Comparison between mechanisms and microstructures of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>α</mml:mi><mml:mtext>−</mml:mtext><mml:mi>γ</mml:mi></mml:mrow><mml:mo>,</mml:mo><mml:mo> </mml:mo><mml:mrow><mml:mi>γ</mml:mi><mml:mtext>−</mml:mtext><mml:mi>ε</mml:mi></mml:mrow></mml:math>, and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>α</mml:mi><mml:mtext>−</mml:mtext><mml:mi>ε</mml:mi><mml:mtext>−</mml:mtext><mml:mi>α</mml:mi></mml:mrow></mml:math> phase transitions in iron
Robin Fréville, Agnès Dewaele, Nicolas Bruzy, Volodymyr Svitlyk, Gastón Garbarino
Abstract
Iron $\ensuremath{\alpha}\ensuremath{\leftrightarrow}\ensuremath{\epsilon}, \ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\gamma}$, and $\ensuremath{\gamma}\ensuremath{\rightarrow}\ensuremath{\epsilon}$ transformations have been characterized in diamond anvil cells under hydrostatic compression conditions. In situ x-ray diffraction of single or oligocrystals and ex situ SEM-EBSD measurements have been analyzed with multigrain techniques. The mechanisms of $\ensuremath{\alpha}\ensuremath{\leftrightarrow}\ensuremath{\epsilon}$ transitions are martensitic, following Burgers paths which requires a high plastic activity. A memory effect of the reversion exists in the vast majority of the sample: the starting orientation of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}$ single crystal is recovered. Small grains of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}$ exhibit a new orientation compatible with Burgers path, possibly associated with twinning in $\ensuremath{\epsilon}\text{\ensuremath{-}}\mathrm{Fe}$. Close to the $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\epsilon}\text{\ensuremath{-}}\mathrm{Fe}$ triple point (8.7 GPa, 750 K), the $\ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\gamma}$ transformation occurs via diffusion and reconstruction of $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Fe}$, and $\ensuremath{\gamma}\ensuremath{\rightarrow}\ensuremath{\epsilon}$ transformation is martensitic but involves no plasticity. As a result, the microstructures in $\ensuremath{\epsilon}\text{\ensuremath{-}}\mathrm{Fe}$ produced by a direct $\ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\epsilon}$ transformation and by $\ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\rightarrow}\ensuremath{\epsilon}$ transitions path are very different.