Experimental investigations and phase-field simulations of triple-phase-separation kinetics within liquid ternary Co-Cu-Pb immiscible alloys

The phase-separation kinetics and microstructure evolution mechanisms of liquid ternary $\mathrm{C}{\mathrm{o}}_{43}\mathrm{C}{\mathrm{u}}_{40}\mathrm{P}{\mathrm{b}}_{17}$ immiscible alloys are investigated by both the drop tube technique and phase-field method. Two successive phase separations take place during droplet falling and lead to the formation of a three-phase three-layer core-shell structure composed of a Co-rich core, a Cu-rich middle layer, and a Pb-rich shell. The Pb-rich shell becomes more and more conspicuous as droplet diameter decreases. Meanwhile, the Co-rich core center gradually moves away from the core-shell center. Theoretical analyses show that a larger temperature gradient inside a smaller alloy droplet induces the accelerated growth of the surface segregation shell during triple-phase separation. The residual Stokes motion and the asymmetric Marangoni convection result in the appearance of an eccentric Co-rich core and the core deviation degree is closely related to the droplet size and initial velocity. A three-dimensional phase-field model of ternary immiscible alloys, which considers the successive phase separations under the combined effects of Marangoni convection and surface segregation, is proposed to explore the formation mechanisms of three-phase core-shell structures. The simulated core-shell morphologies are consistent with the experimental observations, which verifies the model's validity in reproducing the core-shell dynamic evolution. Numerical results reveal that the development of three-phase three-layer core-shell structures can be attributed to the primary and then secondary phase separations dominated simultaneously by Marangoni convection and surface segregation. Furthermore, the effects of droplet temperature gradient on the growth kinetics of the surface segregation shell are analyzed in the light of phase-field theory.

Figure 1

Three-phase three-layer core-shell microstructures composed of a Co-rich core, a Cu-rich middle layer, and a Pb-rich shell in $\mathrm{C}{\mathrm{o}}_{43}\mathrm{C}{\mathrm{u}}_{40}\mathrm{P}{\mathrm{b}}_{17}$ alloy droplets solidified within the drop tube: (a) a thin Pb-rich shell and (b) a thick Pb-rich shell.

Figure 2

Morphology characteristics of the three-phase three-layer core-shell structure versus alloy droplet size: (a) reduced thickness ${\delta }_r$ of the Pb-rich shell, (b) reduced thickness $L_r$ of the Cu-rich layer, (c) reduced radius $R_r$ of the Co-rich core, and (d) deviation degree $e_d$ of the Co-rich core.

Figure 3

Unusual three-phase separated morphologies of $\mathrm{C}{\mathrm{o}}_{43}\mathrm{C}{\mathrm{u}}_{40}\mathrm{P}{\mathrm{b}}_{17}$ alloy droplets under the drop tube condition: (a) multicore-shell structure and (b) dispersed structure.

Figure 4

Temperature-field profiles of $\mathrm{C}{\mathrm{o}}_{43}\mathrm{C}{\mathrm{u}}_{40}\mathrm{P}{\mathrm{b}}_{17}$ alloy droplets versus cooling time $t$ and reduced radial distance $r/R$: (a) schematic diagram of the temperature field, (b) droplet temperature $T-t-r/R$, (c) temperature gradient $G_L-t-r/R$, and (d) cooling rate $R_c-t-r/R$.

Figure 5

Three-dimensional phase-separation processes of $\mathrm{C}{\mathrm{o}}_{43}\mathrm{C}{\mathrm{u}}_{40}\mathrm{P}{\mathrm{b}}_{17}$ alloy droplets at different conditions: (a) neither surface segregation nor Marangoni convection, (b) with surface segregation but no Marangoni convection, and (c) both surface segregation and Marangoni convection. Here red (dark gray) shows the Co-rich phase, yellow (bright white) shows the Cu-rich phase, and blue (light gray) shows for the Pb-rich phase.

Figure 6

Concentration curves of solutes and solvent along the radial direction of the $\mathrm{C}{\mathrm{o}}_{43}\mathrm{C}{\mathrm{u}}_{40}\mathrm{P}{\mathrm{b}}_{17}$ alloy droplet at various phase-separation moments: (a) solvent Co, (b) solute Cu, and (c) solute Pb.

Figure 7

Effects of droplet temperature gradient on the growth of the Pb-rich shell: (a) reduced thickness ${\delta }_r$ of the Pb-rich shell versus evolution time τ and (b) and (c) cross sections of 3D phase separation at temperature gradients ${\overline{G}}_L$ of 0 and 21.4 K/mm, respectively.

Figure 8

Three-dimensional chemical-potential evolution of $\mathrm{C}{\mathrm{o}}_{43}\mathrm{C}{\mathrm{u}}_{40}\mathrm{P}{\mathrm{b}}_{17}$ alloy droplets: (a) solvent Co, (b) solute Cu, and (c) solute Pb.

Figure 9

Chemical-potential curves of solutes and solvent along the radial direction of the $\mathrm{C}{\mathrm{o}}_{43}\mathrm{C}{\mathrm{u}}_{40}\mathrm{P}{\mathrm{b}}_{17}$ alloy droplet at different evolution moments: (a) chemical potential of solvent Co, (b) chemical potential of solute Cu, and (c) chemical potential of solute Pb.

Figure 10

Theoretical prediction and experimental confirmation of distribution patterns for Co-, Cu-, and Pb-rich globules within $\mathrm{C}{\mathrm{o}}_{43}\mathrm{C}{\mathrm{u}}_{40}\mathrm{P}{\mathrm{b}}_{17}$ alloy droplets under microgravity condition: (a) the velocity sketches of thermal Marangoni migration $V_{\mathrm{M}}$ and Stokes motion $V_{\mathrm{S}}$ and (b) experimental evidence.

Figure 11

Migration mechanisms of Co-rich globules within falling droplets of $\mathrm{C}{\mathrm{o}}_{43}\mathrm{C}{\mathrm{u}}_{40}\mathrm{P}{\mathrm{b}}_{17}$ alloy versus initial velocity $V_0$, droplet diameter $D$, and falling time $t$: (a) residual gravity $g_r-V_0-t$ at $D=598\mu \mathrm{m}$, (b) residual gravity $g_r-D-t$ at $V_0=5\mathrm{m}/\mathrm{s}$, (c) the ratio ${\beta }_{\mathrm{MS}}-V_0-t$ of Marangoni migration velocity $V_{\mathrm{M}}$ and Stokes motion velocity $V_{\mathrm{S}}$ at $D=598\mu \mathrm{m}$, and (d) ${\beta }_{\mathrm{MS}}-D-t$ at $V_0=5\mathrm{m}/\mathrm{s}$.