Session L38: Materials in Extremes: Strength and Plasticity

First-principles thermoelasticity of bcc lead at high pressure: equation of state and strength of materials

Calculations of thermodynamic properties of materials from first-principles theory are critical for equation of state (EOS) and material strength modeling. Here, we present the thermoelastic properties of bcc lead metal based on density functional theory (DFT) molecular dynamics (MD) method. We make use of the fluctuation formulas associated with the canonical ensemble form of DFT-MD simulation to address the anharmonic contributions to the equilibrium thermodynamic properties. In the context of constitutive strength models, we have calculated the temperature and density dependence of the shear modulus from the Debye temperature up to near the melt temperature for bcc lead. The results can provide more accurate first-principles DFT based high-pressure and –temperature constitutive modeling applications[1].
[1] Robert E. Rudd et al., APS SCCM Proceedings, 2017.   

Dynamic Impact Tests of Tantalum Single Crystals

In this work, Taylor impact experiments are conducted using tantalum single crystals to understand their dynamic behaviors and plastic anisotropy. Tantalum single crystals oriented along [100], [110], [111] and [149] directions are tested at different impact velocities to examine their orientation dependent mechanical behaviors at various strain rates and temperatures. Single crystals showed highly localized deformation and strong plastic anisotropy compared to polycrystalline tantalum. In addition, [100], [110] and [111] single crystals exhibited four, two and three-fold symmetries in their impact surfaces, respectively. A simple crystallographic analysis based on the Schmid law is used to understand the observed symmetries in deformed single crystal projectiles. The first Taylor impact tests using single crystals suggest that the crystallography strongly influences deformation behaviors at high strain rates and temperature regimes.
  

Integrated Modeling and Experiments for Strength in Tantalum: A Tri-lab Effort

Recent strength investigations showcase the growing role of computational modeling in both the design and the interpretation of high-rate dynamic response experiments in metals. Focused simulations at length scales from continuum to atomistic are expanding our understanding of high-rate deformation and challenging our simple definitions of material strength.

An overview of three current experimental platforms at the national labs for measuring strength in tantalum at high-pressures and high-rates will be presented and the mechanisms and models associated with simulating deformation in these extreme regimes will be discussed.

Our ongoing tri-lab effort in tantalum strength combines measurements on LANL’s gas guns, Sandia’s Z machine, and LLNL’s NIF/Omega platforms over a wide range of strain rates and peak pressures. Recent successes in integrating dislocations, grain boundaries and microstructure into modeling plasticity and strength at continuum, mesoscale, and atomistic scales will be summarized, with a focus on challenges and multiscale modeling directions.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.   

Dislocation generation under extreme crystal conditions: small volume and high stress

The emerging nanoscale pristine crystals represent an extreme state of crystals: free of pre-existing dislocations and high surface-to-volume ratio. These crystals often show ultra-high strength and catastrophic plasticity. Here, we demonstrate two important surface dislocation sources underlying the ultra-high strength and dramatic plasticity, i.e., thermally activated nucleation (TAN) of surface dislocations and surface rebound sustained (SRS) generation of high-speed dislocations. The very first dislocation is created by TAN from surface and has been shown sensitive to surface stresses, leading to ultra-high crystal strength strongly dependent on sample size. However, subsequent dislocations governing significant plasticity are often generated in a strongly correlated SRS manner, i.e., under stresses that are a significant fraction of the ideal strength, the nucleated dislocation is accelerated to approach the sound speed and rebound into new dislocations when hitting free surfaces. The rebounded dislocations continue the relay until the sample is significantly relaxed. Our work bridges the experimental phenomena and atomistic insights by revealing dislocation physics on short timescale and calculating dislocation activation events under experiment-relevant conditions.   

Modeling High Strain Rate Plasticity in BCC Lead

We report on the strength (flow stress) of lead at high pressure and high strain rate. We focus on the high-pressure body-centered cubic (bcc) phase of lead. There are two models of lead strength at high strain rate that have been available previously. Both models were constructed using data from the low-pressure, face-centered cubic phase of lead. Plasticity in bcc and fcc crystals can be very different. Recent experiments conducted at the National Ignition Facility have used ramp-compression to drive Rayleigh-Taylor instability and have measured the ripple growth to infer lead strength at high pressure. Those experiments are dominated by behavior in the bcc phase. We have developed an Improved Steinberg-Guinan model for bcc lead strength [1] using ab initio calculations of the shear modulus at pressure, as well as hardening parameters from other bcc metals. We compare the predictions of the new model with those from the two previous models and preliminary results from the experiments.
[1] Robert E. Rudd et al., APS SCCM Proceedings, 2017.   

