Mechanisms of eutectic lamellar destabilization upon rapid solidification of an undercooled Ag-39.9 at.% Cu eutectic alloy

doi:10.1016/j.jmst.2020.05.019

Abstract

Abstract:

The eutectic Ag-Cu alloys exhibiting fine Ag-Cu lamellar eutectic structure formed upon rapid solidification have great potentials being used in various engineering fields. However, the desired fine primary lamellar eutectic structure (PLES) is usually replaced by a coarse anomalous eutectic structure (AES) when the undercooling prior to solidification exceeds a certain value. The forming mechanism of AES in the undercooled eutectic Ag-Cu alloy has been a controversial issue. In this work, the undercooled Ag-39.9 at.% Cu eutectic alloy is solidified under different cooling conditions by using techniques of melt fluxing and copper mold casting. The results show that the coupled eutectic growth of this alloy undergoes a transition from a slow eutectic-cellular growth (ECG) to a rapid eutectic-dendritic growth (EDG) above a undercooling of 72 K, accompanying with an abrupt change of the distribution and amount of AES in as-solidified microstructures. Two kinds of primary lamellar eutectic structures are formed by ECG and EDG during recalescence, respectively. The destabilization of PLES that causes the formation of AES is ascribed to two different mechanisms based on the microstructural examination and theoretical calculations. Below 72 K, the destabilization of PLES formed by slow ECG is caused by the mechanism of “termination migration” driven by interfacial energy. While above 72 K, the destabilization of PLES formed by rapid EDG is attributed to the unstable perturbation of interface driven by interfacial energy and solute supersaturation.

Key words: Ag-Cu alloys, Rapid solidification, Undercooling, Eutectics, Destabilization

H. Dong, Y.Z. Chen, Z.R. Zhang, G.B. Shan, W.X. Zhang, F. Liu. Mechanisms of eutectic lamellar destabilization upon rapid solidification of an undercooled Ag-39.9 at.% Cu eutectic alloy[J]. J. Mater. Sci. Technol., 2020, 59: 173-179.

Figures/Tables 7

Fig. 1. Recalescence rate as a function of undercooling for the undercooled Ag-39.9 at.% Cu eutectic alloy.

Fig. 2. OM images of the undercooled Ag-39.9 at.% Cu eutectic alloy solidified at (a) and (b) ΔT = 68 K and (c) and (d) ΔT = 72 K under condition of natural cooling.

Fig. 3. OM images of the undercooled Ag-39.9 at.% Cu eutectic alloy solidified at (a) ΔT = 68 K and (b) ΔT = 72 K under condition of quenching in the copper mold.

Fig. 4. SEM images of the microstructures of the undercooled Ag-39.9 at.% Cu eutectic alloy quenched in the copper mold at ΔT = 68 K (a) and then annealed at 1003 K for 1 h (b) and 2 h (c). The white arrows point to the inner region of the lamellae (cell center). The insets are the magnified images of the regions at the cell boundary.

Fig. 5. SEM images of the microstructures of the undercooled Ag-39.9 at.% Cu eutectic alloy quenched in the copper mold at ΔT = 72 K (a) and then annealed at 1003 K for 1 h (b) and 2 h (c).

Fig. 6. (a) Calculated V as a function of ΔT, (b) the calculated λ as a function of ΔT, (c) the calculated |Gcα| as a function of ΔT, and (d) the comparison of the calculated Δτ and the measured Δtpr as a function of ΔT.

Fig. 7. Schematic diagram of the ‘termination migration’ induced morphological transition of the PLES formed by ECG. The arrow points to the inner region of the eutectic cell.

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