High-Rate Plastic Deformation of Nanocrystalline Tantalum to Large Strains: Molecular Dynamics Simulation

Abstract

Recent advances in the ability to generate extremes of pressure and temperature in dynamic experiments and to probe the response of materials has motivated the need for special materials optimized for those conditions as well as a need for a much deeper understanding of the behavior of materials subjected to high pressure and/or temperature. Of particular importance is the understanding of rate effects at the extremely high rates encountered in those experiments, especially with the next generation of laser drives such as at the National Ignition Facility. Here we use large-scale molecular dynamics (MD) simulations of the high-rate deformation of nanocrystalline tantalum to investigate the processes associated with plastic deformation for strains up to 100%. We use initial atomic configurations that were produced through simulations of solidification in the work of Streitz et al [Phys. Rev. Lett. 96, (2006) 225701]. These 3D polycrystalline systems have typical grain sizes of 10-20 nm. We also study a rapidly quenched liquid (amorphous solid) tantalum. We apply a constant volume (isochoric), constant temperature (isothermal) shear deformation over a range of strain rates, and compute the resulting stress-strain curves to large strains for both uniaxial and biaxial compression. We study the rate dependence and identify plastic deformation mechanisms. The identification of the mechanisms is facilitated through a novel technique that computes the local grain orientation, returning it as a quaternion for each atom. This analysis technique is robust and fast, and has been used to compute the orientations on the fly during our parallel MD simulations on supercomputers. We find both dislocation and twinning processes are important, and they interact in the weak strain hardening in these extremely fine-grained microstructures.

Grain-size dependent mechanical behavior of nanocrystalline metals

“…The first integration of an orientation imaging map to atomistic simulations was accomplished by Rudd [107] and can be seen in Figure 17. Ravelo …”

Theory and simulation of 1D TO 3D plastic relaxation in tantalum

“…The threshold for homogeneous nucleation in the shock, ∼ 65 GPa, is considerably lower than in isothermal compression at room temperature, ∼ 85 GPa. The state following homogeneous nucleation on the Hugoniot is very disordered and quite different than that in 300 K isothermal compression where shear loops and twins are observed with orientation analysis [13].…”

“…The simulation is held at that volume and internal energy, and the ensuing stresses are output. We also output the various structural information for each atom in order to verify whether defects have nucleated or not [13].…”

Grain-Size-Independent Plastic Flow at Ultrahigh Pressures and Strain Rates

“…Molecular dynamics (MD) simulations of nano-crystalline fcc copper have been done with grain sizes up to tens of nm. Atomistic modeling has suggested very high flow stress under dynamic conditions in the ultra-fine grained limit at pressures below ∼100 GPa [21,22]. There have been no dynamic experiments to test these predictions so far.…”