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

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.

Introduction

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, such as advanced lead frames in large-scale integrated circuits [1], high-field magnet design electronic devices [2] and contacts and interconnect layers in semiconductor industry [3]. However, the desired fine primary lamellar eutectic structure (PLES) is often subject to destabilization and replaced by a coarse granular morphology called as anomalous eutectic structure (AES) when undercooling (ΔT) exceeds a certain value [[4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]]. The forming mechanism of AES upon rapid solidification of the undercooled Ag-39.9 at.% Cu eutectic alloy has been a controversial issue and aroused intensive investigations since it was first observed in the 1960′s.

Based on the observation of the as-solidified microstructures, previous models interpreting the formation of the AES in the undercooled Ag-39.9 at.% Cu eutectic alloy include repeated nucleation of Cu-rich phase ahead of the growing matrix of Ag-rich phase [4] and uncoupled dendritic growth of Ag-rich and Cu-rich phases [5]. Considering that the recalescence rate abruptly increases when ΔT is larger than 76 K, Walder and Ryder [6,7] speculated that the growth of lamellar eutectic structure might be replaced by the growth of a metastable γ’ phase during recalescence, and the remelting of the metastable γ’ phase during post-recalescence period leads to the formation of the AES. With the assistances of electron backscatter diffraction and high-speed camera techniques, a few authors [[8], [9], [10], [11], [12]] proposed that the remelting of the PLES is responsible for the formation of AES in the undercooled Ag-39.9 at.% Cu eutectic alloy. Recently, based on a quantitative analysis of the spatial distribution of AES in the Ag-39.9 at.% Cu eutectic alloy undercooled to a range of 10 K–60 K, Mullis and Coplet [13] challenged the remelting model and proposed that the formation of AES should be ascribed to a kinetic shift in the eutectic point during rapid solidification. More recently, Liu et al. [14] proposed that the formation of AES in the undercooled Ag-39.9 at.% Cu eutectic alloy was caused by remelting of the PLES by examining the microstructure of the sample which was solidified in a “thin-gauge” and subsequently reheated to a temperature between the nucleation temperature and the eutectic plateau temperature.

It should be noted that the above models all consider that the formation of AES within the undercooling range of eutectic coupled growth of Ag-39.9 at.% Cu eutectic alloy is attributed to a single mechanism. Unlike other typical binary eutectic systems (such as Ni-18.7 at.% Sn eutectic alloys, etc.), within the undercooling range of eutectic coupled growth, the AES in these systems distributes relatively uniformly in the as-solidified sample and its amount gradually increases with increasing ΔT [15,16]. While for the Ag-Cu eutectic alloy, the reported experimental results show that the distribution and amount of AES undergoes an abrupt change when the undercooling exceeds a critical value of 70−76 K [[6], [7], [8], [9],11]. Below this critical undercooling, the as-solidified microstructure exhibits eutectic cells where the regular lamellar eutectic structure appears in the cell center and the AES forms in the cell boundaries [9,11]. While above this critical undercooling, the as-solidified microstructure consists of a large amount of AES in the eutectic dendrite stems and arms and only a small amount of lamellar eutectic structure forms in the periphery region of the AES [[6], [7], [8]]. These two types of AES may be originated from different forming mechanisms because of their significantly different microstructural characteristics. The purpose of the present work aims to clarify the forming mechanisms of these two types of AES formed upon rapid solidification of the undercooled Ag-39.9 at.% Cu eutectic alloy within the undercooling range of eutectic coupled growth.