Development of novel ferritic steels strengthened by the Co₁₆X₆Si₇-G phase : A theoretical and experimental study

This work aims to develop a novel Co₁₆X₆Si₇-G (X = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re) phase strengthened ferritic steel with the help of first-principles calculations. Ddensity function theory calculation results demonstrated that the Co₁₆X₆Si₇-G phase possesses the advantage of good thermodynamic stability and low misfit with the ferritic matrix. On the basis of calculation, we successfully prepared three model alloys, i.e., the Fe20Cr5Co2Si1Ti, Fe20Cr5Co2Si0.75Nb and Fe20Cr5Co2Si1.5Ta (wt.%), who make a good trade-off between strength, ductility and good oxidation/corrosion resistance. Our observation verified that the Co₁₆X₆Si₇-G phase can precipitate within 1 h with proper heat treatment, showing a cube-on-cube orientation relationship with its parent matrix. The introduction of G phase precipitates amazingly increases the micro-hardness of model alloys by ∼ 160 HV after aging at 873 K. And the precipitates within the ferrite phase have not been observed to grow coarser even after 96 hrs aging treatment. Furthermore, excellent mechanical properties of ∼ 1.2 GPa tensile strength and ∼ 10 % total elongation have been achieved after 6 hrs aging time at 873 K. These findings indicate the Co₁₆X₆Si₇-G phase would be a promising candidate as a strengthening medium and provide valuable insight into the screen of precipitates for the development of high-strength and ductile steels.