Lawrence Livermore National Laboratory



Luke Hsiung

Overview

The ability to determine material deformation mechanisms will have a significant impact on the development of predictive constitutive materials models for Lawrence Livermore National Laboratory's weapons applications. We studied the deformation mechanisms of single-crystal tantalum at four crystal orientations and shock compressed at two peak pressures using a gas gun flyer-plate impact method. Our goal was to describe the shock-induced shear transformations, specifically deformation twinning and the alpha body-centered cubic (bcc) to omega (pseudo-hexagonal) transition. These occurred in shock-deformed polycrystalline tantalum at 30 GPa when the formation of low-energy cellular dislocation structure and polygonization caused by dynamic recovery was suppressed. Our study emphasized the effects of crystal orientation and strain rate on shock-induced dynamic recovery and shear transformations. We proposed novel mechanisms for the formation of cellular dislocation structure and polygonization based on the coupling reactions of specified coplanar dislocations in {-101} slip planes to describe the competition between dynamic recovery and shear transformations.

Background and Research Objectives

The ability to determine material deformation mechanisms is an important technology that will have a significant impact on the development of predictive constitutive materials models for weapons applications. With the increasing computational power of supercomputers and the development of multiscale modeling techniques, it will soon be possible to develop constitutive models (e.g., strength and martensitic transformation models) parameterized entirely from first principles. These models must be validated by experimental data, but often the data are from integrated dynamic experiments that are difficult to conduct. Experiments that directly determine material deformation mechanisms can have tremendous impact. For example, a model that assumes slip-based plasticity might need to be extended to account for the effect of crystalline twinning or phase transformation on the plastic flow rate and microstructural changes.

Deformation of a crystalline material can always be described as the sum of a change in volume and a change in shape at constant volume (i.e., shear). Assuming a specific structure, the change in a material's volume can be recovered when the load is removed. The change in shape may or may not be recovered. The part of shear that is recoverable is elastic, and the part that remains is plastic; thus, plastic deformation through shear by dislocation glide plays a vital role in shock-induced deformation twinning and alpha–omega transition. Line density and arrangement of high-density stored dislocations (which can alter the free energy significantly by changing the internal energy) is a crucial factor in determining the onset of shock-induced deformation twinning and alpha–omega transition (i.e., shear transformations) in tantalum. A novel dislocation-based mechanism has been proposed and reported by other researchers to rationalize the transition of dislocation glide to shear transformations in shock-deformed tantalum: shear transformations. Twinning and the alpha–omega transition take place as an alternative deformation mechanism to accommodate the high strain rate when the shear stress required for dislocation multiplication exceeds the threshold shear stresses for twinning and the alpha–omega transition.

The goals of this project were to (1) determine the underlying mechanisms for the development of low-energy cellular structure and polygonization and (2) clarify the governing factors for the suppression of dynamic recovery. We emphasized the effect of crystal orientation and strain rate on the competition between dynamic recovery and shear transformations.

Impact on Mission

Our research supports the advancement of the science, technology, and engineering competencies that are the foundation of the NNSA mission, particularly as they pertain to issues related to stockpile stewardship and broader national security challenges. Our research also addresses the Laboratory's research and development challenges in high explosive physics and material science.

Conclusion

We verified that shock-induced polygonization and shear transformations (i.e., deformation twinning and the alpha–omega [pseudo-hexagonal] transition) occur in single-crystal tantalum specimens at four crystal orientations and shocked at two peak pressures, 50 GPa and 65 GPa. Shock-induced polygonization mainly took place in the [001]- and [-111]-oriented specimens, in which the coplanar coupling reactions for dynamic recovery were prevalent. The suppression of dynamic recovery at high strain rate occurred in the [001]- and [-111]-oriented specimens shocked at 50 GPa; however, the thermally activated polygonization became prevalent when the shock pressure further increased to 65 GPa because of adiabatic heating. Shock-induced shear transformations prevailed in the [011]- and the [-123]-oriented specimens shocked at both pressures in which the coupling reactions for dynamic recovery were suppressed due to lack of active coplanar slip systems. We proposed novel mechanisms caused by the coupling reactions of the ½<111> coplanar dislocations in the {-101} slip planes to describe the effects of crystal orientation and strain rate on the competition between dynamic recovery and shear transformations.

Publications and Presentations

Hsiung, L. and G. H. Campbell. 2017. "Transition of Dislocation Glide to Shear Transformation in Shocked Tantalum." MRS Advances 2(27): 1417–28. doi: 10.1557/adv.2017.236. LLNL-JRNL-710038.