Tuning B2-NiAl precipitate evolution in Cr-Fe-Ni-Al BCC-superalloys: CALPHAD design, thermal stability, and mechanical implications

Next-generation concentrated solar power (CSP) systems place stringent demands on structural materials that must operate reliably at temperatures exceeding 800 °C. Chromium-based BCC-superalloys strengthened by B2-NiAl intermetallic compounds are promising candidates for meeting these high-temperature requirements. Previous studies on Cr-5Ni-5Al and Cr-5Ni-5Al-10Fe (at.%) alloys have shown only a modest volume fraction of ~5% B2-NiAl precipitates. This work aims to increase the intermetallic volume fraction and assess the resultant microstructure and mechanical properties. Phase diagram calculations (CALPHAD) revealed that the addition of 20 at.% Fe allows for complete homogenisation of Cr-10Ni-10Al into single phase (A2) at 1400 °C. Ageing treatments at 800, 1000 and 1200 °C produced coherent B2-NiAl precipitates, reaching a maximum area fraction of ~16% at 1200 °C for 20 h, which is a net increase over previously achieved 5% inCr-5Ni-5Al-10Fe. The precipitates grew via a diffusion-controlled process with exceptionally low coarsening rates. Synchrotron X-ray diffraction results confirmed low lattice misfit ~ −0.1% in the Cr-10Ni-10Al-20Fe alloy. Furthermore, microhardness measurements demonstrated that the combined effects of Fe solid solution strengthening and enhanced B2 precipitation strengthening yield superior hardness in Cr-10Ni-10Al-20Fe alloy at room temperature compared to Cr-10Ni-10Al ternary alloy for the vast majority of heat treatment temperatures. These findings provide guidance for designing Cr-based BCC-superalloys with thermal stability and mechanical robustness for service in demanding high-temperature energy-conversion environments.