Platinum metal was shock compressed to 660 GPa using a two-stage light-gas gun to qualify this material as an ultrahigh-pressure standard for both dynamic and static experiments. The shock velocity data are consistent with most of the previously measured low-pressure data, and an overall linear us−up relationship is found over the range 32–660 GPa. As a part of this work, we have also extended the Hugoniot of the tantalum standard we use to 560 GPa; we have included these data into a new linear fit of the tantalum Hugoniot between 55–560 GPa. We also present the results of a first-principles theoretical treatment of compressed platinum. The fcc phase is predicted to remain stable to beyond 550 GPa. In addition, we have calculated the 300-K pressure-volume isotherm and the Hugoniot. The latter is in excellent agreement with experimental results and qualifies the former to at least 10% accuracy.
We present an overview of recent work on quantum-based atomistic
simulation of materials properties in transition metals
performed in the Metals and Alloys Group at Lawrence Livermore
National Laboratory. Central to much of this effort has been
the development, from fundamental quantum mechanics, of robust
many-body interatomic potentials for bcc transition metals via
model generalized pseudopotential theory (MGPT), providing close
linkage between ab initio electronic-structure
calculations and large-scale static and dynamic atomistic
simulations. In the case of tantalum (Ta), accurate MGPT
potentials have been so obtained that are applicable to
structural, thermodynamic, defect, and mechanical properties
over wide ranges of pressure and temperature. Successful
application areas discussed include structural phase stability,
equation of state, melting, rapid resolidification,
high-pressure elastic moduli, ideal shear strength, vacancy and
self-interstitial formation and migration, grain-boundary atomic
structure, and dislocation core structure and mobility. A
number of the simulated properties allow detailed validation of
the Ta potentials through comparisons with experiment and/or
parallel electronic-structure calculations. Elastic and
dislocation properties provide direct input into
higher-length-scale multiscale simulations of plasticity and
strength. Corresponding effort has also been initiated on the
multiscale materials modelling of fracture and failure. Here
large-scale atomistic simulations and novel real-time
characterization techniques are being used to study void
nucleation, growth, interaction, and coalescence in series-end
fcc transition metals. We have so investigated the microscopic
mechanisms of void nucleation in polycrystalline copper (Cu),
and void growth in single-crystal and polycrystalline Cu,
undergoing triaxial expansion at a large, constant strain rate - a
process central to the initial phase of dynamic fracture.
The influence of pre-existing microstructure on the void growth
has been characterized both for nucleation and for growth, and
these processes are found to be in agreement with the general
features of void distributions observed in experiment. We have
also examined some of the microscopic mechanisms of plasticity
associated with void growth.
Quasicrystals are metal alloys whose noncrystallographic symmetry and lack of structural periodicity challenge methods of experimental structure determination. Here we employ quantum-based total-energy calculations to predict the structure of a decagonal quasicrystal from first principles considerations.We employ Monte Carlo simulations, taking as input the knowledge that a decagonal phase occurs in Al-Ni-Co near a given composition, and using a few features of the experimental Patterson function. The resulting structure obeys a nearly deterministic decoration of tiles on a hierarchy of length scales related by powers of τ , the golden mean.
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