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We have used a table-top scale laser to dynamically compress iron at strain rates in excess of 109 s-1. Using an embedded ultrafast interferometer, we have measured corresponding free surface histories with a time resolution of approximately 10 ps. We have analyzed the surface histories using a method that accounts for nonsteady wave propagation and time-dependent material behavior. We show that at these strain rates, the α→∈ polymorphic transition begins within 100 ps after an initial very large (∼10 GPa) and mostly elastic compression and appears largely complete within a similar time thereafter. The corresponding deviatoric stress before the transition begins can exceed 3 GPa, while the transition stress itself is up to 25 GPa, nearly twice the value measured at low strain rates. We use these results to propose a systematic variation with loading time of the normal-stress/relative-volume curve followed by iron during rapid compression. [ABSTRACT FROM AUTHOR]

We have used a 45 μJ laser pulse to accelerate the free surface of fine-grained tantalum films up to peak velocities of ~1.2 km s-1. The films had thicknesses of ~1'2 μm and in-plane grain widths of ~75'150nm. Using ultrafast interferometry, we have measured the time history of the velocity of the surface at different spatial positions across the accelerated region. The initial part of the histories (assumed to correspond to the "elastic precursor" observed previously) exhibited measured strain rates of ~0.6 to ~3.2 × 109 s-1 and stresses of ~4 to ~22GPa. Importantly, we find that elastic amplitudes exhibit little variation with strain rate for a constant peak surface velocity, even though, via covariation of the strain rate with peak surface velocity, they vary with strain rate. Furthermore, by comparison with data obtained at lower strain rates, we find that amplitudes are much better predicted by peak velocities rather than by either strain rate or sample thickness. [ABSTRACT FROM AUTHOR]

We have used a table-top scale laser to dynamically compress iron at strain rates in excess of 109 s-1. Using an embedded ultrafast interferometer, we have measured corresponding free surface histories with a time resolution of approximately 10 ps. We have analyzed the surface histories using a method that accounts for nonsteady wave propagation and time-dependent material behavior. We show that at these strain rates, the α→∈ polymorphic transition begins within 100 ps after an initial very large (∼10 GPa) and mostly elastic compression and appears largely complete within a similar time thereafter. The corresponding deviatoric stress before the transition begins can exceed 3 GPa, while the transition stress itself is up to 25 GPa, nearly twice the value measured at low strain rates. We use these results to propose a systematic variation with loading time of the normal-stress/relative-volume curve followed by iron during rapid compression. [ABSTRACT FROM AUTHOR]

We have used a 45 μJ laser pulse to accelerate the free surface of fine-grained tantalum films up to peak velocities of ~1.2 km s-1. The films had thicknesses of ~1'2 μm and in-plane grain widths of ~75'150nm. Using ultrafast interferometry, we have measured the time history of the velocity of the surface at different spatial positions across the accelerated region. The initial part of the histories (assumed to correspond to the "elastic precursor" observed previously) exhibited measured strain rates of ~0.6 to ~3.2 × 109 s-1 and stresses of ~4 to ~22GPa. Importantly, we find that elastic amplitudes exhibit little variation with strain rate for a constant peak surface velocity, even though, via covariation of the strain rate with peak surface velocity, they vary with strain rate. Furthermore, by comparison with data obtained at lower strain rates, we find that amplitudes are much better predicted by peak velocities rather than by either strain rate or sample thickness. [ABSTRACT FROM AUTHOR]