First principles FF for Ta; the nanoscale description of spallation failure

Alejandro Strachan, Tahir Çağın, and William A. Goddard III

Materials and Process Simulation Center, Beckman Institute (139-74)

California Institute of Technology, Pasadena, California 91125

We developed a many-body embedded atom model force field (denoted qEAM-FF) for Tantalum, purely from first principles quantum mechanics (QM) data. The qEAM-FF is based on the following QM data: zero temperature equation of state for different phases (BCC, FCC and A15) in a wide pressure range, elastic constants, vacancy formation energy and energy of homogeneously sheared BCC crystal.

Using this first principles FF with molecular dynamics we calculate the melting curve of Ta for pressures up to 500 GPa. The zero pressure melting temperature obtained is in very good agreement with the experimental value of .

We calculate for Ta the energetics of vacancy formation and migration in different environments: vacancy in bulk BCC crystal, vacancy near a surface and vacancy near a ½<111> screw dislocation.

We also studied, using molecular dynamics with the qEAM-FF, the rapid expansion of Ta metal following the high compression (50 to 100 GPa) induced by high velocity (2 to 4 km/s) impact. We find that catastrophic spall failure in this system coincides with a critical behavior characterized by a void distribution of the form , with . This corresponds to a threshold in which percolation of the voids results in tensile failure. We define an order parameter (, the ratio of the volume of the largest void to the total void volume) which changes rapidly from ~0 to ~1 when the metal fails and scales as with exponent , where is the total void fraction. We found similar behavior for FCC Ni suggesting that this critical behavior is a universal characteristic for failure of solids in rapid expansion.



This research was funded by a grant from DOE-ASCI-ASAP.

The facilities of the MSC are also supported by grants from NSF

(MRI CHE 99), ARO (MURI), ARO (DURIP), NASA, BP Amoco, Exxon, Dow Chemical,

Seiko Epson, Avery Dennison, Chevron Corp.,

Asahi Chemical, 3M, and Beckman Institute.



Spall failure in tantalum





Spall: Void volume distribution for different times.

Spall: time evolution of volume of the largest void (a) and stress in the load direction (b). Inset: order parameter.