Using Neutron Diffraction to Investigate Texture Evolution During Consolidation of Deuterated Triaminotrinitrobenzene (d-TATB) Explosive Powder
Triaminotrinitrobenzene (TATB) is a highly anisotropic molecular crystal used in several plastic-bonded explosive (PBX) formulations. A complete understanding of the orientation distribution of TATB particles throughout a PBX charge is required to understand spatially variable, anisotropic macroscale properties of the charge. Although texture of these materials can be measured after they have been subjected to mechanical or thermal loads, measuring texture evolution in situ is important in
... s important in order to identify mechanisms of crystal deformation and reorientation used to better inform thermomechanical models. Neutron diffraction measurements were used to estimate crystallographic reorientation while deuterated TATB (d-TATB) powder was consolidated into a cylindrical pellet via a uniaxial die-pressing operation at room temperature. Both the final texture of the pressed pellet and the in situ evolution of texture during pressing were measured, showing that the d-TATB grains reorient such that (001) poles become preferentially aligned with the pressing direction. A compaction model is used to predict the evolution of texture in the pellet during the pressing process, finding that the original model overpredicted the texture strength compared to these measurements. The theory was extended to account for initial particle shape and pore space, bringing the results into good agreement with the data. Crystals 2017, 7, 138 2 of 17 Capturing this behavior in macroscale constitutive models requires understanding the anisotropy of single crystals as well as their collective interaction. There are three sources of anisotropy in TATB crystals that complicate mechanisms of polycrystal response. First, TATB (C 6 H 6 N 6 O 6 ) has a triclinic crystal structure with space group P1 that is a source of crystal anisotropy [5, 6] . As depicted in Figure 1 , each triclinic unit cell of TATB possesses two TATB molecules, each comprising a modified benzene ring. The bonds contained within the modified benzene ring (colored brown in Figure 1 ) are sufficiently strong so that the molecules remain flat and aligned in layers along the (001) crystallographic plane (highlighted in blue). The intermolecular bonds formed within these molecular layers of TATB are strong hydrogen bonds, while the bonds between adjacent layers are of relatively weaker van der Waals type, thus providing a second source of anisotropy. For example, TATB thermal expansion is approximately 20 times higher along the c-axis than for the a-and b-lattice directions    . Finally, the geometric morphology of TATB crystals is also anisotropic; depending on the synthesis route, they can be high-aspect ratio graphite-like platelets corresponding to crystallographic (001) planes [9, 10] . Los Alamos National Laboratory programs especially funding support from conceived and designed the study; Amanda L. Higginbotham Duque synthesized the deuterated TATB material; John D. Yeager and Bjørn Clausen performed the consolidation experiments; Sven C. Vogel performed the final texture measurements on the HIPPO instrument; Darby J. Luscher developed the theory and carried out simulations; all authors discussed and interpreted the results and reviewed the manuscript; Darby J. Luscher and John D. Yeager wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.