Following the phase transitions of iron in 3D under extreme conditions Despite its fundamental importance and its FIGURE 1 wide applications in material science, the nature of the mechanisms involved in phase transitions under various conditions are still unclear even in relatively simple systems, such as pure Fe. Here, we developed a new approach to characterize, for the first time in situ, the micro-structures induced by phase transitions of iron at high pressure and temperature. We show that in α-Fe ↔ ∈-Fe A TWO STEPS -FE ∈-FE TRANSFORMATION transitions a displacive transformation induces typical martensitic microstructures, The α-Fe to ∈-Fe transformation in iron was induced by compressing a single crystal of iron up to /home/webapps/asp_fr/data/asp/publications/synchrotron-soleil/synchrotron-soleil-2020/soleil-highlights-2020-hd-ss-tc10 GPa and heating but is followed by a second reconstructive to 800 K (Figure 1). This transformation is first characterized by step which transforms most of the sample. the burst of numerous and large platelets of up to 30 μm that extend to the whole sample volume. The coupling of XRD with Transformations of iron at high pressure and temperature have3D imaging in situ allows us to determine the orientations of significant implications in Fe-based materials technology as these platelets in relation to the starting single crystal orientation. well as in planetary science due to its prominent presence inThese different orientations, or variants, are materialized by terrestrial planet cores. The mechanism and kinematic of a phasedifferent colors in Figure 2. transition depend heavily on the crystal structures involved. TheFIGURE 2 iron phase diagram presents three crystalline phases (Figure 1): α-Fe, a body centered cubic structure that is stable at ambient conditions; γ-Fe, a face centered cubic structure that is formed when heated above 1185 K at ambient pressure; and ∈-Fe, a hexagonal close-packed structure formed when compressed above 13 GPa at ambient temperature. Despite their wide applications for materials science, the microscopic mechanism of the different transitions under various pressure – temperature conditions remains unclear. In fact, microstructural studies have only been performed ex situ on recovered samples therefore after two transformations (for example after α-Fe →∈ → α-Fe transformations). Analyses on the micro-structure on the recovered sample are nontrivial and a detailed understanding of the micro and meso-scale mechanism is lacking. Here, we combine X-ray diffraction (XRD) and 3D fast micro- tomography to monitor in situ the orientations and mobility of interfaces of coexistence of parent and daughter phases of iron upon transitions around the α-γ-∈ triple point in pure iron (» 8.7 GPa, 750 K). 88