Thermodynamically stable hR20 structure of the Al3Er phase studied by varying solidification rate and polymorphic transformation

Jianzhu Wang; Kunyuan Gao; Xiangyuan Xiong; Jing Ning; Xiaolan Wu; Shengping Wen; Wu Wei; Hui Huang; Zuoren Nie; Dejing Zhou

doi:10.20517/microstructures.2026.105

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Research Article Open Access 15 Oct 2026

Thermodynamically stable hR20 structure of the Al₃Er phase studied by varying solidification rate and polymorphic transformation

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Jianzhu Wang¹

,

Kunyuan Gao¹

, ...

Dejing Zhou²

Microstructures 2026, 6, 20260130. 10.20517/microstructures.2026.105

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Abstract

In this study, the crystal structure evolution and microstructure of the Al₃Er phase in an Al-30 wt% Er alloy solidified at different cooling rates were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The XRD and EBSD results show that as the cooling rate decreases, the Al₃Er phase transforms from the cP4 structure (60 °C/h) to a mixture of cP4 and hR20 structures (50-40 °C/h), and finally to the hR20 structure (30-5 °C/h). The cP4-structured Al₃Er obtained at 60 °C/h is single-crystalline with an average grain size of 211 μm and exhibits straight boundaries, whereas the hR20-structured Al₃Er obtained at 5 °C/h is polycrystalline with an average grain size of 92 μm and displays wavy boundaries. TEM analysis shows the orientation relationship between the two structures: $$ [1 \overline{2} 1]_{\mathrm{cP} 4} / /[1 \overline{1} 00]_{\mathrm{hR} 20} $$, (111)_cP4//(0006)_hR20. These findings confirm that the stable structure of the Al₃Er phase is hR20. Classical nucleation theory calculation excludes the possibility of direct precipitation of the hR20-structured Al₃Er from the liquid, indicating that it forms through a polymorphic transformation of the cP4-structured Al₃Er. Johnson-Mehl-Avrami-Kolmogorov kinetic analysis yields a critical cooling rate of 36 °C/h for this transformation, consistent with the experimental observations. The high Avrami exponent (n = 6.8) reveals a diffusion-displacive coupled phase transformation mechanism of polymorphic transformation. This work provides a theoretical basis for understanding the structural evolution of the Al₃Er phase and for designing Al-Er alloys.

Keywords

Al₃Er phasepolymorphic transformationthermodynamic stabilitycritical cooling rateautocatalytic nucleation

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Research Article

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Thermodynamically stable hR20 structure of the Al₃Er phase studied by varying solidification rate and polymorphic transformation

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Wang, J.; Gao, K.; Xiong, X.; Ning, J.; Wu, X.; Wen, S.; Wei, W.; Huang, H.; Nie, Z.; Zhou, D. Thermodynamically stable hR20 structure of the Al₃Er phase studied by varying solidification rate and polymorphic transformation. Microstructures 2026, 6, 20260130. https://dx.doi.org/10.20517/microstructures.2026.105

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DOI: https://doi.org/10.20517/microstructures.2026.105

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