The Effect of Yttrium Addition on the Solidification Microstructure and Sigma Phase Precipitation Behavior of S32654 Super Austenitic Stainless Steel
This study focuses on S32654 super austenitic stainless steel (SASS) and systematically characterizes the morphology of the sigma (σ) phase and the segregation behavior of alloying elements in its as-cast microstructure. High-temperature confocal scanning laser microscopy (HT-CSLM) was employed to i...
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2025-07-01
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| author | Jun Xiao Geng Tian Di Wang Shaoguang Yang Kuo Cao Jianhua Wei Aimin Zhao |
| author_facet | Jun Xiao Geng Tian Di Wang Shaoguang Yang Kuo Cao Jianhua Wei Aimin Zhao |
| author_sort | Jun Xiao |
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| description | This study focuses on S32654 super austenitic stainless steel (SASS) and systematically characterizes the morphology of the sigma (σ) phase and the segregation behavior of alloying elements in its as-cast microstructure. High-temperature confocal scanning laser microscopy (HT-CSLM) was employed to investigate the effect of the rare earth element yttrium (Y) on the solidification microstructure and σ phase precipitation behavior of SASS. The results show that the microstructure of SASS consists of austenite dendrites and interdendritic eutectoid structures. The eutectoid structures mainly comprise the σ phase and the γ<sub>2</sub> phase, exhibiting lamellar or honeycomb-like morphologies. Regarding elemental distribution, molybdenum displays a “concave” distribution pattern within the dendrites, with lower concentrations at the center and higher concentrations at the sides; when Mo locally exceeds beyond a certain threshold, it easily induces the formation of eutectoid structures. Mo is the most significant segregating element, with a segregation ratio as high as 1.69. The formation mechanism of the σ phase is attributed to the solid-state phase transformation of austenite (γ → γ<sub>2</sub> + σ). In the late stages of solidification, the concentration of chromium and Mo in the residual liquid phase increases, and due to insufficient diffusion, there are significant compositional differences between the interdendritic regions and the matrix. The enriched Cr and Mo cause the interdendritic austenite to become supersaturated, leading to solid-state phase transformation during subsequent cooling, thereby promoting σ phase precipitation. The overall phase transformation process can be summarized as L → L + γ → γ → γ + γ<sub>2</sub> + σ. Y microalloying has a significant influence on the solidification process. The addition of Y increases the nucleation temperature of austenite, raises nucleation density, and refines the solidification microstructure. However, Y addition also leads to an increased amount of eutectoid structures. This is primarily because Y broadens the solidification temperature range of the alloy and prolongs grain growth perio, which aggravates the microsegregation of elements such as Cr and Mo. Moreover, Y raises the initial precipitation temperature of the σ phase and enhances atomic diffusion during solidification, further promoting σ phase precipitation during the subsequent eutectoid transformation. |
| format | Article |
| id | doaj-art-d25f082f5ded4fdfac3b30b253527f98 |
| institution | DOAJ |
| issn | 2075-4701 |
| language | English |
| publishDate | 2025-07-01 |
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| spelling | doaj-art-d25f082f5ded4fdfac3b30b253527f982025-08-20T03:08:02ZengMDPI AGMetals2075-47012025-07-0115779810.3390/met15070798The Effect of Yttrium Addition on the Solidification Microstructure and Sigma Phase Precipitation Behavior of S32654 Super Austenitic Stainless SteelJun Xiao0Geng Tian1Di Wang2Shaoguang Yang3Kuo Cao4Jianhua Wei5Aimin Zhao6Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, ChinaCollaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, ChinaCollaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, ChinaCollaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, ChinaCollaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, ChinaCollaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, ChinaCollaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, ChinaThis study focuses on S32654 super austenitic stainless steel (SASS) and systematically characterizes the morphology of the sigma (σ) phase and the segregation behavior of alloying elements in its as-cast microstructure. High-temperature confocal scanning laser microscopy (HT-CSLM) was employed to investigate the effect of the rare earth element yttrium (Y) on the solidification microstructure and σ phase precipitation behavior of SASS. The results show that the microstructure of SASS consists of austenite dendrites and interdendritic eutectoid structures. The eutectoid structures mainly comprise the σ phase and the γ<sub>2</sub> phase, exhibiting lamellar or honeycomb-like morphologies. Regarding elemental distribution, molybdenum displays a “concave” distribution pattern within the dendrites, with lower concentrations at the center and higher concentrations at the sides; when Mo locally exceeds beyond a certain threshold, it easily induces the formation of eutectoid structures. Mo is the most significant segregating element, with a segregation ratio as high as 1.69. The formation mechanism of the σ phase is attributed to the solid-state phase transformation of austenite (γ → γ<sub>2</sub> + σ). In the late stages of solidification, the concentration of chromium and Mo in the residual liquid phase increases, and due to insufficient diffusion, there are significant compositional differences between the interdendritic regions and the matrix. The enriched Cr and Mo cause the interdendritic austenite to become supersaturated, leading to solid-state phase transformation during subsequent cooling, thereby promoting σ phase precipitation. The overall phase transformation process can be summarized as L → L + γ → γ → γ + γ<sub>2</sub> + σ. Y microalloying has a significant influence on the solidification process. The addition of Y increases the nucleation temperature of austenite, raises nucleation density, and refines the solidification microstructure. However, Y addition also leads to an increased amount of eutectoid structures. This is primarily because Y broadens the solidification temperature range of the alloy and prolongs grain growth perio, which aggravates the microsegregation of elements such as Cr and Mo. Moreover, Y raises the initial precipitation temperature of the σ phase and enhances atomic diffusion during solidification, further promoting σ phase precipitation during the subsequent eutectoid transformation.https://www.mdpi.com/2075-4701/15/7/798super austenitic stainless steelσ phaseHT-CLSMprecipitation mechanismrare earth Y |
| spellingShingle | Jun Xiao Geng Tian Di Wang Shaoguang Yang Kuo Cao Jianhua Wei Aimin Zhao The Effect of Yttrium Addition on the Solidification Microstructure and Sigma Phase Precipitation Behavior of S32654 Super Austenitic Stainless Steel Metals super austenitic stainless steel σ phase HT-CLSM precipitation mechanism rare earth Y |
| title | The Effect of Yttrium Addition on the Solidification Microstructure and Sigma Phase Precipitation Behavior of S32654 Super Austenitic Stainless Steel |
| title_full | The Effect of Yttrium Addition on the Solidification Microstructure and Sigma Phase Precipitation Behavior of S32654 Super Austenitic Stainless Steel |
| title_fullStr | The Effect of Yttrium Addition on the Solidification Microstructure and Sigma Phase Precipitation Behavior of S32654 Super Austenitic Stainless Steel |
| title_full_unstemmed | The Effect of Yttrium Addition on the Solidification Microstructure and Sigma Phase Precipitation Behavior of S32654 Super Austenitic Stainless Steel |
| title_short | The Effect of Yttrium Addition on the Solidification Microstructure and Sigma Phase Precipitation Behavior of S32654 Super Austenitic Stainless Steel |
| title_sort | effect of yttrium addition on the solidification microstructure and sigma phase precipitation behavior of s32654 super austenitic stainless steel |
| topic | super austenitic stainless steel σ phase HT-CLSM precipitation mechanism rare earth Y |
| url | https://www.mdpi.com/2075-4701/15/7/798 |
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