Morphologies of intermetallic compound phases in Sn−Cu and Sn−Co peritectic alloys during directional solidification

Research & Development CHINA FOUNDRY www.springer.com/41230 https://doi.org/10.1007/s41230-022-1109-z Morphologies of intermetallic compound phases in Sn-Cu and Sn-Co peritectic alloys during directional solidification *Peng Peng1, 2, 3, Jin-mian Yue4, An-qiao Zhang1, Jia-tai Wang3, and Jiang-lei Fan5 1. School of Physics and Electronic Information Engineering, Qinghai Normal University, Xining 753000, China 2. School of Materials and Energy, Lanzhou University, Lanzhou 730000, China 3. Northwest Rare Metal Materials Research Institute, Shizuishan 753099, Ningxia, China 4. Shanxi Taigang Stainless Steel Co., Ltd., Taiyuan 030003, China 5. School of Mechanical and Electrical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450000, China Abstract: The morphologies of intermetallic phases (IMCs) during directional solidification of the Sn-Cu (L+Cu3Sn→Cu6Sn5) and Sn-Co (L+CoSn→CoSn2) peritectic systems were analyzed. The primary Cu3Sn and peritectic Cu6Sn5 phases in Sn-Cu alloy are IMCs whose solubility ranges are narrow, while both the primary CoSn and peritectic CoSn2 phases in Sn-Co alloy are IMCs whose solubility ranges are nil in equilibrium condition. The experimental results before acid corrosion shows that the dendritic morphology of both the Cu6Sn5 and CoSn2 phases can be observed. The investigation on the local dendritic morphology after deep acid corrosion shows that these dendrites are composed of small sub-structures with faceted feature. Faceted growth of the primary Cu3Sn and CoSn phases is also confirmed, and a faceted to non-faceted transition in their morphologies is observed with increasing growth velocities. Further analysis shows that the dendritic morphology is formed in the solidified phases whose solubility range is larger during peritectic solidification. Keywords: intermetallic compound phase; peritectic alloy; directional solidification; solubility range; sub-structure CLC numbers: TG146.1+4 Document code: A 1 Introduction Numerous research works have been performed for peritectic alloys which can be used in broad industrial applications [1-6]. The peritectic reaction: L+α→β occurs during solidification of these systems, where α is the primary phase and β is the peritectic phase [7]. Three different types of peritectic systems can be defined depending on whether the solid phases involved are solid solution phases or intermetallic compound phases (IMCs) with nil solubility or narrow solubility range [7]. Different from the solid solution phases which have been commonly analyzed, the growth of IMCs from melt is of practical interest because the appropriate morphology, size, and distribution of IMCs can lead to significant optimizing properties of alloys [8-13]. Furthermore, IMCs *Peng Peng Ph. D., Professor. His research interests mainly focus on the solidification theory of nonferrous alloys (Sn-base, and TiAl alloys) during directional solidification, and advanced casting processes of structural superalloys (Ni-base alloys). E-mail: Received: 2021-08-28; Accepted: 2022-04-29 exhibit more complex morphologies as compared with the solid solution phases: planar, cellular to dendritic [14]. Faceted growth with a strong anisotropy can often be observed during solidification of IMCs. In addition, the transition from faceted growth to nonfaceted growth with increasing cooling rates have also been confirmed [15, 16]. However, the current analyses on the morphology of solidification microstructure are usually based on two-dimensional observation [17, 18], which is limited since the three-dimensional information of the growth morphology can not be fully revealed [19]. In recent decades, many methods have been developed to characterize the three-dimensional morphology of solid phases. Three of these methods have been most widely used: three-dimensional reconstruction on the basis of three-dimensional successive sectioning [20, 21], synchrotron radiation [22], and the deep etching method using appropriate acid/alkali corrosion. Among them, the deep etching method has shown strong applicability, and its only difficulty lies in choosing an appropriate corrosive to better exhibit the morphology information. For this reason, the morphology information of many phases [8-13] including both solid solution phase and IMCs have been obtained through the deep etching method. CHINA FOUNDRY Research & Development The IMCs can be frequently encountered in solidification of different peritectic systems. Among them, the Sn-Cu and Sn-Co alloys have shown wide application. As one of the most popular lead-free solders, Sn-Cu alloys are commonly applied in the electronics industry owing to their excellent weldability and non-toxic property [23-29]. Investigation on the Sn-Cu solders of high Sn content is of great interest since the operating temperature of solders can be enhanced by increasing the content of Cu. Sn-Co alloys are also widely used in lead-free solders, negative electrode materials, etc [30-35]. In Sn-based solders used in electronic packaging, Co can form a good diffusion barrier between solder and substrate due to its low solubility [36-38]. Despite the numerous reports on Sn-Cu and Sn-Co peritectic systems, the study on the three-dimensional morphology of these peritectic systems containing IMCs has been rarely reported. Further experimental evidences are required for understanding the growth of the IMCs in detail in these peritectic alloys. In this work, the morphology features of IMCs were analyzed in the Sn-Cu and Sn-Co peritectic alloys through deep etching method. Furthermore, the dependence of the formation of dendritic morphology on the solubility range of the solidified phases was also investigated. 2 Experimental procedure Pure copper (99.9wt.%), pure tin (99.9wt.%) and cobalt (99.9wt.%) were used as raw materials to prepare the Sn-32at.% Cu and Sn-9at.% Co alloys by melting in a vacuum induction furnace. The rods (Φ6 mm×80 mm) were cut from the ingot and placed into the Al2O3 tubes [φ6.5(Φ7.5) mm×110 mm]. Directional solidification was carried out in a Bridgman-type apparatus. First, the furnace was heated up above their melting temperature (600 ºC for Sn-Cu and 850 ºC for Sn-Co), and held for 30 min. Second, the Sn-32at.% Cu and Sn-9at.% Co samples were fabricated at different growth velocities: 10, 20 μm·s-1 for Sn-32at.% Cu alloy and 1, 5, 10, 100 μm·s-1 for Sn-9at.% Co alloy. After that, the tube was quickly quenched into the liquid Ga-In-Sn alloy. The temperature gradient during directional (a) solidification of both alloys is 32 K·mm -1 , which was obtained from the temperature profiles of the PtRh30-PtRh6 thermocouples near the solid/liquid interface. Finally, the microstructures on the longitudinal sections of the samples were analyzed using scanning electron microscopy (SEM, Apreo-S) equipped with energy dispersive spectrometer (EDS). An etchant solution of 10 g FeCl3+40 mL HCl+160 mL C2H5OH was used to dissolve t (...truncated)