Trans. Nonferrous Met. Soc. China 23(2013) 289-293
Solidification process and microstructure evolution of bulk undercooled Co-Sn alloys
Li LIU1,2, Xiao-li MA3, Qi-sen HUANG1, Jin-fu LI1, Xian-hua CHENG2, Yao-he ZHOU1
1. State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China;
2. School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China;
3. School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
Received 31 December 2011; accepted 23 May 2012
Abstract: A series of Co-Sn alloys with Sn content ranging from 12% to 32% (mole fraction) were undercooled to different degrees below the equilibrium liquidus temperature and the solidification behaviors were investigated by monitoring the temperature recalescence and examing the solidification microstructures. A boundary clearly exists, which separates the coupled growth zone from the decoupled growth zone of eutectic phases for the alloys with Sn content ranging from 14% to 31% (mole fraction). The other Co-Sn alloys out of this content range are hard to be undercooled into the coupled growth zone in the experiment. It is found that the so-called non-reciprocal nucleation phenomenon does not happen in the solidification of undercooled Co-Sn off-eutectic alloys.
Key words: Co-Sn alloy; undercooling; recalescence; coupled growth zone; solidification; microstructure evolution
1 Introduction
Rapid solidification of deeply undercooled alloy melts is far from equilibrium. As a result, competitions between various phases or different growth modes exist in the rapid solidification [1-7]. One of them is the selection between single-phase growth and eutectic coupled growth in the rapid solidification of the undercooled alloys around eutectic points. It was revealed that there is a coupled growth zone of eutectic alloys below the eutectic line. When the eutectic alloys are undercooled into the zone prior to nucleation, regular eutectic microstructure forms as the primary phase. Such a finding considerably improved the understanding of the solidification microstructure formation [8,9]. Co-Sn eutectic alloy is often used for studying the solidification behavior of undercooled eutectic alloys [10-12]. Up to now, however, the coupled eutectic zone in the alloy system is not still established. One purpose of this work is to determine the undercooling at which the primary eutectic will form in the solidification of Co-Sn alloys.
During investigating the solidification micro- structure of off-eutectic alloys, it was also found that the primary phase was surrounded by a Halo of another phase but the opposite is not true. Such a phenomenon is usually called non-reciprocal nucleation [13-15], meaning that one eutectic phase can trigger the nucleation of the other phase, but not vise versa. If we note that heterogeneous nucleation is realized through crystal lattice matching between the substrates and the crystal to nucleate, the viewpoint of non-reciprocal nucleation is doubtful.
2 Experimental
Co-Sn alloys with compositions ranging from 12%-32% Sn (mole fraction, the same below if not mentioned), including the Co-24%Sn eutectic alloy, were chosen. The raw materials are pure Co and Sn with 99.999% and 99.99% purities (mass fraction), respectively. Undercooling experiment was carried out in a vacuum chamber back-filled with ultra pure argon. The raw materials placed in a quartz glass crucible was induction melted, and then cyclically superheated and cooled under the protection of a molten flux until the desired undercooling was achieved. The superheating of melt was controlled to be about 300 K by adjusting the input power of the induction coil. A two-color infrared pyrometer with an accuracy of 1 K and a response time of 1 ms was utilized to monitor the thermal history of the sample during the entire experimental cycle. The temperature data were recorded in computer, from which the cooling curves can be redrawn.
The surface of the as-cast sample was observed without any etching by a JSM7600F field emission gun scanning electron microscope (SEM). Then the sample was cross-sectioned, polished and etched for structural observation under an optical microscope (OM). The etching agent was a mixed solution of CuSO4, HCl and ethanol. Compositions of various phases were analyzed by OXFORD INCA EDX apparatus equipped on the JSM7600F SEM.
3 Results
3.1 Cooling curves
During the rapid solidification of undercooled melts, the growing solid releases latent heat rapidly towards the remaining melt while little heat is transferred into the surrounding, leading to a fast temperature rise of the system, i.e., recalescence. Recalescence behavior therefore reflects the feature of rapid solidification. Co-18%Sn hypo-eutectic alloy, Co-24%Sn eutectic alloy and Co-30%Sn hyper-eutectic alloy were taken as examples, and the corresponding experimental results were described. Defining undercooling as the difference between the equilibrium liquidus temperature TL and the on-set temperature of solidification, Fig. 1 shows the typical cooling curves as a function of undercooling for the alloys. It can be seen clearly that Co-24%Sn eutectic alloy always shows one recalescence in the whole undercooling range studied in this work. In contrast, the cooling curves of Co-18%Sn hypo-eutectic alloy and Co-30%Sn hyper-eutectic alloy exhibit two temperature recalescences when the undercooling is below a critical value ΔTc, but only one temperature recalescence at larger undercoolings. The ΔTc for the two alloys are 170 K and 108 K, respectively.
