A study of elastic precursor decay in FCC, BCC and HCP metals under shock loading

The evolution of the elastic precursor with strain rate and distance contains rich information regarding the origins of yielding under intense dynamic loading. Despite being a topic of study for over 40 years, the lattice and microstructural origins of this behaviour remain obscured and strongly material dependent. Efforts to analytically model the decay of the precursor based upon elementary dislocation theory typically result in an overprediction of the initial mobile dislocation density of at least 2 orders of magnitude. This phenomenon has been attributed to a number of possible sources, including the high-rate activation of high order slip systems, the rapid nucleation of dislocations during the finite rise of the stress wave, and transonic dislocation velocities. Despite a wealth of experimental studies of Al and Fe; extension to other FCC or BCC materials, to higher symmetry HCP systems, or indeed materials considering microstructural variation have been few, limiting careful study of the dependence of factors such as Peiriel's stress, thermal activation, and stacking-fault energy.
Means to carefully study precursor decay lies in the unique capabilities of the highly instrumented ISP 100 mm bore single stage gas gun. Preliminary experiments performed on single crystal and polycrystalline tantalum, commerically pure aluminium, and Ma2 magnesium alloy have revealed the potential for performing simultaneous loading of multiple targets, allowing direct comparison between samples of differing thickness, composition, or defect density. This PhD seeks to build upon these pilot studies, by extending this research to the study of specific FCC, BCC, and HCP metals of interest.
This PhD will investigate the kinetics of dislocation generation and mobility through a systematic study of the elastic precursor decay phenomenon. Using the suite of ISP experimental platforms, from the large-bore gas gun, mesoscale gas launcher, to the long-pulse laser shock driver, this experimentally based PhD project will perform a comprehensive study of specific FCC (Al, Cu, Ni), BCC (Ta, Mo, W), and HCP (Ti, Zr, Mg) metal systems, across decades of strain-rate (10^4 to 10^10 s^-1) and target thickness (cm to mu). The large format of the 100 mm bore ISP gas gun will enable simultaneous loading of multiple targets, facilitating direct scrutiny of the effects of strain-rate and target thick- ness, crystal system, texture, and in the case of single crystal targets, orientation. This approach is suitable for targets on the several mm to sub-mm length scale, and will employ frequency-shifted HetV. Moving to thinner samples presents a challenge due to the relatively low velocities associated with the peak elastic states, and the extremely rapid decay rates near the loading surface. These lie at the boundary of both sensitivity and accuracy for both conventional displacement-mode (i.e. HetV) and velocity-mode (VISAR) interferometry techniques. Experiments on the mesoscale gas launcher and long-pulse laser will allow interrogation of this challenging regime, and trialling of a novel imaging displacement interferometer.
As this project seeks to correlate lattice and microstructure to plastic relaxation, a thorough understanding of the initial condition of the targets is of critical importance. To this end, samples will be fully characterised prior to testing using the extensive X-ray and metallography suite at Imperial College, to reveal initial dislocation densities, grain size, texture (polycrystals), and crystal orientation (single crystals).
The results of these experiments will guide the Dynamic Discrete Dislocation Plasticity (D3P) code recently developed at Imperial, which will enable specific dislocation behaviours to be explored. This will in turn shed light on the incipient stages of defect generation and instability formations at extreme conditions.