THE SPALL STRENGTH AND HUGONIOT ELASTIC LIMIT OF TANTALUM WITH VARIOUS GRAIN SIZE

The VISAR free surface velocity histories have been measured for commercial grade coarse grain (CG, 50 - 60 mu m) and ultra fine grained (UFG, similar to 0.5 mu m grain size) after severe plastic deformation tantalum and for comparison tantalum single crystals, at peak stresses around 12-14 GPa and strain rates of 10(5)-10(6) s(-1). The decrease in the grain size, which resulted in similar to 25% increase of the hardness did not cause any significant influence on the HEL, the value of which is similar to 2 GPa, but increases slightly the spall strength of the UFG tantalum (7.4 GPa) in comparison with the CG samples (similar to 7 GPa). In both cases the spall strength does not noticeably vary with increase of the peak shock stress up to 70 GPa. The experiments using samples precompressed at 40 and 100 GPa peak pressure have confirmed weak influence of preceding shock compression on the tantalum spall strength. The tantalum single crystals display the highest spall strength equal to similar to 10 GPa. The influences of the grain size on static and dynamic yield stresses are discussed in terms of general strain rate effects.