Recalescence features of all the investigated alloys at different undercoolings are depicted in the Co-Sn equilibrium phase diagram shown in Fig. 2, where the blank circles represent that the alloy melt undercooled to the temperature pointed solidifies with double recalescences, while the solid circles represent the solidification with single recalescence. Clearly, the value of ΔTc increases gradually as the alloy composition deviates from the eutectic point of Co-24%Sn. As shown in Fig. 2, there is no ΔTc to be detected for Co-12%Sn, Co-13%Sn and Co-32%Sn alloys. Double recalescence constantly happens in the whole undercooling range studied. It is uncertain at present whether ΔTc exists in the alloys out of the composition range 14%-31% Sn due to the undercooling limits achieved.
Fig. 1 Cooling curves at different undercoolings
3.2 Solidification structure
Figures 3-5 show the cross-sectional micro- structures of Co-24%Sn, Co-18%Sn and Co-30%Sn alloys solidified at different undercoolings, respectively.
Fig. 2 Recalescence type of cooling curve for different alloys with various undercoolings
As shown in Fig. 3(a), the solidification structure of Co-24%Sn eutectic alloy is composed of regular lamellar eutectic completely at a very low undercooling. When the undercooling increases up to 21 K or higher, anomalous eutectic structures start to form in the center of the eutectic colony, and regular lamellar eutectic grows outwards radically from the edge of anomalous eutectic region (Figs. 3(b) and (c)).
Compared with the Co-24%Sn eutectic alloy, more complex solidification microstructures were observed in the Co-Sn off-eutectic alloys as the undercooling increases. At undercooling below ΔTc where double recalescence takes place, the primary solid formed in rapid solidification is α-Co single phase dendrite for Co-18%Sn hypo-eutectic alloy. Its branches are surrounded by β-Co3Sn2 halos. Lamellar eutectics distribute in the interdendritic regions. As the undercooling increases, more and more α-Co dendrite branches are broken, and anomalous eutectic forms (Fig. 4(b)). At undercooling larger than ΔTc, similar anomalous eutectics to largely undercooled Co-24%Sn eutectic alloy form (Fig. 4(c)). Outside the anomalous eutectics are residual branches of α-Co and lamellar eutectics.
Fig. 3 Cross-sectional microstructures of Co-24%Sn eutectic alloy undercooled at 15 K (a), 43 K (b) and 110 K (c)
Fig. 4 Cross-sectional microstructures of Co-18%Sn hypo-eutectic alloy undercooled at 63 K (a), 170 K (b) and 210 K (c)
Fig. 5 Cross-sectional microstructures of Co-30%Sn hyper-eutectic alloy undercooled at 20 K (a), 108 K (b) and 118 K (c)
The microstructure evolution of the Co-30%Sn hyper-eutectic alloy is similar to the Co-18%Sn hypo-eutectic alloy except that the primary β-Co3Sn2 instead of α-Co forms at undercooling below ΔTc (Figs. 5(a) and (b)), and branches of β-Co3Sn2 encircle the anomalous eutectics at a large undercooling (Fig. 5(c)). Figure 6 shows the surface morphology of the Co-18%Sn hypo-eutectic alloy and Co-30%Sn hyper-eutectic alloy. For both alloys, the coarse primary phase is surrounded by a halo of the other phase.
Fig. 6 SEM images of alloy sample surface
4 Discussion
4.1 Eutectic coupled zone
When the undercooling is lower than ΔTc, the microstructures of off-eutectic alloys consist of coarse single-phase branches and lamellar eutectics surrounding them. Obviously, the primary solid is the single phase, i.e., α-Co for hypo-eutectic alloy and β-Co3Sn2 for hyper-eutectic alloy. The solidification results in two temperature recalescences. In our previous work, it had already been confirmed that regular lamellar eutectic forms as the primary solid in the rapid solidification of Co-24%Sn eutectic alloy is in the whole undercooling range investigated (0-203 K) [12]. Correspondingly, only one recalescence occurs during solidification. In the solidification structures of the Co-18%Sn hypo-eutectic alloy and Co-30%Sn hyper-eutectic alloy undercooled above ΔTc, anomalous eutectics are located at the center of the eutectic colony, and their morphologies are similar to the anomalous eutectics in the Co-24%Sn eutectic alloy. At the meantime, single recalescence rather than double recalescence occurs in the corresponding cooling curve. Thus, it is clear that the off-eutectic alloys first solidify into one of the eutectic phase when undercooling is less than ΔTc, resulting in the first temperature recalescence. As the primary phase grows, a solute-enriched layer is established around it till the temperature falls to a relatively low value at which the other eutectic phase nucleates in the remaining liquid and subsequently growths with the first phase in a coupled way (i.e. lamellar eutectic growth), triggering the second temperature recalescence. Due to the solute enrichment around the primary phase, a halo of the other phase forms surrounding it (Figs. 6(a) and (b)). The fact that the primary phase is always surrounded by a halo of the other phase and double recalescence constantly occurs at undercooling below ΔTc whether the alloy is hypo- eutectic or hyper-eutectic indicate that the so-called non-reciprocal nucleation phenomenon does not happen in the solidification of undercooled Co-Sn off-eutectic alloys.
