In-situ X-ray observations of dendritic solidification

Motivation

Fluid flow plays an important role in solidification processes, since the convective transport of heat and solute influences the undercooling at the solidification front and thus determines the growth conditions of the solid phase decisively. A sufficient understanding of the strongly coupled interaction between the melt flow and the solidification requires diagnostic tools that allow in-situ X-ray observations of the process and provide high resolution data of the flow field, the concentration field and the solidified structure.

The melting temperature of industrial alloys is typically very high which imposes various constraints on the experimental instrumentation and makes in-situ observations of solidification difficult. For this reason, low-melting model alloys are often used in experimental studies. In our visualization experiments we deploy the binary gallium-indium system that has a eutectic point at T = 29.77℃ for Ga-21.4wt%In and remains liquid at room temperature for a certain composition range. The thermophysical properties of the Ga-In system are similar to those of industrial alloys. In particular, Ga-In alloy and Ni-based superalloy in liquid state have nearly similar density, dynamic viscosity and thermal conductivity.

Goal

The main research activities in the solidification projects focus on the effects of melt convection on dendritic growth and on the question of whether effective control of microstructure formation can be achieved with the elaborate application of electromagnetic fields.

Two phenomena formed the focus of the investigations, the development of segregation structures and the fragmentation of primary dendrites, since both processes have a significant influence on the mechanical properties of the solidified alloy. The solidification experiments for validation of the numerical models were conducted at different flow conditions [1-4].

Representative in situ experiments are considered as a benchmark for many numerical groups: Prof. C. Beckermann (University of Iowa) [5, 6]; Dr. A. Kao, Prof. K. Pericleous (University of Greenwich) [7-10]; Prof. P. D. Lee (University College London) [9, 11].

1. Fluid flow effects on dendritic solidification

The model Ga-In alloys serve for the understanding of solidification processes in industrially relevant alloys under the influence of natural convection. In situ X-ray radiography delivers experimental data for validation of the numerical models. Figure 1 shows a comparison of numerical (a–c) using a microscopic parallelized Cellular Automata Lattice Boltzmann Method and experimental (d–f) results of the microstructure time evolution, leading to the formation of a stable channel. Although thermal buoyancy now drives large-scale flow circulations, the results reveal that this is not the primary mechanism for the formation of the stable channel. Instead, the mechanism is driven by solute buoyancy forces.

2. Coarsening stage

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.

Selected Publications

[1] N. Shevchenko, J. Grenzer, O. Keplinger, A. Rack, and S. Eckert, Observation of side arm splitting studied by high resolution X-ray radiography
International Journal of Materials Research, vol. 111, no. 1, 2020, pp. 11-16.
[2] 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
[3] 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
[4] Shevchenko, N., Boden, S., Gerbeth, G., Eckert, S., Chimney formation in solidifying Ga-25wt pct in alloys under the influence of thermosolutal melt convection
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 44 (8) (2013), 3797-3808
[5] 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 Mater, vol. 146, 2018
[6] Neumann-Heyme, H.; Shevchenko, N.; Grenzer, J.; Eckert, K.; Beckermann, C.; Eckert, S., In-situ measurements of dendrite tip shape selection in a metallic alloy Physical Review Materials 6(2022), 063401
[7] A. Kao, N. Shevchenko, M. Alexandrakis, I. Krastins, S. Eckert, and K. Pericleous, Thermal dependence of large-scale freckle defect formation,
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, vol. 377, no. 2143, 2019
[8]A. Kao, I. Krastins, M. Alexandrakis, N. Shevchenko, S. Eckert, and K. Pericleous, A Parallel Cellular Automata Lattice Boltzmann Method for Convection-Driven Solidification,
JOM, vol. 71, no. 1, 2019
[9] X. Fan et al., Controlling solute channel formation using magnetic fields,
Acta Mater, vol. 256, p. 119107, Sep. 2023
[10] Soar, P.; Kao, A.; Shevchenko, N.; Eckert, S.; Djambazov, G.; Pericleous, K., Predicting Concurrent Structural Mechanical Mechanisms During Microstructure Evolution,
Philosophical Transactions of the Royal Society A 380(2022)2217, 20210149
[11] Karagadde, S., Yuan, L., Shevchenko, N., Eckert, S., Lee, P.D., 3-D microstructural model of freckle formation validated using in situ experiments
Acta Materialia, 79 (2014), 168-180