Session B2: MS-1: Materials Science 1

Effect of microstructure on the dynamic tensile response of a Cu-Bi alloy

Plate-impact experiments were conducted to examine the dynamic tensile response of samples containing 99\% (atomic) of copper, with the remainder composed of bismuth dispersed along the grain boundaries and in the form of dendritic precipitates. Free surface velocity histories from these experiments possessed several features not typically observed in spall experiments, which precluded a straightforward interpretation. Therefore, recovery experiments were also carried out. The results, using the recovered samples, suggest that dynamic tensile failure is dictated primarily by the location and orientation of the grain boundaries, with intergranular fracture being the predominant mode of failure. If no properly oriented grain boundary was present, the state of tension resulted in intragranular fracture, with the Bi precipitates dictating the location of the damage. The voids and cracks coalesced to form well defined regions of damage oriented parallel to the (001) Cu planes. Both free surface velocities and recovery experiment results provide a consistent picture. Work supported by DOE.   

Dislocation mechanics based constitutive equation descriptions for copper and iron in high rate deformation tests

Different constitutive equations apply for the loading rate dependence of shock-induced plastic deformations in copper as compared with high rate shockless deformations in isentropic compression experiments (ICEs). In the shock case, the rate dependence is attributed to thermally-activated generation of a nanoscale dislocation structure at the propagating front. Exceptionally high shear-induced dislocation densities are produced. In high rate ICE-type shockless loading, different dislocation dynamics apply for mobile dislocations activated from within the originally-resident density. The lower density necessitates a higher dislocation velocity and, thus, a much higher drag-controlled flow stress is needed to sustain even lower strain rates than apply for shocks. A quasi-ICE strength result for copper is near to the theoretical limit. For iron, shock-induced plate impact results show competition at the Hugoniot elastic limit between different \underline {grain-size-dependent} slip and deformation twinning stresses. The follow-on plastic strain rate is controlled by generation of a \underline {grain-size-independent} nanoscale deformation twinning structure, consistent with dislocation mechanics considerations.   

Texture Dependency of High Strain Rate Properties of Ti-6Al-4V

Over the last few decades the characterisation of Titanium alloys has become increasingly important, mainly due to the requirement for better understanding of lightweight structural materials in aerospace applications. This trend is further strengthened by the emergence of new manufacturing and processing technologies promising Titanium alloys at a lower price, placing them within the range of automotive and consumer product manufacturers. A key aspect of fully understanding the behaviour of Titanium alloys is to determine how varying microstructure affects high strain rate properties. This paper reports the data from high strain rate characterisation tests that have been carried out on four Ti-6Al-4V plates with differing microstructures in both tension (longitudinal, transverse directions) and compression (longitudinal, transverse, through thickness directions). Tension and compression Split-Hopkinson Pressure Bars were used to achieve strain rates of 10$^{3}$ s$^{-1}$. The data from these characterisation tests can then be used to evaluate the affect of microstructure on the anisotropic properties of Ti-6Al-4V.   

Crystallographic Damage Formation at the Breakout Surface in Copper during Shock Loading

We are using Transient Imaging Displacement Interferometry (TIDI) to unravel the dynamics of damage formation in copper. The images obtained from TIDI reveal a rich and complex dynamics during shock and release at the specimen's breakout surface in flyer plate-launched shock experiments. Included in the observations are elastic movement of grain boundaries and onset of damage at grain boundary junctions that remains in the material post-shot. In single crystal targets, we have also observed localized regions of damage having crystallographic symmetry that appear in the leading compression edge of the shock-up, persist through the release and remain in the material post-shot. This talk will focus on our single-crystal results and observations concerning the unexpected early-time phenomenon.   

Transmission Electron Microscopy in the study of Shock Compression

The application of TEM to Shock Compression will be reviewed, with particular reference to specimen preparation and the avoidance of dislocation rearrangement. General principles will be illustrated by a specific study of Ti6Al4V. Ti-6Al-4V has been deformed by means of one-dimensional plate impact to stresses of 5 and 10 GPa. In one case, a Ti-6Al-4V flyer plate delivered a square edged shock pulse, whilst in the other, a layered composite flyer delivered a pseudo-ramped pulse in the form of a series of shock steps. Dislocation and twin densities were studied as a function of pulse height and shape.