Further tomograms were collected while maintaining the sample at high MODELING, METHODOLOGY pressure and temperature for /home/webapps/asp_fr/data/asp/publications/synchrotron-soleil/synchrotron-soleil-2020/soleil-highlights-2020-hd-ss-tc30 minutes. These images show that AND INSTRUMENTATION while the platelets formed first remain identical, new ∈-Fe grains with SYNCHROTRON SOLEIL HIGHLIGHTS 2020 round and smooth surfaces appear between the platelets and grow until they fill the whole volume. XRD analyses show that these grains share the same crystallographic orientation as the first stage platelets. PSICHE BEAMLINE Two steps are thus clearly evidenced, the first where platelets appear, and the second where grains appears and grow. Associated publication EVOLUTION OF THE MECHANISMS INVOLVED UPON THE Following the phase transitions of iron in 3D with X-ray tomography and α-FE →∈-FE TRANSFORMATION diffraction under extreme conditions. Using a kinematic compatibility model, the possible interfaces for the α-Fe E. Boulard, C. Denoual, A. Dewaele, and ∈-Fe lattices were calculated (Ball et al., 1987) and compared to A. King, Y. Le Godec & N. Guignot. the measurement of the ∈-Fe platelets surfaces orientation: the match is perfect. This indicates that the first step is displacive (or martensitic) and Acta Materialia, 192, 30(2020). follows the Burgers path (Burgers 1934). In this mechanism, collective movements of atoms of less than an interatomic distance produce the References daughter phase (figure 1, right hand). It produces a typical martensitic [1] W.J. Padilla et al. , Rev. Sci. Inst., micro-structure. 75, 4710 (2004). The smooth surfaces of ∈-Fe grains formed in the second step indicate [2] N. Maleeva et al., Nature Comm. 9, that the transformation becomes rather reconstructive (involving atomic 3889 (2018). diffusion to form the daughter phase). [3] Row et al. Rev. Sci. Inst., 87, 033105 (2016). CONCLUSION [4] P. Roy et al., Infrared Physics & Technology, 49, 139 (2006). Our study evidences a change in the mechanisms involved upon the transformation of α-Fe into ∈-Fe, which is only possible through an in Corresponding author situ monitoring of the transformation microstructures. The kinematics of Eglantine Boulard phase transformation are therefore not solely dependent on the crystal Institut de minéralogie, de physique structure since the transformation can switch from a rapid, diffusionless des matériaux et de cosmochimie to a very slow (with diffusion) mechanism. UMR 7590, Sorbonne Université 4, place Jussieu - BC 115 75252 Paris Cedex 5, France eglantine.boulard@upmc.fr Captions FIGURE 1: The Fe phase diagram (left) and schematic of the α-Fe → ∈-Fe Burgers transformation path (right). It involves a vertical compression followed by a shuffle of every other (110)bcctransforms into (0001)hplane (white arrows). (110)bcc plane cpbasal plane. FIGURE 2: a) schematic of the sample assembly in the Paris-Edinburg press. b) 3D rendering of the density (yellow: ∈-Fe, green: ∈-Fe) in the sample volume upon the -Fe ∈-Fe transition. c) ∈-Fe platelets orientation variants, materialized by different colors, within a 2D slice of the reconstructed volume. d) Reconstructed absorption contrast 2D slice. Dark (light) zones correspond to ∈-Fe (α-Fe). 89