Laser compression of nanocrystalline tantalum

Abstract

Abstract. Nanocrystalline tantalum was prepared by high pressure torsion from monocrystalline [100] stock, yielding a grain size of 70nm. It was subjected to laser driven compression at energy levels of ~ 350 J to ~ 850 J in the Omega facility (LLE, U. of Rochester) with corresponding pressures as high as ~ 170 GPa. The laser beam created a crater of significant depth (~ 100 µm). Transmission electron microscopy (TEM) revealed dislocations in the grains but no twins in contrast with monocrystalline tantalum. Hardness measurements were conducted and show the same trend as single crystalline tantalum. The grain size was found to increase close to the energy deposition surface due to the thermomechanical excursion.

The defect evolution in shock loaded tantalum single crystals

“…The dislocation density measured by every author from their own experiment shows the grain size has a strong e↵ect on the dislocation density: materials with large grains (or single crystals) generate more dislocations in shock loading. This is probably because the annihilation of dislocations at the grain boundaries reduces the density of dislocations in the polycrystalline material [28]. However the results from di↵erent author don't follow.…”

“…It can be seen that Rittel's compression test results are a little higher than the prediction from Sherwood's experiments. This is due to the interstitial element difference of the materials [28]. The calculation of the flow stress in compression shows that the dislocation flow stress in the shock front of the current experiment is around 600MP a, which is lower than the shear stress required for twin activation.…”

“…The residual dislocation substructure after shock loading is summarised in Table 2.2. The dislocation structure 16: TEM dislocation micrographs of (a) 35 GP a laser shocked tantalum single crystal [28]; (b) 15 GP a plate impacted tantalum [29]; (c) 45 GP a plate impacted tantalum single crystal [29]; (d) 7 GP a plate impacted tantalum [36]; (e) 45 GP a plate impacted tantalum [29]; (f) 20 GP a plate impacted tantalum [36] …”

“…They are from the twinning shear stress results of Sherwood [77]. The activation shear stress of twinning is considered to be independent of the strain rate [32] [28]. The exponential rising curve is the flow stress for the dislocations.…”

“…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.…”