Effect of strain rate and dislocation density on the twinning behavior in tantalum

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.

Direct numerical simulation of deformation twinning in polycrystals

“…That is followed by Section 3.3 where we present a large scale simulation of a tantalum polycrystal. As discussed in [4], these results for tantalum capture interesting aspects of experimentally observed behavior. A comparison between high energy diffraction microscopy data and simulations for a magnesium alloy is presented in Section 4.…”

“…General loading conditions will therefore tend to produce a combination of deformation by twinning and slip. Additionally, even in high symmetry crystals with ample slip systems such as face centered cubic (FCC) and body centered cubic (BCC) crystals, it is well known that low temperatures and/or high strain rates tends to promote deformation twinning [1,2,3,4]. For example, in BCC tantalum, it is postulated that twinning occurs when the strain rate is sufficiently high that the usual dislocation mechanisms do not provide rapid enough relaxation to prevent the stress from building.…”

“…For example, in BCC tantalum, it is postulated that twinning occurs when the strain rate is sufficiently high that the usual dislocation mechanisms do not provide rapid enough relaxation to prevent the stress from building. There is also experimental evidence to suggest a complex interplay between dislocation density evolution and deformation twinning found in BCC tantalum [4], making tantalum a very interesting material to consider. In what follows, our focus will be concentrated on HCP magnesium alloy AZ31, as well as BCC tantalum.…”

Contrasting the effects of cold rolling on the shock response of typical face centred cubic and body centred cubic metals

“…In contrast, after a cold rolling treatment, no evidence of twining is observed (Figure 4a-b), although the microstructure has a significant number of long, low angle boundaries present. c) In a previous work [8] (also noted by others [7,9]) it was observed that the upper and low yield points at the HEL were removed by cold rolling. It was suggested that this was due to an accumulation of interstitial oxygen atoms around dislocations, effectively pinning them in place, and thus requiring a high stress to remove them.…”

“…For example, a reduction in stacking fault energy in fcc metals (either in a pure metal like silver [4], or via alloying in copper alloys [5]) can result in a shift in deformation mechanism from dislocation formation to twinning, whilst in bcc metals, a reduction in Peierls stress (such as in niobium) can result in a much higher proportion of the deformation being accommodated by dislocation generation rather than motion of pre-existing dislocations [6]. The effects of increasing dislocation density before shock loading has been much less studied, although a series of papers have reported on the role of dislocation density on tantalum [7][8][9]. In this paper, we examine the effects of dislocation density on the shock response of two near ideal fcc and bcc metals, copper and tantalum.…”