Mechanical and Civil Engineering Seminar

Monday February 10, 2020 4:00 PM

Shock Compression of Molybdenum Single Crystals to High Stresses Tomoyuki Oniyama, Graduate Student, Mechanical Engineering, Ph.D. Thesis Defense

Location: Gates-Thomas 135

Abstract

To investigate the role of crystal anisotropy and the impact stress on the shock induced elastic-plastic deformation of BCC single crystals at high stresses, molybdenum single crystals were shock compressed along [100], [111], and [110] orientations. A series of plate impact experiments were conducted with various impact stresses (23 ~ 190 GPa) along each orientation. Along the [100] and [111] orientations, two wave structure - an elastic shock wave trailed by a plastic shock wave - was observed to 110 GPa. Along the [110] orientation, the two wave structure was observed only up to 90 GPa.

Based on the measured quantities, in-material quantities at the elastic limit and at the peak state were calculated. The elastic wave amplitudes were analyzed to determine the crystal anisotropy effects, the impact stress dependence, and the activated slip systems on the elastic limit. The elastic wave amplitude increased linearly with increasing impact stress, and that was significantly larger along the [111] orientation compared to the other orientations. The difference between calculated maximum resolved shear stresses at the elastic limit and corresponding Peierls stress suggested the activation of {110}<111> slip systems.

At the peak state, the Hugoniot relations were calculated along each orientation and compared with polycrystalline molybdenum Hugoniot relations. The Hugoniot relations along three orientations were within experimental uncertainties, even though the elastic limit showed considerable anisotropy. Also, they agreed reasonably well with the polycrystalline molybdenum data. This implied that the in-material quantities at the peak state does not depend on crystal orientation or the presence of grain boundaries.

In addition to the plate impact experiments, we conducted finite element simulations of shock compressed molybdenum single crystals using Abaqus Explicit in order to gain insight into deformation mechanisms activated during the elastic-plastic deformation. Shear strains on slip systems were explicitly considered by the crystal plasticity model implemented using Abaqus VUMAT subroutine. The results of FEM simulations indicated that {110}<111> systems were likely to be operating at the elastic limit. This observation was consistent with the experimental results from the present study.

Series Mechanical and Civil Engineering Seminar Series

Contact: Holly Golcher golcher@caltech.edu