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<p>This paper reports on phase transformations, microstructural evolution, and mechanical properties of novel Ni–Mo–Cr–W–Al–Ti superalloys developed as candidate structural materials for molten salt nuclear reactors. Four alloys (UNT-1 to UNT-4), containing 6 to 18 wt pct Mo along with W (6 to 7 wt pct), Al (3 to 3.5 wt pct), and Ti (1 to 1.5 wt pct), were cast, homogenized, hot-rolled, and air-cooled. Effects of aging treatments at 750&#xa0;°C for up to 400 hours were studied using hardness testing, X-ray diffraction, scanning and transmission electron microscopy, and mechanical evaluation by profilometry-based indentation plastometry (PIP) and small-scale tensile testing at room temperature and up to 750&#xa0;°C. The as-rolled and air-cooled alloys exhibited recrystallized equiaxed grains whose size varied with Mo content. Hardness ranged from 370 to 460 HV, with the higher values attributed to <i>γ</i>′ precipitation during post-rolling cooling. Subsequent aging produced further hardening, reaching ~ 520 HV, which remained stable to 400 hours, with hardness increasing proportionally to Mo content. Yield strengths were exceptionally high, ranging from ~ 1.1 to ~ 1.5&#xa0;GPa at room temperature and ~ 1.1 to 1.2 GPa at 750&#xa0;°C. Distinct microstructural changes were observed across the alloys after aging, including the formation of SRO, LRO Ni<sub>2</sub> (MoCrW), <i>γ</i>′-L1<sub>2</sub> precipitates, Mo/W-rich BCC particles, μ and<i> σ</i> phases, with the latter two more pronounced in Mo-rich alloys. Strengthening in all compositions was primarily derived from a high-volume fraction of fine, stable <i>γ</i>′ precipitates. The results highlight the role of Mo concentration in conjunction with Al and Ti additions on ordered phase formation and strengthening mechanisms. The unique microstructural features and outstanding mechanical properties achieved underscore the potential of these alloys to meet the stringent requirements of next-generation molten salt reactor structural applications.</p>

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Microstructure Evolution and Mechanical Properties of Ni–Cr–Mo–W–Al–Ti Superalloys with Varying Mo for High Temperature Structural Applications

  • 期刊论文

  • 发表日期：

    2026年2月4日

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  • Sonali Ravikumar,
  • N. Naveen Kumar,
  • Jie Song,
  • Vishal Soni,
  • Boateng Twum Donkor,
  • Gopal B. Viswanathan,
  • Matthew A. Steiner,
  • Steven J. Zinkle,
  • Vilupanur A. Ravi,
  • Govindarajan Muralidharan,
  • Vijay K. Vasudevan

摘要

This paper reports on phase transformations, microstructural evolution, and mechanical properties of novel Ni–Mo–Cr–W–Al–Ti superalloys developed as candidate structural materials for molten salt nuclear reactors. Four alloys (UNT-1 to UNT-4), containing 6 to 18 wt pct Mo along with W (6 to 7 wt pct), Al (3 to 3.5 wt pct), and Ti (1 to 1.5 wt pct), were cast, homogenized, hot-rolled, and air-cooled. Effects of aging treatments at 750 °C for up to 400 hours were studied using hardness testing, X-ray diffraction, scanning and transmission electron microscopy, and mechanical evaluation by profilometry-based indentation plastometry (PIP) and small-scale tensile testing at room temperature and up to 750 °C. The as-rolled and air-cooled alloys exhibited recrystallized equiaxed grains whose size varied with Mo content. Hardness ranged from 370 to 460 HV, with the higher values attributed to γ′ precipitation during post-rolling cooling. Subsequent aging produced further hardening, reaching ~ 520 HV, which remained stable to 400 hours, with hardness increasing proportionally to Mo content. Yield strengths were exceptionally high, ranging from ~ 1.1 to ~ 1.5 GPa at room temperature and ~ 1.1 to 1.2 GPa at 750 °C. Distinct microstructural changes were observed across the alloys after aging, including the formation of SRO, LRO Ni2 (MoCrW), γ′-L12 precipitates, Mo/W-rich BCC particles, μ and σ phases, with the latter two more pronounced in Mo-rich alloys. Strengthening in all compositions was primarily derived from a high-volume fraction of fine, stable γ′ precipitates. The results highlight the role of Mo concentration in conjunction with Al and Ti additions on ordered phase formation and strengthening mechanisms. The unique microstructural features and outstanding mechanical properties achieved underscore the potential of these alloys to meet the stringent requirements of next-generation molten salt reactor structural applications.