Dendritic growth / Coarsening
The solidified microstructure of metal alloys ensues largely from the growth and coarsening of dendrites. During their initial growth into an undercooled melt, dendrites form a characteristic tree-like structure of primary stems and higher-order branches. At a later stage, when the surrounding melt approaches equilibrium, the dendritic structures undergo a slow coarsening process that continuously reduces the number of sidebranches and leads to an increase in the average microstructural length scale.
Our visualisation experiments uncover a complex interaction between dendritic growth and melt convection. Melt flow induces various effects on grain morphology primarily caused by convective transport of solute, such as a facilitation of the growth of primary trunks or lateral branches, dendrite remelting or fragmentation depending on the dendrite orientation, the flow direction and intensity.
The growth speed of the dendrites is strongly correlated with the local solute concentration at the dendrite tip. Figure 1 shows three snapshots of a dendrite tip. A strong variation of the solute concentration layer becomes obvious which is strongly governed by the melt flow in the upper part of the mushy zone.
Figure 1: Snapshots of a propagating dendrite tip: (a) accelerated growth at low gallium concentration; (b) Decelerated growth at higher gallium concentration; (c) further increase of the gallium concentration stops growth.
The coarsening of dendritic structures is characterized by transformation of the side-arm morphology present after growth. It typically proceeds by three mechanisms: (i) retraction of small sidebranches towards their parent stem, (ii) pinch-off or detachment of sidebranches at the narrow neck with the parent stem, and (iii) coalescence of neighboring sidebranches.
The direct investigation of dendritic sidearm evolution during coarsening appears to be rather complex and impose high requirements with respect to the spatial and temporal resolution and sensitivity of the detector. The synchrotron radiography experiments with solidifying Ga-25wt%In alloy were performed at BM20 and ID19 (ESRF) at the at a spatial resolution of < 1 µm. The present measurements provide real-time in-situ data on two phenomena that are of major importance in coarsening of dendrites: sidearm retraction and pinch-off.
Figure 2 shows an example of the evolution of the shape of the sidearms during a pinch-off process.
Figure 2: Images showing the development of secondary branches at different times during isothermal coarsening: (a) 5380 s, (b) 1670 s before- and (c) at the moment of pinch-off.
The coarsening of dendritic structures is studied in collaboration with other theoretical and experimental groups.
H. Neumann-Heyme, K. Eckert (FWDT, HZDR), C. Beckermann (University of Iowa); J. Grenzer (FWI, HZDR & ROBL, ESRF); A. Rack (ID19, ESRF).
Shevchenko, N., Neumann-Heyme, H., Pickmann, C., Schaberger-Zimmermann, E., Zimmermann, G., Eckert, K., Eckert, S.
Investigations of fluid flow effects on dendritic solidification: Consequences on fragmentation, macrosegregation and the influence of electromagnetic stirring
IOP Conference Series: Materials Science and Engineering 228(2017), 012005
Neumann-Heyme, H.; Shevchenko, N.; Lei, Z.; Eckert, K.; Keplinger, O.; Grenzer, J.; Beckermann, C.; Eckert, S.
Coarsening evolution of dendritic sidearms: from synchrotron experiments to quantitative modeling
Acta Materialia (2018) accepted
Shevchenko, N.; Roshchupkina, O.; Sokolova, O.; Eckert, S.
The effect of natural and forced melt convection on dendritic solidification in Ga-In alloys
Journal of Crystal Growth 417(2015), 1-8
Shevchenko, N.; Boden, S.; Eckert, S.; Borin, D.; Heinze, M.; Odenbach, S.
Application of X-ray radioscopic methods for characterisation of two-phase phenomena and solidfication processes in metallic melts
Eur. Phys. J. Special Topics 220 (2013) 63-77
Boden, S.; Eckert, S.; Willers, B.; Gerbeth, G.
X-ray radioscopic visualization of the solutal convection during solidification of a Ga-30 wt pct In alloy
Met. Mater Trans A. 39A (2008) 613–623