High strain-rate plastic flow in Al and Fe

Thin Fe and Al foils were ramp-compressed over several to tens of ns timescales to study the time-dependence associated with the onset of plastic flow. Peak stress states of 15-200 GPa were achieved through laser ramp-compression where the strain rate was varied, shot-to-shot, between 10(6) to 10(8) s(-1). Our data combined with data from other dynamic compression platforms reveals a strong correlation between the peak elastic precursor stress, sigma(E), and the strain rate at the onset of plastic flow, <(epsilon)over dot>(p). In fcc Al, phonon drag dislocation flow dominates above <(epsilon)over dot>(p) similar to 10(3) s(-1) and sigma(E) similar to 0.03 GPa where sigma(E) scales as <(epsilon)over dot>(0.43)(p). By contrast, the Al alloy 6061-T6 exhibits a relatively weak dependency of sigma(E) with <(epsilon)over dot>(p) up to strain rates of similar to 10(7) s(-1). Our Fe data, reveals a sharp increase in sigma(E) at <(epsilon)over dot>(p) > 5 x 10(6) s(-1). This is consistent with a transition in plastic flow to a phonon drag regime. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3670001]