At undercooling above ΔTc, lamellar α-Co/β-Co3Sn2 eutectic solidifies from the undercooled melt as the primary phase. In this case, although one phase is needed to grow in the remaining liquid once coupled eutectic growth stops, fresh nucleation is unnecessary since both phases have been present. Therefore, there is only one recalescence to be observed. In the phase diagram ΔTc constitutes the upper boundary of the eutectic coupled zone. Limited by the maximum undercooling obtained in this experiment, the lower boundary was not determined. For the alloys with Sn content less than 14% or more than 31%, coupled eutectic as the primary phase was not discovered in the experiment.
4.2 Formation of anomalous eutectic
For the Co-24%Sn eutectic alloy regular lamellar eutectic is the primary solid in the rapid solidification. When undercooling is very low, the recalescence is too weak to break the initial lamellar eutectic morphology under the action of superheating and remelting. When the undercooling exceeds 21 K, the recalescence becomes strong enough to remelt part of the primary lamellar eutectic, leading to the formation of anomalous eutectic. As undercooling increases, more and more primary lamellar eutectics solidify in the rapid solidification, and the volume fraction of anomalous eutectics enlarges gradually.
Due to complex solidification mode, the formation of anomalous eutectic in off-eutectic Co-Sn alloys is more complicated than that in the Co-24%Sn eutectic alloy. When undercooling is high enough but still lower than ΔTc, both the primary single phase solid formed in the first recalescence and the regular lamellar eutectic formed in the second recalescence will be disintegrated into anomalous eutectics owing to superheating and remelting. When the undercooling exceeds ΔTc, coupled eutectic growth first happens in the undercooled melt as done in the Co-24%Sn eutectic alloy. In this case, anomalous eutectics only result from the partial remelting of the primary lamellar eutectics.
5 Conclusions
1) The upper boundary of coupled growth zone of eutectic phases is determined for the Co-Sn alloys whose Sn content ranges from 14% to 31%. For the alloy in the composition range, lamellar eutectic instead of single phase primarily forms during solidification once the alloy melt is undercooled below the boundary accompanied by a single recalescence.
2) The simultaneous presence of double recale- scences and halo-like structure surrounding the primary phase in the hypo-eutectic and hyper-eutectic Co-Sn alloys indicates that the so-called non-reciprocal nucleation phenomenon does not happen in the solidification.
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大体积深过冷Co-Sn合金的凝固过程与组织演化
刘 礼1,2，马晓丽3，黄起森1，李金富1，程先华2，周尧和1
1. 上海交通大学 材料科学与工程学院，金属基复合材料国家重点实验室，上海 200240；
2. 上海交通大学 机械与动力工程学院，上海 200240；
3. 上海交通大学 材料科学与工程学院，上海 200240；
摘 要：将含12%-32%Sn(摩尔分数)的系列Co-Sn合金熔体过冷至平衡液相线以下不同温度进行凝固实验，通过监测快速凝固过程中的温度再辉与凝固组织分析不同过冷度下各合金的凝固行为进行研究。确定了Sn含量从14%到31%范围内Co-Sn合金凝固时共晶两相进行耦合生长和非耦合生长的分界线，在此成分范围之外的Co-Sn合金则很难被过冷至共晶共生区。在非共晶成分Co-Sn合金的深过冷凝固过程中不存在非互惠形核现象。
关键词：Co-Sn合金；过冷度；再辉；共晶共生区；凝固；组织演化
(Edited by Xiang-qun LI)
Foundation item: Project (50874073) supported by the National Natural Science Foundation of China; Project (2011CB610405) supported by the National Basic Research Program of China; Project (2011M500074) supported by China Postdoctoral Science Foundation
Corresponding author: Jin-fu LI; Tel:+86-21-54748530; E-mail: jfli@sjtu.edu.cn
DOI: 10.1016/S1003-6326(13)62459-0
