temperatures well below the glass transition proceeds via the formation of shear bands. In this contribution, we investigated shear bands originating from in situ tensile tests of Al88Y7Fe5 melt-spun ribbons performed under a transmission electron microscope. The observed contrasts of the shear bands were found to be related to a thickness reduction rather than to density changes. This result should alert the community of the possibility of thickness changes occurring during in situ shear band formation that may affect interpretation of shear band properties such as the local density. The observation of a spearhead-like shear front suggests a propagation front mechanism for shear band initiation here
Grain boundary diffusion in severely deformed Al-based alloy
Grain boundary diffusion in severely deformed Al-based AA5024 alloy is
investigated. Different states are prepared by combination of equal channel
angular processing and heat treatments, with the radioisotope Co being
employed as a sensitive probe of a given grain boundary state. Its diffusion
rates near room temperature (320~K) are utilized to quantify the effects of
severe plastic deformation and a presumed formation of a previously reported
deformation-modified state of grain boundaries, solute segregation at the
interfaces, increased dislocation content after deformation and of the
precipitation behavior on the transport phenomena along grain boundaries. The
dominant effect of nano-sized AlSc-based precipitates is evaluated using
density functional theory and the Eshelby model for the determination of
elastic stresses around the precipitates.Comment: 13 pages, 7 figure
Low temperature features in the heat capacity of unary metals and intermetallics for the example of bulk aluminum and AlSc
We explore the competition and coupling of vibrational and electronic
contributions to the heat capacity of Al and AlSc at temperatures below 50
K combining experimental calorimetry with highly converged finite temperature
density functional theory calculations. We find that semilocal exchange
correlation functionals accurately describe the rich feature set observed for
these temperatures, including electron-phonon coupling. Using different
representations of the heat capacity, we are therefore able to identify and
explain deviations from the Debye behaviour in the low-temperature limit and in
the temperature regime 30 - 50 K as well as the reduction of these features due
to the addition of Sc.Comment: 10 pages, 6 figures in total, paper submitted to Physical Review
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