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<p>The effects of long-term aging on the microstructure and mechanical properties of a novel developed Ni-Cr-Fe superalloy for automotive exhaust valves were investigated. During prolonged aging at 750 °C, the coarsening of the spheroidal γ′ phase followed the Lifshitz–Slyozov–Wagner (LSW) law, and its coarsening rate constants (<i>k</i>) are 294.37 nm<sup>3</sup>/h (0-3000&#xa0;h) and 92.68 nm<sup>3</sup>/h (3000-7000&#xa0;h), respectively, which exhibits segmented linear behavior. Notably, the intergranular (M<sub>23</sub>C<sub>6</sub>) and intragranular (MC) carbides remained stable with γ′ during the initial 1000&#xa0;h. Between 1000 and 2000&#xa0;h, primary α-Cr precipitates preferentially nucleated at the interfaces of MC-type carbides. During this period, the material strength remained largely unchanged, whereas the fracture elongation decreased from 26% to approximately 10%. After 2000 h, the grain boundary M<sub>23</sub>C<sub>6</sub> underwent abnormal coarsening, with the mean width increasing from 410 to 1328&#xa0;nm, and the α-Cr gradually transformed from fine-grained to acicular/lamellar morphologies. Mechanical property degradation was primarily attributed to γ′ coarsening (shifting dislocation-γ′ interactions from shearing to Orowan looping) and accelerated intergranular crack propagation by grain boundary carbide coarsening and α-Cr precipitation in the matrix.</p>

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Effect of Long-Term Thermal Exposure on Microstructure and Performance of Ni-Cr-Fe Superalloy for Automotive Exhaust Valves

  • 期刊论文

  • 发表日期：

    2026年8月5日

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  • Yuguo Tu,
  • Jie Pan,
  • Xueshan Xiao,
  • Wei Peng,
  • Zhiyuan Zhu

摘要

The effects of long-term aging on the microstructure and mechanical properties of a novel developed Ni-Cr-Fe superalloy for automotive exhaust valves were investigated. During prolonged aging at 750 °C, the coarsening of the spheroidal γ′ phase followed the Lifshitz–Slyozov–Wagner (LSW) law, and its coarsening rate constants (k) are 294.37 nm3/h (0-3000 h) and 92.68 nm3/h (3000-7000 h), respectively, which exhibits segmented linear behavior. Notably, the intergranular (M23C6) and intragranular (MC) carbides remained stable with γ′ during the initial 1000 h. Between 1000 and 2000 h, primary α-Cr precipitates preferentially nucleated at the interfaces of MC-type carbides. During this period, the material strength remained largely unchanged, whereas the fracture elongation decreased from 26% to approximately 10%. After 2000 h, the grain boundary M23C6 underwent abnormal coarsening, with the mean width increasing from 410 to 1328 nm, and the α-Cr gradually transformed from fine-grained to acicular/lamellar morphologies. Mechanical property degradation was primarily attributed to γ′ coarsening (shifting dislocation-γ′ interactions from shearing to Orowan looping) and accelerated intergranular crack propagation by grain boundary carbide coarsening and α-Cr precipitation in the matrix.