Introduction of L1₂-Ordered Precipitation to Alumina-Forming Austenitic Heat-Resistant Steels With Low Ni Content

Alumina-forming austenitic (AFA) heat-resistant steels have been reported as a promising new class of steels in recent years with potential applications in advanced ultra-supercritical power plants. It is well known that L1₂-ordered γ’ phase is the most important precipitate for high-temperature strengthening in Ni-based superalloys and it can be stabilized by increasing the Ni content in heat-resistant steels. In the current work, the evolution of L1₂-ordered precipitates were compared in the Cu-bearing AFA alloys with 20, 27 and 35 wt.% Ni. After slow tensile tests at 700°C (∼2 × 10⁻⁵ s⁻¹), L1₂-ordered precipitates occurred in all the alloys. Alloy AFA27 displayed the most densely distributed L1₂-particles in the matrix, whose ultimate tensile strength was also the highest. However, the L1₂-ordered precipitates were only observed in alloy AFA27 after the slow tensile test at 750°C due to the thermodynamic and kinetic reasons. Flow curves of slow tensile tests indicated different precipitation behaviors at 700°C and 750°C. Chemical composition analysis and thermodynamic calculation revealed that the occurrence of L1₂-ordered Ni-Cu-Al phase depends on temperature, Ni content and the atomic ratio of Ni/Al. This opens up new opportunities to promote the formation of L1₂-ordered phase in Fe-based austenitic heat-resistant steels with low Ni content and benefits high-temperature strengthening.