Key Takeaways & Executive Findings
- •• Micro X-ray computed tomography enables non-destructive 3D visualization of intermetallic phases in magnesium alloys, overcoming limitations of 2D metallography. • The study characterizes Al-Mn and eutectic intermetallics in both industrial Mg-Al-Zn and novel Mg-Ni-Gd-Y alloys, revealing their nucleation, growth, and spatial distribution. • 3D analysis provides insights into the morphology and interconnectivity of intermetallic networks, which are critical for optimizing solidification processes and alloy properties. • Findings have significant implications for improving melt purity, corrosion resistance, and microstructural uniformity in magnesium alloy production and recycling.
Abstract
This comprehensive study investigates the formation and evolution of intermetallic compounds during the solidification process of magnesium alloys using advanced micro X-ray computed tomography. By analyzing both common industrial Mg-Al-Zn alloys and a novel rare earth-containing Mg-Ni-Gd-Y alloy, we aim to characterize the nucleation, growth, and distribution of Al-Mn and eutectic intermetallics across various stages of solidification. The non-destructive imaging technique employed in this research provides high-resolution, three-dimensional insights into the microstructural development, allowing for a detailed examination of the morphology, spatial arrangement, and interconnectivity of intermetallic phases. This approach overcomes limitations of traditional two-dimensional metallographic methods, offering a more comprehensive understanding of the complex three-dimensional structures formed during solidification.
1. Introduction
Presently, Mg-Al alloys are extensively utilized in gravity casting (GC), direct chill (DC) casting, low-pressure casting, and high pressure die-casting (HPDC) processes [1, 2]. The formation of numerous divorced eutectic Mg17Al12 phases during non-equilibrium solidification leads to an instable microstructure and significantly limits the alloy’s thermal stability and high-temperature creep properties [3 −5]. During the early stages of the solidification process before the eutectic reaction, the microstructural evolution was mainly characterized by the growth and coarsening of α-Mg dendrites. This led to the development of a eutectic network confined within the limited interdendritic spaces after dendrite coherency [6 −9]. There are significant differences in the morphology of α-Mg dendrites and the distribution of the discrete eutectic phase Mg17Al12 as functions of alloy composition and cooling conditions [1, 7].
Apart from divorced eutectic structure in cast microstructure, another important feature in magnesium alloy is that Fe is the primary impurity in the smelting and melting of magnesium alloys, which adversely affects corrosion resistance. To mitigate this, an appropriate amount of anhydrous MnCl2 is often added during the refinement/melting process of Mg alloys. This addition leads to the formation of Al-Mn-(Fe) compounds [10−12], which can then be removed through sedimentation or filtration. Maintaining the Fe/Mn mass ratio below 0.032 is crucial for ensuring the corrosion resistance of AZ91 magnesium alloy [13]. These compounds form during the early stages of solidification in the AZ91 alloy and combat with Fe impurities. Due to differences in density, Al-Mn-(Fe) compounds tend to nucleate from the melt and gravitate towards oxide inclusions [14, 15]. The formation, growth, and distribution of Al-Mn-(Fe) compounds during the melting and casting of Mg-Al alloys are crucial for producing AZ91 alloy ingots with refined grains, uniform structure, and high purity, as well as for the efficient recycling of magnesium alloy scrap [16].
Recent studies have focused on refining and achieving uniform microstructures in Al-Mn intermetallic compounds within AZ91 alloys by examining: 1) The distribution and morphology of Al8Mn5 particles under varying cooling conditions [12, 17−20]; 2) The kinetics and crystallography of Al8Mn5 nucleation and growth [11, 12, 15, 16, 20−23]; 3) The correlation between Al8Mn5 and grain size of Mg-Al alloys [24, 25]; and 4) the segregation and settling behaviors of Al8Mn5 in the melt [15], which all significantly impact the microstructure and composition uniformity. Therefore, it is of great importance to visualize the 3D morphology and distribution of intermetallics forming during solidification. The results have strong implications to resolve the industry ambiguities, optimize magnesium alloy melting processes, improve melt purity and corrosion resistance, and enhance the microstructure and regeneration of magnesium alloys.
Meanwhile, X-ray imaging technology has seen advancements globally over the past decade [26 −41], leveraging in-situ or ex-situ synchrotron radiation/lab source X-ray computed tomography (XCT) technology for quantitatively analyzing the kinetics of ma
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SUN Wei, HU Xiao-juan, DENG Yang-chao, YANG Yang, YAO Hu, ZHANG Yong-hong, ZHANG Rui-feng, ZENG Guang (2026). Three-dimensional characterization of intermetallic compound formation in magnesium alloys with micro X-ray computed tomography. Journal of Central South University. https://doi.org/10.1007/s11771-025-6125-x
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Frequently Asked Questions
What is the main objective of this study?
The study aims to characterize the formation and evolution of intermetallic compounds during solidification of magnesium alloys using micro X-ray computed tomography, providing 3D insights into their morphology, distribution, and interconnectivity.
Which alloys were investigated in this research?
The research investigates common industrial Mg-Al-Zn alloys and a novel rare earth-containing Mg-Ni-Gd-Y alloy.
What are the advantages of using micro X-ray computed tomography over traditional methods?
Micro X-ray computed tomography is non-destructive and provides high-resolution 3D imaging, overcoming limitations of 2D metallographic methods and enabling detailed analysis of complex 3D microstructures.
What are the key findings regarding intermetallic compounds?
The study reveals the nucleation, growth, and distribution of Al-Mn and eutectic intermetallics, highlighting their 3D morphology and spatial arrangement, which are critical for optimizing solidification processes and improving alloy properties.
How do the results impact industrial applications?
The findings have significant implications for improving melt purity, corrosion resistance, and microstructural uniformity in magnesium alloy production and recycling, aiding in process optimization and quality control.
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