Chromium-based BCC-superalloys reinforced by B2-CoAl intermetallic precipitates

Chromium (Cr)-based body-centred cubic (BCC)-superalloys strengthened by ordered B2 intermetallic precipitates are promising for structural service at 700-1000 °C, where conventional Ni-based superalloys approach their temperature limits. However, their design space is restricted by the low solubility of B2-forming solutes such as Ni + Al in A2-Cr, which limits the attainable B2 fraction and compositional flexibility. Here, CoAl is introduced as an alternative B2 phase, and precipitation, coarsening, and strengthening in Cr-Co-Al alloys are systematically investigated. Vacuum arc-melted alloys were homogenised at 1450 °C, water quenched, and aged at 1000 °C for up to 240 h. Microstructural analysis reveals strong composition dependence of B2 precipitation. After homogenisation, Cr-5Co-5Al and Cr-10Co remain single-phase A2-Cr, whereas higher-alloyed compositions develop A2 + B2 microstructures. B2-CoAl precipitates remain spherical and reach a maximum area fraction of 34% in Cr-15Co-15Al after 240 h. The A2/B2 lattice misfit is small (-0.3% to -0.6%), indicating favourable coherency for high-temperature performance. Cr-10Co-10Al shows a substantially lower B2 coarsening rate than the reported Cr-10Ni-10Al analogue, indicating improved precipitate stability. All ternary alloys show peak hardness in the homogenised condition: Cr-20Co-10Al reaches 842 HV at room temperature and retains 489 HV at 1000 °C, while B2-free Cr-10Co remains ~510 HV because of strong Co solid-solution strengthening. Ageing softening is attributed to the combined effects of B2 coarsening and Co/Al partitioning, which reduce precipitation and matrix solid-solution strengthening, respectively. These results identify B2-CoAl precipitation as a promising strategy for Cr-based BCC-superalloys.

Journal of Alloys and Compounds, 191435. Elsevier BV