Abstract
The conditions which affect twinning in tantalum have been investigated across a range of strain rates and initial dislocation densities. Tantalum samples were subjected to a range of strain rates, from 10−4/s to 103/s under uniaxial stress conditions, and under laser-induced shock-loading conditions. In this study, twinning was observed at 77K at strain rates from 1/s to 103/s, and during laser-induced shock experiments. The effect of the initial dislocation density, which was imparted by deforming the material to different amounts of pre-strain, was also studied, and it was shown that twinning is suppressed after a given amount of pre-strain, even as the global stress continues to increase. These results indicate that the conditions for twinning cannot be represented solely by a critical global stress value, but are also dependent on the evolution of the dislocation density. In addition, the analysis shows that if twinning is initiated, the nucleated twins may continue to grow as a function of strain, even as the dislocation density continues to increase.
Highlights
The observation of deformation twinning has been reported in numerous studies over the years, including the review article by Christian and Mahajan.[1]
Since it has been reported that twinning in Ta occurs at cryogenic temperatures,[7,8] samples from the as-received material were tested at 77K at strain rates of 10−4/s, 1/s and 103/s to examine the effect of strain rate and the overall stress-strain behavior on twinning
The compressive behavior of polycrystalline Ta samples was investigated under a wide range of strain rates, from 10−4/s to ∼103/s at 77K, and under laser shock loading
Summary
The observation of deformation twinning has been reported in numerous studies over the years, including the review article by Christian and Mahajan.[1]. It is generally accepted that twinning occurs when the deformation conditions are such that the imposed plastic strain rate cannot be accommodated completely by dislocation slip and additional mechanisms are needed. Reasons of falling into this regime include the presence of a lower symmetry crystal structure (orthorhombic, tetragonal, and even hexagonal closed-pack), with limited number of slip systems available, or, for the case of body-centered cubic (bcc) structures, with multiple slip systems, when the strain rate is sufficiently high that the usual dislocation mechanisms are insufficient to maintain the rate. One bcc metal in particular, tantalum, has been the focus of a number of studies.[4,5,6,7,8] Typically, twinning in tantalum has been observed under the conditions of low temperature (
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