Session K7: Materials II: Microstructure 1

Path Dependency of High Pressure Phase Transformations

At high pressures titanium and zirconium are known to undergo a phase transformation from the hexagonal close packed (HCP), alpha-phase to the simple-hexagonal, omega-phase.~ ~ Under conditions of shock loading, the high-pressure omega-phase can be retained upon release. It has been shown that temperature, peak shock stress, and texture can influence the transformation.~ Moreover, under these same loading conditions, plastic processes of slip and twinning are also affected by similar differences in the loading path.~ To understand this path dependency, in-situ velocimetry measurements along with post-mortem metallographic and neutron diffraction characterization of soft recovered specimens have been utilized to qualitatively understand the kinetics of transformation, quantify volume fraction of retained omega-phase and characterize the shocked alpha and omega-phases. Together the work described here can be utilized to map the non-equilibrium phase diagram for these metals and lend insight into the partitioning of plastic processes between phases during high pressure transformation.\\ \\In collaboration with: Frank Addesssio, Curt Bronkhorst, Donald Brown, David Jones, Turab Lookman, Benjamin Morrow, Carl Trujillo, Los Alamos National Lab.; Juan Pablo Escobedo-Diaz, University of New South Wales; Paulo Rigg, Washington State University.   

Instrumented Pressing of HE and Inert Materials to Study the Effect of Particle Size

It is well known that detonation and mechanical properties of high explosives (HE) depend on density. Computationally it has been shown that specific particle-size distributions will lead to better pressed parts. Theoretically this should improve moderate compaction conditions, uniform density and strength. There are many other powder characteristics that are important such as crystal shape and strength. We are interested to explore the role of HE powder characteristics on compaction properties and pellet integrity.$\backslash $pard We have used an instrumented compaction instrument to press inert and HE powders such as TATB and HMX, which have very different crystal structures. The force and displacement measurements from the instrumented press provide information on the quality of compaction of the specimen in the form of Heckel plots, etc. We have evaluated the thermal and mechanical integrity of resultant pellets by measuring the coefficient of thermal expansion and the compressive strength and strain at failure. We have employed micro x-ray computed tomography (CT) to characterize the microstructure and to quantify the number, the size, and the location of voids. The lack of binder in these specimens greatly simplifies the microstructure analysis and makes the data more amenable to modeling and interpretation.   

In Situ Investigation of Mesoscale Mechanics of Energetic Materials Using X-ray Diffraction.

Weak impacts on high explosives (HE) can give rise to either violent reactions or harmless fracture and material dispersal. Predicting this response or the state of damage in the material remains an unsolved technical challenge. \textit{In situ} mesoscale insights to anisotropic dislocation-mediated plasticity, phase transitions, and damage are needed to quantify fundamental structure-property relationships, inform theory, and enable high fidelity simulations. Time-resolved, \textit{in situ} X-ray diffraction during dynamic loading, spanning multiple orders of strain rate, using synchrotron (Advanced Photon Source) and X-ray free electron laser (Linac Coherent Light Source) radiation has been performed for single crystal and plastic bonded formulations of cyclotrimethylene trinitramine (RDX). For the first time, diffraction patterns quantify the average lattice response during elastic-plastic and phase transition and allow for direct comparison of experiments and simulations through measured and computed diagnostics.