The spall strength and Hugoniot elastic limit of tantalum with various grain size

Abstract

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.

The defect evolution in shock loaded tantalum single crystals

“…The HELs of the single crystals are all higher than that of the polycrystal sample. This is consistent with Razorenov's results, in which the HEL of the coarse grained tantalum is significantly higher than that of the ultra-fine grain tantalum [31]. Normally in quasi-static deformation, the grain boundaries are a hardening feature which can block the movement of the dislocations.…”

“…The elastic precursor decay e↵ect reduces the HEL with a longer wave travelling distance, as seen from the Rasorenov's work [31], the material with grain size 50µm…”

“…Lots of other structural changes can occur during the plastic deformation of metals, such as twinning [26] [27] [28], phase transformation [29] [30] and fracture [31]. Mechanical twinning has mainly two e↵ects on plastic deformation: (1) It can subdivide the grains, acting as a barrier for dislocation slip, and therefore increase the work hardening rate.…”

“…The Hugoniot elastic limit of tantalum is 1 3GP a [31][33] [34]. The yield behaviour of tantalum in shock loading is strongly influenced by the material and the shock conditions (e.g.…”

Rate and temperature effects on the flow stress and tensile strength of metals

“…Moreover, it was observed [21] that harder ultra-fine-grained tantalum may demonstrate even lower HEL value than less hard coarse-grained one. Probably it is explained by faster decay in the ultra-fine-grained material where grain boundaries may be the dislocation sources.…”