This paper presents the results of measurements of the strength properties of technically pure tan talum under shock wave loading. It has been found that a decrease in the grain size under severe plastic defor mation leads to an increase in the hardness of the material by approximately 25%, but the experimentally measured values of the dynamic yield stress for the fine grained material prove to be less than those of the ini tial coarse grained specimens. This effect has been explained by a higher rate of stress relaxation in the fine grained material. The hardening of tantalum under shock wave loading at a pressure in the range 40-100 GPa leads to a further increase in the rate of stress relaxation, a decrease in the dynamic yield stress, and the dis appearance of the difference between its values for the coarse grained and fine grained materials. The spall strength of tantalum increases by approximately 5% with a decrease in the grain size and remains unchanged after the shock wave loading. The maximum fracture stresses are observed in tantalum single crystals.

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Abstract. The VISAR free surface velocity histories have been measured for commercial grade coarse grain (CG, 50 -60 μm) and ultra fine grained (UFG, ~0.5 μm 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 ~25 % increase of the hardness did not cause any significant influence on the HEL, the value of which is ~2 GPa, but increases slightly the spall strength of the UFG tantalum (7.4 GPa) in comparison with the CG samples (~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 ~10 GPa. The influences of the grain size on static and dynamic yield stresses are discussed in terms of general strain rate effects.

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