Effects of theα−εphase transition on wave propagation and spallation in laser shock-loaded iron

Abstract

Despite extensive research work on the ␣-phase transition occurring in shock-loaded iron, the kinetics of this transformation remain largely unknown. Here, we present time-resolved free surface velocity measurements in iron foils of thicknesses ranging from 150 to 520 m subjected to laser shocks of peak pressure of about 130 GPa and duration of about 3 ns. The records show an elastic precursor followed by a plastic front, but the double wave structure that is usually associated with the phase change does not appear over such short propagation distances. The measured profiles are compared to the predictions of one-dimensional simulations involving rate-dependent descriptions of both twinning and phase transitions. Such comparisons provide an estimate of a time constant governing the transformation kinetics, which is found to strongly condition the evolution and attenuation of the pressure pulse during its propagation. The spall stress evaluated from the records is shown to be significantly higher after the ␣--␣ cycle. Metallurgical observations of the recovered samples confirm both the phase transition and the spall damage inferred from the velocity profiles. Finally, they show the very clear change in fracture surface morphology to the so-called smooth spall expected above the phase transformation.

Laser-Driven Ramp Compression to Investigate and Model Dynamic Response of Iron at High Strain Rates

“…Then, this calculated ramp profile is used as boundary condition in the 1 D finite difference, Lagrangian hydro code SHYLAC [16,17] to simulate the dynamic response of iron, including the kinetics of the phase transition. The iron sample is discretized into cells of same mass, as recommended in Reference [18], of initial thickness about 0.01 µm.…”

“…with the details of the equation of state for each pure phase described in the References [16,17]. The time evolution of the epsilon mass fraction X is governed by a kinetics law derived from the classical concept of nucleation and growth of phase [23][24][25] …”

High pressure phase transformation in iron under fast compression

“…Iron is a material of significant relevance for many research fields and applications, including the physics of the Earth's core, materials science, metallurgy, etc., and its behavior under shock conditions as well as its phase diagram at high pressures have been studied for decades [20][21][22][23][24][25]. The body-centered-cubic F e(α) to hexagonal-closed-packed F e(ǫ) phase transformation in particular has remained of interest to experimentalists and theorists alike ever since it was first reported in shock wave experiments almost fifty years ago [20].…”

Laser shock experiments to investigate and to model various aspects of the response of metals to shock loading

“…the region beneath the free surface should remain in the α phase, due to pressure decay below the 13 GPa threshold). This leads to drastic differences in the wave profiles, discussed in detail in [2]. The above model is shown to provide consistent velocity profiles with a time constant η set to 5 ns (Fig.…”