Lu Jiang

Precipitation strengthened Al-Co-Cr-Fe-Ni-Ti-based alloys containing a significant volume fraction of fine γ′-Ni3(Al, Ti) in the γ-FCC matrix exhibited promising performance indices compared to other compositionally complex alloys (CCA). The present framework integrates alloy performance with economic, environmental, and societal considerations by assessing the cost, sourcing, and production impacts of alloying elements within the γ-γ′ phase field of the Al-Co-Cr-Fe-Ni-Ti system. Using a CALPHAD-driven design approach, Fe-rich γ′-strengthened alloys were developed to balance affordability (maximising Fe and minimising Co), oxidation resistance (maximising Cr and Al) and high-temperature strength through an extended thermal stability of γ’-phase (> 880 °C). While Ni- and Co-rich alloys provide the highest γ′ volume fractions and strength, they incur greater costs and sustainability impacts. The designed novel γ’-strengthened Fe-rich Fe40Ni39Cr11Al6.5Ti3.5 alloy delivers an optimal combination of strength, excellent oxidation resistance at 900 °C through the formation of a continuous protective oxide scale (Cr2O3 and Al2O3) and reduced societal impacts by optimising the Fe/(Fe+Cr) ratio and minimising the Co/(Co+Ni) ratio. This framework enables sustainability-informed materials discovery by quantifying trade-offs between performance and sustainability indices, thereby improving the prospects for commercial adoption.

Precipitation strengthened Al-Co-Cr-Fe-Ni-Ti-based alloys containing a significant volume fraction of fine γ′-Ni3(Al, Ti) in the γ-FCC matrix exhibited promising performance indices compared to other compositionally complex alloys (CCA). The present framework integrates alloy performance with economic, environmental, and societal considerations by assessing the cost, sourcing, and production impacts of alloying elements within the γ-γ′ phase field of the Al-Co-Cr-Fe-Ni-Ti system. Using a CALPHAD-driven design approach, Fe-rich γ′-strengthened alloys were developed to balance affordability (maximising Fe and minimising Co), oxidation resistance (maximising Cr and Al) and high-temperature strength through an extended thermal stability of γ’-phase (> 880 °C). While Ni- and Co-rich alloys provide the highest γ′ volume fractions and strength, they incur greater costs and sustainability impacts. The designed novel γ’-strengthened Fe-rich Fe40Ni39Cr11Al6.5Ti3.5 alloy delivers an optimal combination of strength, oxidation resistance via a continuous protective oxide layer, and reduced societal impacts by optimising the Fe/(Fe+Cr) ratio and minimising the Co/(Co+Ni) ratio. This framework enables sustainability-informed materials discovery by quantifying trade-offs between performance and sustainability indices, thereby improving the prospects for commercial adoption.

Conventional strengthening strategies for ferrite–martensite dual-phase (DP) steels generally focus on strengthening individual constituent phases, often increasing the strength mismatch between ferrite and martensite and thereby limiting mechanical performance. Here, we propose a dual-precipitation strategy that enables simultaneous precipitation strengthening of both ferrite and martensite through controlled phase transformation followed by tempering. This approach introduces two distinct precipitate populations: interphase precipitates in ferrite and tempering-induced precipitates in martensite. Precipitation behaviour in Ti microalloyed steels, with and without Mo addition, was systematically investigated across varying ferrite–martensite fractions using transmission electron microscopy (TEM), small-angle neutron scattering (SANS), and atom probe tomography (APT). Increasing ferrite fraction progressively reduces precipitation sensitivity during tempering, producing coarser precipitates with lower number density. Mo addition primarily promotes early-stage precipitate nucleation rather than substantially enhancing coarsening resistance, resulting in higher precipitate densities and enhanced strengthening. Precipitate chemistry exhibits pronounced size dependence, with smaller precipitates being increasingly metal-rich, while Mo promotes carbon vacancy incorporation within B1-type precipitates. Strengthening estimation indicates that even ultrafine precipitates strengthen the material predominantly through the Orowan bypassing mechanism. These findings demonstrate that dual-precipitation strategy provides a new microstructural design strategy for tailoring phase-specific strengthening and strength mismatch, offering opportunities to optimise the balance between strength and formability in advanced DP steels.