Key Takeaways & Executive Findings
- •• The study provides quantitative insights into the columnar-to-equiaxed transition (CET) in Mg-Gd-Zn alloys, showing that grain size, dendrite tip radius, and secondary dendrite arm spacing decrease exponentially with increasing cooling rate. • The CET is promoted by lower temperature gradients and higher cooling rates, which is crucial for controlling solidification microstructures in industrial casting processes. • The experimental results align with established models (Hunt, Bouchard-Kirkaldy, and GTK), validating the use of phase-field simulations for predicting dendrite morphology and CET. • Increasing critical nucleation supercooling inhibits equiaxed crystal formation, while higher nucleation density promotes CET, offering strategies to tailor alloy microstructures for improved mechanical properties.
Abstract
Columnar to equiaxial crystal transition (CET) is an important technological feature in many casting processes. This work investigated the CET during the solidification of Mg-Gd-Zn alloys by combining synchrotron radiation in-situ imaging and phase-field method. Results show that the grain size, dendrite tip radius, and secondary dendrite arm spacing (SDAS) all exponentially decrease with an increase in cooling rate (Vc). The variation in the radius of the dendritic tip is similar to the prediction of the Hunt model, while the variation in the SDAS is close to the Bouchard-Kirkaldy model. It is worth noting that the CET is promoted by a decrease in the temperature gradient (G) and an increase in the cooling rate (Vc). In both equiaxed and columnar crystal regions, the dendrite tip growth rate and solid phase volume fraction increase with increasing G and Vc. In addition, the CET process has been predicted by simulation. The results are consistent with the predictions of the GTK model, which is important for the in-depth study of the dendrite morphology in different crystallization regions. In the final stage, the effects of different critical subcooling degrees and nucleation densities on the CET were explored. The results show that increasing the critical nucleation supercooling degree can inhibit the generation of equiaxial crystals, while increasing the nucleation density helps to promote the CET.
1. Introduction
With the growing demand for lightweight materials across various fields, magnesium alloys, called 'green engineering materials in the 21st century', are seeing wider applications due to their low density, high specific strength and stiffness, excellent electromagnetic shielding performance, and outstanding vibration damping capacity [1-2]. The rare-earth elements, such as Gd, Y, Ce, and Nd, can further improve the strength and plastic processing capacity of the magnesium alloys. Recently, Mg-Gd-Zn alloys with a close-packed hexagonal structure have been widely used in aerospace and defense applications due to their high specific strength and excellent plasticity [3-4]. However, poor corrosion resistance and low heat resistance limit the further application of Mg-Gd-Zn alloys. The microstructures formed during the solidification play an important role in determining the final properties of alloys. So, predicting and controlling the formation of solidification microstructures becomes a key to optimizing the alloy properties.
There are two main solidification microstructures including columnar and equiaxed crystal structures. The columnar crystal structure imparts directional properties to castings, whereas the equiaxed crystal structure results in more isotropic and uniform material properties [5-10]. Therefore, studying the columnar-to-equiaxed transition (CET) mechanism in magnesium alloys is of great significance to improve the alloy properties and expand the application of the magnesium alloys.
Over the past decades, extensive studies have been conducted to investigate the CET phenomenon in alloys. Hunt et al. [11] proposed that equiaxial crystals can appear along the columnar crystal front during the directional solidification and predicted the time when all grains can be transformed into equiaxial crystals. Sturz et al. [12] found that the length of the columnar crystal region decreased with the increase of cooling rate in the CET experiments. Zaïdat et al. [13] found that the CET phenomenon became more obvious with the increase of the pulling speed when the temperature gradient was constant. Gäumann et al. [14-15] studied the effects of nucleation undercooling and solute content on the grain structure transition. The nucleation undercooling along the front of the columnar grain boundary was the main factor causing the CET phenomenon. However, above experiments lack real-time observation of the CET phenomenon, and the solidification information might be incomplete. The development of synchrotron radiation technology compensates for the previous limitations in real-time observation of solidification microstructures. Mathiesen et al. [16] observed similar results to those reported by Gäumann et al. [15] using the synchrotron radiation technology. Kang et al. [17] found that the CET of the Al-20wt.%Cu alloy was caused by fragmentation behavior and nucleated dendrites.
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Yong-biao Wang, Bao-qi Ma, Jia-xin Wang, Xin-tian Liu, Ang Zhang, Jian-xiu Liu, Yan Wang, Yu-juan Wu, Li-ming Peng (2025). Quantitative analysis of columnar-to-equiaxed transition in Mg-Gd-Zn alloys. China Foundry. https://doi.org/10.1007/s41230-025-4031-3
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Frequently Asked Questions
What is the columnar-to-equiaxed transition (CET) in alloys?
CET is a phenomenon during solidification where columnar dendrites transition to equiaxed grains. It significantly affects the mechanical properties of castings, with equiaxed structures providing more isotropic properties.
How was the CET studied in Mg-Gd-Zn alloys?
The study combined synchrotron radiation in-situ imaging to observe solidification in real-time and phase-field simulations to model the process. This allowed quantitative analysis of dendrite tip radius, SDAS, and grain size under varying cooling rates and temperature gradients.
What are the key findings regarding cooling rate and temperature gradient?
Higher cooling rates and lower temperature gradients promote the CET. Grain size, dendrite tip radius, and SDAS decrease exponentially with increasing cooling rate, and the CET is enhanced under these conditions.
How do critical nucleation supercooling and nucleation density affect CET?
Increasing critical nucleation supercooling inhibits equiaxed crystal formation, while increasing nucleation density promotes the CET. This provides a means to control the microstructure during casting.
What is the significance of this research for industrial applications?
Understanding CET allows better control of solidification microstructures in Mg-Gd-Zn alloys, leading to improved mechanical properties and broader applications in aerospace and defense where lightweight and high-strength materials are needed.
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