High pressure phase transformation in iron under fast compression

Abstract

We observe kinetic features—velocity loops—at the α to ϵ phase transformation of iron, similar with the ones reported when water is frozen into its ice VII phase under comparable experimental conditions. By using a phase nucleation and growth kinetic model with pressure dependent phase interface velocity we find that the thermodynamic path followed by the sample is strongly dependent on the drive conditions and sample characteristics. The velocity loops become broader and shallower at slower compressions, while on faster time–scales, e.g., for laser drivers, the loops form at higher velocities and may eventually disappear.

Laser-driven quasi-isentropic compression experiments and numerical studies of the iron alpha-epsilon transition in the context of planetology

“…We have observed double wave structures at particle velocities ranging from 0.25 km/s to 0.52 km/s depending on the compression rate. This is consistent with previous observations [12]. According to the ramp used, either a shock formation was observed for high compression rate or a plateau whose duration was function of compression rate.…”

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

“…These variations and the change from isokinetics regime where θ is independent of the loading rate (for thin target) to a regime where θ depends on this rate (for thick target) are the result of a complex interplay of various process with the most important being: (i) The target initial defects density. A thin sample is generally thought to contain higher defects density than thick sample [35,36]. Thus the nucleation rate R might be lower for thick target leading to greater θ; (ii) The creation and propagation of twins during the compression.…”

Time-dependence of the alpha to epsilon phase transformation in iron

“…Due to the time-dependence associated with the transformation, there is a rapid velocity pullback in the u i (t) record at that transition related to stress relaxation caused by an evolving volume collapse into the denser hcp phase. 6 Due to the slightly higher impedance of sapphire, and unlike the free-surface samples, the VISAR is directly observing the phase transformation wave. 13 _ l a!e is defined over the velocity interval u a!e to the minimum pull-back velocity (Fig.…”

“…In addition to the data already discussed, we include in this analysis free-surface measurements obtained with a gas-gun driver 3,5 and those determined from reported Fe/sapphire u i (t) profiles from the Sandia Z-machine. 6,31 The circle and square symbols represent data for ramp and shock loading, respectively. The velocity of the Fe/sapphire interface for the Janus, Jensen 31 and Bastea 6 data was converted to an equivalent longitudinal stress and particle velocity, u, within the bulk Fe using standard impedance matching FIG.…”

“…[2][3][4] Since its discovery, an understanding of the timedependence and nature of the Fe a!e phase transformation has been the goal of research across many experimental platforms and time scales. [2][3][4][5][6][7][8][9][10] Transformation to the eÀphase may be described through a process of nucleation and growth 6,11 where the growth rate is dependent on the flow velocity of martensitic interfaces through the crystal lattice. 12 The rate of plastic accommodation of the new phase is achieved through the movement and generation of martensitic interfacial dislocations.…”