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<p>Deviation of growth orientation from the &lt; 001 &gt; direction is a commonly adopted technical approach in the fabrication of advanced single crystal blades; however, its effect on the microstructural stability of Ni-based single crystal superalloys has not yet been reported, even though microstructural stability plays a critical role in the service performance of single crystal blades. In this study, we systematically investigate the microstructure evolution of DD6 alloy with different deviation angles (0°–45°) from the &lt; 001 &gt; orientation during long-term thermal exposure at 1000&#xa0;°C for up to 1000h. After standard heat treatment, increasing the deviation angle reduces residual eutectic fraction, increases the average size and volume fraction of <i>γ</i>′ precipitates in dendrite cores, but decreases their size uniformity. During thermal exposure, a larger deviation angle significantly accelerates the coarsening rate of <i>γ</i>′ phase and intensifies the reduction of its volume fraction, while having only minor effects on <i>γ</i>′ cubicity and <i>γ</i> channel width. Notably, off-orientation promotes earlier precipitation of topologically close-packed (TCP) phases: for the &lt; 001 &gt; oriented alloy, TCP phases appear after 500h, whereas for a 45° off-orientation (close to &lt; 011 &gt;), TCP phases already precipitate after 50h. With prolonged exposure, the quantity and size of TCP phases increase, but the orientation effect diminishes at later stages. These findings demonstrate that growth orientation deviation from &lt; 001 &gt; accelerates <i>γ</i>′ coarsening and TCP precipitation, thereby degrading microstructural stability. This growth orientation-dependent degradation of microstructural stability is mainly attributed to the variation in solidification-induced elemental segregation and the redistribution of refractory elements. These changes modify the diffusion kinetics during thermal exposure, thereby accelerating <i>γ</i>′ coarsening and promoting earlier TCP phase precipitation. The results provide critical guidance for controlling growth orientation in single crystal blade manufacturing, particularly for complex geometries requiring off-axis solidification.</p>

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Off-orientation accelerates coarsening and TCP precipitation: microstructural evolution of Ni-based single crystal superalloys during thermal exposure process

  • 期刊论文

  • 发表日期：

    2026年7月28日

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  • Zhiqiang Yang,
  • Songsong Hu,
  • Weimin Bai,
  • Pengfei Qu,
  • Wendao Li,
  • Yingzheng Chen,
  • Jiefeng Li,
  • Haojie Xie,
  • Fengjun Fan,
  • Xinming Wang

摘要

Deviation of growth orientation from the < 001 > direction is a commonly adopted technical approach in the fabrication of advanced single crystal blades; however, its effect on the microstructural stability of Ni-based single crystal superalloys has not yet been reported, even though microstructural stability plays a critical role in the service performance of single crystal blades. In this study, we systematically investigate the microstructure evolution of DD6 alloy with different deviation angles (0°–45°) from the < 001 > orientation during long-term thermal exposure at 1000 °C for up to 1000h. After standard heat treatment, increasing the deviation angle reduces residual eutectic fraction, increases the average size and volume fraction of γ′ precipitates in dendrite cores, but decreases their size uniformity. During thermal exposure, a larger deviation angle significantly accelerates the coarsening rate of γ′ phase and intensifies the reduction of its volume fraction, while having only minor effects on γ′ cubicity and γ channel width. Notably, off-orientation promotes earlier precipitation of topologically close-packed (TCP) phases: for the < 001 > oriented alloy, TCP phases appear after 500h, whereas for a 45° off-orientation (close to < 011 >), TCP phases already precipitate after 50h. With prolonged exposure, the quantity and size of TCP phases increase, but the orientation effect diminishes at later stages. These findings demonstrate that growth orientation deviation from < 001 > accelerates γ′ coarsening and TCP precipitation, thereby degrading microstructural stability. This growth orientation-dependent degradation of microstructural stability is mainly attributed to the variation in solidification-induced elemental segregation and the redistribution of refractory elements. These changes modify the diffusion kinetics during thermal exposure, thereby accelerating γ′ coarsening and promoting earlier TCP phase precipitation. The results provide critical guidance for controlling growth orientation in single crystal blade manufacturing, particularly for complex geometries requiring off-axis solidification.