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Confined spinodal fluctuations at crystal defects evidenced in Fe-Mn alloys

In this ongoing project, we investigate spinodal fluctuations at crystal defects such as grain boundaries and dislocations in Fe-Mn alloys using atom probe tomography, electron microscopy and thermodynamic modeling [1,2].

Analysis and design of materials and fluids requires understanding of the fundamental relationships between structure, composition and properties. Dislocations and grain boundaries influence microstructure evolution through the enhancement of diffusion and by facilitating heterogeneous nucleation, where atoms must overcome a potential barrier to enable the early stage of formation of a phase. Adsorption and spinodal decomposition are known precursor states to nucleation and phase-transition, however nucleation remains the less well understood step in the complete thermodynamic sequence that shapes a microstructure. Here, we report near-atomic-scale observations of a phase transition mechanism which consists of solute adsorption to crystalline defects followed by linear and planar spinodal fluctuations in a Fe-Mn model alloy. These fluctuations provide a pathway for austenite nucleation due to the higher driving force for phase transition in the solute-rich regions. Our observations are supported by thermodynamics calculations, which predict the possibility of spinodal decomposition due to magnetic ordering.

Atom probe tomography analysis of dislocations decorated with Mn after 6h at 450°C. (a) 12.5 at.%Mn iso-concentration surfaces (12.5 at. % Mn threshold) showing two grain boundaries and numerous dislocations decorated by Mn. Scale bar, 40 nm. (b) Close-up on the middle section of the dataset indicated by the purple dashed line showing decorated dislocations. Scale bar, 30 nm (c) 1D composition profiles along the two dislocations marked by the arrows in (b). The points are the experimental values that are connected by the blue lines for better visualization. The blue shading represents the statistical error associated with the calculation of the composition. The dashed lines show the typical values of compositional fluctuation.

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