In situ synchrotron diffraction and modeling of non-equilibrium solidification of a MnFeCoNiCu alloy

Abstract

The solidification mechanism and segregation behavior of laser-melted Mn35Fe5Co20Ni20Cu20 was firstly investigated via in situ synchrotron x-ray diffraction at millisecond temporal resolution. The transient composition evolution of the random solid solution during sequential solidification of dendritic and interdendritic regions complicates the analysis of synchrotron diffraction data via any single conventional tool, such as Rietveld refinement. Therefore, a novel approach combining a hard-sphere approximation model, thermodynamic simulation, thermal expansion measurement and microstructural characterization was developed to assist in a fundamental understanding of the evolution of local composition, lattice parameter, and dendrite volume fraction corresponding to the diffraction data. This methodology yields self-consistent results across different methods. Via this approach, four distinct stages were identified, including: (I) FCC dendrite solidification, (II) solidification of FCC interdendritic region, (III) solid-state interdiffusion and (IV) final cooling with marginal diffusion. It was found out that in Stage I, Cu and Mn were rejected into liquid as Mn35Fe5Co20Ni20Cu20 solidified dendritically. During Stage II, the lattice parameter disparity between dendrite and interdendritic region escalated as Cu and Mn continued segregating into the interdendritic region. After complete solidification, during Stage III, the lattice parameter disparity gradually decreases, demonstrating a degree of composition homogenization. The volume fraction of dendrites slightly grew from 58.3 to 65.5%, based on the evolving composition profile across a dendrite/interdendritic interface in diffusion calculations. Postmortem metallography further confirmed that dendrites have a volume fraction of 64.7% ± 5.3% in the final microstructure.

X-ray Imaging of Alloy Solidification: Crystal Formation, Growth, Instability and Defects

“…Although the current review focuses principally on in situ X-ray imaging studies, it is worth noting that in situ X-ray diffraction is also increasingly applied to solidification studies, including in tandem with imaging [65][66][67][68]. Synchrotron X-ray diffraction provides information on structural evolution throughout solidification of both single-phase and multi-phase alloys, and is particularly helpful when trying to understand non-equilibrium phase transformations such as the identification of intermediate, metastable phases [65].…”

“…In situ Xray imaging with high temporal resolution (microseconds) provided by synchrotron X-ray sources is being used to investigate the process-structure relationships in these processes, such as the high speed melt pool behaviour, heat source-powder bed interactions and defect formation [48][49][50][51][52][53][54]. It is also being used to shed light on other rapid solidification processes such as laser spot welding [68,142,143].…”

“…Radiograph sequences showing the interaction between powder particles and the melt pool during DED process for (h-j) a particle with a lower velocity estimated at ∼1 m s −1 and (k,l) a particle with a higher velocity estimated at ∼10 m s −1 [54]. Debye-Scherrer rings of a Mn-Fe-Co-Ni-Cu high-entropy alloy (m) in a cold-rolled state and (n) after laser welding [68]. (o) Time-resolved 1D diffraction pattern as a function of the reciprocal lattice vector q.…”

“…(o) Time-resolved 1D diffraction pattern as a function of the reciprocal lattice vector q. Time t = 0 indicates the onset of solidification [68]. (p) Evolution of the (200) peak at a selected time interval (indicated by a box in (o)) [68].…”

“…Time t = 0 indicates the onset of solidification [68]. (p) Evolution of the (200) peak at a selected time interval (indicated by a box in (o)) [68].…”