On the Ultimate Tensile Strength of Tantalum
Eric N. Hahn1,2, Timothy C. Germann2, Ramon Ravelo2, 3, T. Remington1,4, B. A. Remington4, Shiteng Zhao1, James E. Hammerberg2, Marc A. Meyers1,
1 University of California San Diego, La Jolla, CA
2 Los Alamos National Laboratory, Los Alamos, NM
3University of Texas, El Paso, TX
4Lawrence Livermore National Laboratory, Livermore CA
Pulsed lasers can generate tensile pulses range of pico to nanoseconds, orders of magnitude lower than gas-gun and explosively driven experiments. This enables the exploration of spalling in this extreme regime. Using non-equilibrium molecular dynamics simulations, we characterize the ductile tensile failure of a model body-centered cubic metal, tantalum, over six orders of magnitude in strain rate. Molecular dynamics calculations combined with experimental measurements using pulsed lasers show power-law kinetic relationships that vary as a function of dominant defect mechanism and grain size. The maximum sustained tensile stress, or spall strength, increases with increasing strain rate, before ultimately saturating at ultra-high strain rates, i.e. those approaching or exceeding the Debye frequency. The upper limit of tensile strength can be well estimated by the cohesive energy, or the energy required to separate atoms from one another. At strain rates below the Debye frequency, the spall strength of nanocrystalline Ta is less than single crystalline tantalum. This occurs in part due to the decreased flow stress of the grain boundaries; stress concentrations at grain boundaries that arise due to compatibility requirements; and the growing fraction of grain-boundary atoms as grain size is decreased into the nanocrystalline regime. In the present cases, voids nucleate at defect structures present in the microstructure. The exact makeup and distribution of defects is controlled by the initial microstructure and the plastic deformation during both compression and expansion, where grain boundaries and grain orientation play critical roles.