Elastic-Plastic Deformation of Molybdenum Single Crystals Shocked to 12.5 GPa

Anirban Mandal

To examine and understand the elastic-plastic deformation of shock compressed molybdenum (Mo) – a body-centered cubic (BCC) metal, high purity single crystal samples were shocked along [100], [111], and [110] crystallographic orientations to an elastic impact stress of 12.5 GPa. Elastic-plastic wave profiles, measured at different propagation distances using laser interferometry, showed an anisotropic, time-dependent material response. The measured elastic wave amplitudes exhibited large and rapid attenuation – from the calculated elastic impact stress – at propagation distances close to the impact surface before reaching a threshold value (elastic limit). The decay rates along [100] and [110] orientations were faster compared to [111], while the elastic limit along [111] (~5.3 GPa) was significantly larger than those along [100] and [110] orientations (~3.6 GPa). The observed orientation dependence of the elastic wave amplitudes was understood by examining the corresponding resolved shear stresses (τ) on {110}<111> and {112}<111> slip systems determined from quasi-static studies. A comparison of the resolved shear stresses (τ_el) on the operative slip planes at the elastic limit and the reported Peierls stress (τ_P) for screw dislocations in Mo suggested that elastic wave amplitudes attenuate rapidly for τ > τ_P, but no measurable attenuation occurs after τ ≈ τ_P. Resolved shear stresses corresponding to elastic limits under shock loading were ~10-50 times larger than the shear stresses under quasi-static loading, a likely consequence of the large Peierls stress in Mo crystals. Numerical simulations of the measured wave profiles, performed using a dislocation-based continuum model, suggested that {110}<111> and/or {112}<111> slip systems are operative under shock loading. A dislocation generation mechanism operative for τ > τ_el, in addition to regenerative dislocation multiplication, was required to model the rapid elastic wave attenuation observed near the impact surface. Although a physical justification for this mechanism was not established for the single crystals used in this study, the dislocation-based continuum model resulted in a good overall match to the measured wave profiles for all three orientations. Numerical simulations have provided insight into the nature of plastic deformation on the slip systems and showed that Mo single crystals do not work harden significantly under shock loading.

Elastic-Plastic Deformation of Molybdenum Single Crystals Shocked to 12.5 GPa

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