PHYSICAL REVIEW B 93, 214108 (2016) Kinetics of the iron α-ε phase transition at high-strain rates: Experiment and model N. Amadou, 1, 2, 3 , * T. de Resseguier, 2 E. Brambrink, 1 T. Vinci, 1 A. Benuzzi-Mounaix, 1 G. Huser, 4 G. Morard, 5 F. Guyot, 5 K. Miyanishi, 6 N. Ozaki, 6, 7 R. Kodama, 6, 7 and M. Koenig 1, 8 1 LULI–CNRS, Ecole Polytechnique, CEA: Universit´ e Paris-Saclay; UPMC Universit´ e Paris 06: Sorbonne Universit´ es, F-91128 Palaiseau Cedex, France 2 Institut Pprime, CNRS, ENSMA, Universit´ e de Poitiers, F-86000 Poitiers, France 3 D´ epartement de Physique, Universit´ e Abdou Moumouni de Niamey, BP. 10662 Niamey, Niger 4 CEA, DAM, DIF, F-91297 Arpajon, France 5 Institut de Minralogie, de Physique des Matriaux et Cosmochimie (IMPMC), MNHN, CNRS, UPMC, IRD, Sorbonne Universit´ es, F-75005 Paris, France 6 Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan 7 Photon Pioneers Center, Osaka University, Suita, Osaka 565-0871, Japan 8 Institute for Academic Initiatives, Osaka University, Suita, Osaka 565-0871, Japan (Received 27 January 2016; revised manuscript received 22 April 2016; published 14 June 2016) In this article, we investigate the kinetics of the iron α-ε transition under laser-driven ramp compression for deformation rates ranging from 3 to 9 × 10 7 s −1 . As in previous work, we observe a plateau in the rear surface velocity profile at the transition. With increasing deformation rate the transition onset pressure raises from 11 to 25 GPa, while the plateau duration decreases. These kinetic effects are well reproduced by an Avrami-type kinetics model of nucleation and growth with a constant, nanosecond scale completion time, which suggests an isokinetic regime over the explored range of strain rates. DOI: 10.1103/PhysRevB.93.214108 I. INTRODUCTION Solid-solid polymorphic phase transitions under dynamic compression play key roles for (i) material science where they offer ways to synthesize materials for technological applica- tions [1,2]; (ii) planetary science (including geophysics) where they are fundamental for understanding Earth and telluric exoplanet internal structure [3–5]; and (iii) condensed matter physics, where they give insight into atomic rearrangement under extreme conditions [6–9]. In particular, laser-driven compression experiments can provide insight into some specific features of phase transformation such as new phase nucleation and growth rates, the transition characteristic time, the transition stress and compression, etc. [6,10,11]. The most famous and widely studied example of such solid-solid phase transition is the martensitic transformation of iron from the ground state body-centered cubic (bcc) structure (α phase) to the high pressure hexagonal close-packed (hcp) structure (ε phase) around a pressure of 13 GPa. Since its discovery by Bancroft et al. in 1956 [12], this transition has been extensively studied under both static [9] and standard shock compression [6,10,13–15]. Recently, efforts have been focused on investigating the phase transition kinetics and dynamical behavior using ramp compression which gives a continuous information along the compression path. Fast compressed diamond anvil cells have provided in situ data on the progress of the transition at low strain rates in the 10 −2 − 10 2 s −1 range [16]. At much higher rates, Bastea et al. [17] have reported Z-pinch quasi-isentropic compression of iron, for thick targets (≈500 μm) and long compression times (≈200 ns), where transition kinetics has been explored by varying the target initial temperature. By * nourou.amadou@polytechnique.edu using a phase nucleation and growth kinetic model with pressure dependent phase interface velocity, they found that the thermodynamic path followed by the sample is strongly dependent on the drive conditions and probably on the target characteristics. In a previous article, we have reported on the kinetic effects of the iron α-ε transition under different laser ramp loading conditions [18] where it was shown that the transition leads to a plateau in the VISAR measured velocity profile. Upon increasing the loading rate, this plateau is shifted to higher velocities while its duration decreases. In a subsequent article by Smith et al. [11], a time-dependence study of the α-ε phase transformation under wide experimental conditions (compression time between 3 and 300 ns and sample thickness from 10 μm up to millimeter) was reported. One pointing out of this latter work was that treating the kinetics of the phase transformation locally as a function of the difference in free energy with a constant time scale for the transition reproduces quite poorly the velocity histories which suggests the need for more explicit representation of the nucleation of daughter phase within the parent followed by growth from the nuclei to capture the response of the material under highly dynamic deformation. Here, we report on a complementary experimental and numerical investigation of the iron α-ε dynamical phase transition kinematics under different laser ramp compression loading conditions. The deformation rates range from ˙ ε = 3.2 to 8.6 × 10 7 s −1 . Over this relatively narrow range it is found that an Avrami-type transition model based on a theoretical description of nucleation and growth mechanism reproduces well the measured velocity profiles with a constant characteristic time, which indicates an isokinetic regime in which the transformation kinetics remains invariant. The article layout is as follows: In Sec. II we describe the experiment setup and results while in Sec. III we present 2469-9950/2016/93(21)/214108(6) 214108-1 ©2016 American Physical Society