27 research outputs found
Limiting velocities and transonic dislocations in Mg
To accurately predict the mechanical response of materials, especially at
high strain rates, it is important to account for dislocation velocities in
these regimes. Under these extreme conditions, it has been hypothesized that
dislocations can move faster than the speed of sound. However, the presence of
such dislocations remains elusive due to challenges associated with measuring
these experimentally. In this work, molecular dynamics simulations were used to
investigate the dislocation velocities for the basal edge, basal screw,
prismatic edge, and prismatic screw dislocations in Mg in the sub-, trans-, and
supersonic regimes. Our results show that only prismatic edge dislocations
achieve supersonic velocities. Furthermore, this work demonstrates that the
discrepancy between the theoretical limiting velocity and the MD results for Mg
is due to its sensitivity to large hydrostatic stress around the dislocation
core, which was not the case for fcc metals such as Cu.Comment: 7 pages, 4 figures; v2 clarifications and additional result
Machine Learning Based Approach to Predict Ductile Damage Model Parameters for Polycrystalline Metals
Damage models for ductile materials typically need to be parameterized, often
with the appropriate parameters changing for a given material depending on the
loading conditions. This can make parameterizing these models computationally
expensive, since an inverse problem must be solved for each loading condition.
Using standard inverse modeling techniques typically requires hundreds or
thousands of high-fidelity computer simulations to estimate the optimal
parameters. Additionally, the time of a human expert is required to set up the
inverse model. Machine learning has recently emerged as an alternative approach
to inverse modeling in these settings, where the machine learning model is
trained in an offline manner and new parameters can be quickly generated on the
fly, after training is complete. This work utilizes such a workflow to enable
the rapid parameterization of a ductile damage model called TEPLA with a
machine learning inverse model. The machine learning model can efficiently
estimate the model parameters much faster, as compared to previously employed
methods, such as Bayesian calibration. The results demonstrate good accuracy on
a synthetic test dataset and is validated against experimental data.Comment: 13 pages, 9 figures; v2 minor revisio
Structural disjoining potential for grain boundary premelting and grain coalescence from molecular-dynamics simulations
We describe a molecular dynamics framework for the direct calculation of the
short-ranged structural forces underlying grain-boundary premelting and
grain-coalescence in solidification. The method is applied in a comparative
study of (i) a Sigma 9 120 degress twist and (ii) a Sigma 9 {411}
symmetric tilt boundary in a classical embedded-atom model of elemental Ni.
Although both boundaries feature highly disordered structures near the melting
point, the nature of the temperature dependence of the width of the disordered
regions in these boundaries is qualitatively different. The former boundary
displays behavior consistent with a logarithmically diverging premelted layer
thickness as the melting temperature is approached from below, while the latter
displays behavior featuring a finite grain-boundary width at the melting point.
It is demonstrated that both types of behavior can be quantitatively described
within a sharp-interface thermodynamic formalism involving a width-dependent
interfacial free energy, referred to as the disjoining potential. The
disjoining potential for boundary (i) is calculated to display a monotonic
exponential dependence on width, while that of boundary (ii) features a weak
attractive minimum. The results of this work are discussed in relation to
recent simulation and theoretical studies of the thermodynamic forces
underlying grain-boundary premelting.Comment: 24 pages, 8 figures, 1 tabl
Perspectives on Novel Refractory Amorphous High-Entropy Alloys in Extreme Environments
Two new refractory amorphous high-entropy alloys (RAHEAs) within the
W--Ta--Cr--V and W--Ta--Cr--V--Hf systems were herein synthesized using
magnetron-sputtering and tested under high-temperature annealing and displacing
irradiation using \textit{in situ} Transmission Electron Microscopy. While the
WTaCrV RAHEA was found to be unstable under such tests, additions of Hf in this
system composing a new quinary WTaCrVHf RAHEA was found to be a route to
achieve stability both under annealing and irradiation. A new effect of
nanoprecipitate reassembling observed to take place within the WTaCrVHf RAHEA
under irradiation indicates that a duplex microstructure composed of an
amorphous matrix with crystalline nanometer-sized precipitates enhances the
radiation response of the system. It is demonstrated that tunable chemical
complexity arises as a new alloy design strategy to foster the use of novel
RAHEAs within extreme environments. New perspectives for the alloy design and
application of chemically-complex amorphous metallic alloys in extreme
environments are presented with focus on their thermodynamic phase stability
when subjected to high-temperature annealing and displacing irradiation
Photonic Doppler velocimetry probe used to measure grain boundaries of dynamic shocked materials
Author Institution: Mission Support and Test Services, LLC; Los Alamos National LaboratorySlides presented at the 2018 Photonic Doppler Velocimetry (PDV) Users Workshop, Drury Plaza Hotel, Santa Fe, New Mexico, May 16-18, 2018
Strain-rate effects and dynamic behavior of high entropy alloys
The novel class of multicomponent alloys, also known as high-entropy alloys (HEAs) exhibits excellent properties under low strain-rate conditions. These are especially revealed in the high strength of nanocrystalline CoCrFeMnNi and AlNbTiV alloys, and in the high fracture toughness of AlCoCrCuFeNi and NbMoTaW alloys. Nevertheless, up to now, the dynamic behavior of these high-entropy alloys has not been investigated to the same extent as the quasi-static response. A significantly different mechanical response, such as spallation failure and shear localization, manifests itself when materials are subjected to dynamic loading. Shear localization is an essential precursor to shear failure; studies addressing retardation of its onset are important because of their relevance to applications such as armor for military use. The resistance to shear localization is associated with the extensive work hardening ability enabled by dislocation slip, twinning, and phase transformations which override thermal softening. Apart from shear localization, in contrast to the conventional fracture failure dictated by fracture toughness, spallation resulting from tensile pulses and involving propagating micro-cracks and/or micro-voids also plays an important role in dynamic performance. Although distinctive behaviors have also been reported for other conventional metallic materials under dynamic loading, the unique characteristics of HEAs warrant this review
Microstructure Based Failure Criterion For Ductile Materials
For ductile metals, the process of dynamic fracture occurs through nucleation, growth and coalescence of voids. The stress required to nucleate these voids is inferred from the velocimetry data (using the acoustic approach) and termed as the spall strength. This is a key parameter that is used to evaluate a material’s susceptibility to damage and failure. However, it is also well recognized that the dynamic parameters used to generate the shock state such as pulse duration, tensile strain-rate and peak stress coupled with material microstructure itself affect the material response in a complex manner. Yet, it is impossible to capture all this information by assessing only the spall strength measured from simple one-dimensional Photon Doppler Velocimetry measurements. Although, there exist widely used corrections proposed by Kanel et. al. that allow for the inclusion of some of these complexities into the measured spall strength but still does not take the microstructure into account. In this work, we propose another scheme for normalization of spall strength with a damage area to capture the complexities included in the damage and failure process especially pertaining to microstructure. We will also demonstrate the application of this scheme by applying to examples of materials such as Copper, Copper-24 wt%Ag, Copper-15 wt% Nb and additively manufactured 316L SS