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The VISAR free surface velocity histories have been measured for commercial grade coarse grain (CG, 50 – 60 m) and ultra fine grained (UFG, ~0.5 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 ~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. Introduction It is well known that dynamic deformation leads to hardening of metals and alloys due to storage of defects [1] and reduction of grain sizes under severe plastic deformation [2]. On the other hand, the data on the influence of these factors on flow stresses and material strength under high strain rates is very sparse and often contradictory [3,4]. The interest in the dynamic strength of tantalum is connected with its industrial applications under strong impulse stresses. Tantalum has a high density (16.65 g/cm 3 ), melting temperature (2996 0 C) and is unique in it’s combination of high hardness with high plasticity. Extensive research and publications are dedicated to investigations of the elastic-plastic properties and spall fracture of tantalum under shock-wave loading [5-9], but the question about the influence of structural factors on these processes remains unanswered. In this work, the VISAR free surface velocity histories have been measured for commercial grade tantalum with various grain sizes and structural defects, and for comparison, tantalum single crystals to get information about the elastic-plastic transition and the resistance to spall fracture. Material and experiment The tantalum samples in as-received state (CG) had an average grain size ~50-60 m. The ultrafine grain structure (UFG) was obtained by forging in three directions at decreasing temperature starting from 800C. As a result of severe plastic deformation, a uniform structure was achieved with an average grain size ~0.5 – 0.7 m. The microstructure of CG and UFG tantalum samples are presented in Figure 1. It is seen from these pictures, that the deformed samples have rather uniform structure without strongly marked zones of different granularity. However, as the electron microscopy displays, the structure contains less than 1 % of larger extended grains with the size up to 4 m. After the three-dimensional forging the twins in the grain structure of tantalum are not observed. a b Fig.1. Microstructures of as-received (a) and ultra fine grain (b) tantalum tested. The unsoundness of CG and UFG tantalum samples was varied by means of shock wave of ~40 and ~100 GPa intensity. Figure 2 presents the photograph of microstructure of as-received tantalum sample recovered after shock loading of ~25 GPa intensity. Fig.2. Microstructure of as-received tantalum sample subjected by shock wave loading of 25 GPa amplitude. As it can see from this figure, the forming of the bands of local deformations (twins) is observed practically in all grains. Metallography of recovered preshock samples has shown up the twin structure of 10-20% concentration at 40 – 60 GPa, which decreased under pressure growth due to the annealing of material with increasing of shock temperature. In UFG samples, the concentration of twins was found close to zero. Dislocation density increased from 0.5×10 10 cm -2 to 2×10 10 cm -2 at 100 GPa. A few experiments were done with tantalum single crystals of uncertain orientation. The data about hardness measured for all tantalum samples tested are presented in Table 1. From Table it is clear, that both severe static plastic deformation and preshock deformation increase hardness of CG samples by ~25%, but the preshock deformation did not change the hardness of UFG samples. Table 1. The hardness of tantalum in various structural states. Sample Hardness, HRB As-received (CG) (grain size ~55 μm) 76 – 79 Ultra-fine grained (UFG) (~0.5 μm) 103 – 104 Shock precompressed CG (~40 GPa) 103 Shock precompressed CG (~100 GPa) 97 – 99 Shock precompressed UFG (~40 GPa) 104 – 105 Shock precompressed UFG (~100 GPa) 103 – 105

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