Using the Richtmyer-Meshkov flow to infer the strength of LY-12 at extreme conditions

An improved analytical model of the Richtmyer-Meshkov (RM) flow in the elastoplastic materials is presented in this paper. This model describes the stabilization by yield strength effect on the RM flow in solids and linear relationships between initial configurations of perturbation and the growth. Then we make use of the model to analysis the explosion driven RM flow experiments with solid Cu and test our model by comparing the predicted values of existing strength models. Finally, we perform a plate impact experiment with solid LY12 aluminium alloy to validate our model and infer Y=1.23 GPa for a 28 GPa shock and a strain rate of 7.5×10⁶ s^-1.   

Experimental studies of shock-induced plasticity and shock-wave structure in FCC and BCC metals

Our studies of the precursor decay in pure FCC and BCC metals revealed striking differences in the behavior of the two groups of solids. In FCC metals (Al, Cu, Ag) the decay τ_HEL (h) (HEL, Hugoniot elastic limit) is found to be smooth; over a wide temperature (RT – melting) range and propagation distances h = 0.03-3 mm it may be reasonably well fit by a two-parameter function, τ_HEL (h) = τ_HEL0(h/h₀)^-α with α = 0.35-0.7. Moreover, the growth of τ_HEL with temperature implies that the motion of dislocations in these metals is controlled by phonon viscous drag. In the case of any of seven studied BCC metals (Ta, Nb, V, W, Mo, Cr, Fe) the dependence τ_HEL (h) is not smooth. At relatively small, less than 1-1.5 mm, propagation distances the decay of elastic precursor wave in BCC metals is similar to that in FCC ones, i.e. the exponent α also lies somewhere between 0.35 and 0.7. But as soon as the elastic wave propagates beyond this, ~1-mm, threshold the character of the decay changes dramatically; the value of the decay exponent α abruptly becomes smaller than 0.1. Such change of the decay rate corresponds to the transition of the control of plastic deformation from phonon viscous drag to thermally activated (slower but much energy-saving) generation of dislocation double-kinks. The stress τ_HEL* at which the transition takes place at given temperature is the Peierls stress τ_P of the metal. Our studies of seven above mentioned BCC metals show that the dependences of their normalized Peierls stress τ_P(T)/τ_P(T=0) on normalized temperature T/T_m are described by a single function. This should be considered as an evidence of similarity of the dislocations core structure in BCC metals.   

Dual domain material point method for nonequilibrium systems

A multiscale numerical method is developed for materials undergoing a large deformation with a high strain rate. The continuum level equation of motion is solved using the dual domain material point method (DDMP) to avoid numerical difficulties caused by large deformations. In a DDMP calculation material points are used to carry the material deformation history, which is often important in thermodynamically nonequilibrium systems. The closure quantities, such as stress, are obtained from numerical simulations of small domains surrounding the material points considering the lower scale physics, such as the molecular dynamics (MD).

To preserve the deformation history, re-initialization of these MD systems is not allowed. New numerical schemes are needed to treat the large deformation of the simulation domain while ensuring the physical consistence between the calculations performed at different scales. These schemes are studied. Results from the combined DDMP-MD calculations are compared to a pure MD simulation. Although the size of the example problem is only a few hundred nanometers due to the limitation of the pure MD simulation, we show that the method of the combined simulation is applicable to macroscopic systems.   

Low-stress Wave Speed Measurements

Measurements of wave speeds at low stresses are often overlooked due to the difficulty of data collection and analysis. For some materials a simple extrapolation of higher-stress data to low stress states is reasonable, but for others phase changes and other complex behaviors can make this approach difficult and inaccurate. We present here a discussion of a new technique to measure wave speeds at low stress in copper and steel. This technique allows low stress wave speeds to be determined with an uncertainty of better than 1%. We will present results from measurements using this technique. We will compare polycrystalline and single crystal (of various orientations) copper wave speeds at low stresses. We will also put forward results from measurements of polycrystalline steel alloys.
  

Ab initio shock loading on poly (p-phenylene terephthalamide) (PPTA) and its implications for Kevlar and other aramid-based fibers performance

Ab initio molecular dynamics simulations using the multi-scale shock technique are applied in the study of the dynamic response of poly (p-phenylene terephthalamide) (PPTA) crystals to shock loading. PPTA crystals form the bulk of para-aramid fibers, such as Kevlar and Twaron, and are responsible for their outstanding strength-to-weight ratio. Strong shock loadings describe the shock response from elastic to polymer decomposition. Results reveal an anisotropic response for shocks perpendicular to the crystal symmetry axis (aramid fiber axis) including stress release mechanisms combining structural phase transformation (SPT) and production of paracrystallinity. SPT is observed for shocks along the [100] direction and are triggered by shock-induced coplanarity of amide and phenylene groups resulting in reorganization of PPTA sheet stacking. Generation of paracrystallinity is triggered by [010] shock-induced scission of hydrogen bonds, trans-cis polymer conformation change, and disruption of chain sheets. While the SPT preserves crystalline order and PPTA properties the generation of paracrystallinity strongly affects PPTA strength. The simulation results provide an atomistic view on the effects of shock in para-aramid fibers.