Operando study of the evolution of peritectic structures in metal solidification by quasi-simultaneous synchrotron X-ray diffraction and tomography

Using quasi-simultaneous synchrotron X-ray diffraction and tomography techniques, we have studied in-situ and in real-time the nucleation and co-growth dynamics of the peritectic structures in an Al-Mn alloy during solidification. We collected ∼30 TB 4D datasets which allow us to elucidate the phases’ co-growth dynamics and their spatial, crystallographic and compositional relationship. The primary Al 4 Mn hexagonal prisms nucleate and grow with high kinetic anisotropy -70 times faster in the axial direction than that in the radial direction. In all cases, a ∼5 µm Mn-rich diffusion layer forms at the liquid-solid interface, creating a sharp local solute gradient that governs subsequent phase transformation. The peritectic Al 6 Mn phases nucleate epitaxially within this diffusion zone, initially forming a thin shell surrounding the Al 4 Mn with an orientation relationship of {10 1 ¯ 0} HCP // {110} O , [0001] HCP // [001] O . Such ∼5 µm Mn-rich diffusion layers also cause solute depletion at the liquid side of the liquid-solid interface, limiting further epitaxial phase growth, but prompting phase re-nucleation and branching at crystal edges, resulting tetragonal prism structures that no longer follow the initial orientation relationship. The anisotropic interfacial kinetics and local region latent heat release also led to the formation of liquid-filled core defects at the centre of both phases. Furthermore, increasing cooling rate from 0.17 to 20°C/s can disrupt the stability of the solute diffusion zone, effectively suppressing the formation of the core defects and forcing a transition from faceted to non-faceted morphologies. Our work provides systematic new knowledge and practical approach for tailoring and controlling the peritectic structures in metallic alloys through the solidification processes.