A novel ferritic steel family hardened by intermetallic compound G-phase

In this paper, G-phase precipitation and the resulting hardening effect on Fe-20Cr-3Ni-1Mn-3Si ferritic alloys by the addition of Ti, Nb, Ta and Zr were individually studied by electron microscopy and atom probe tomography, combined with thermodynamic and first principle calculations. The high resolution scanning electron and transmission electron microscopy observations confirmed that four kinds of Ni₁₆M₆Si₇ (M = Ti, Nb, Ta and Zr) G-phase particles were distributed uniformly in the matrix of the four alloys aged at 560–860 °C. The 3D-APT results revealed the formation of the four different nanoscale precipitates, with the highest number density (6.05 × 10²³ m⁻³) and smallest radius (1.64 ± 0.45 nm) in the Ti added alloy. The nanoscale sized precipitates stability (~3 nm for Ti added alloy; ~5 nm for Nb added alloy and Zr added alloy; ~25 nm for Ta added alloy) was attributed to their particular cube-cube orientation relationships, which led to a very low interfacial energy. The effects of the G-phase on precipitation hardening and quasi steady-state deformation resistance were examined. The 560 °C-aging hardening experiments suggested a rapid precipitation process of the nano-particles. The peak hardness values of the four alloys are in the descending order: Ti ˃˃ Nb > Ta ˃˃ Zr. The 660 °C quasi steady-state deformation experiments showed threshold stresses of 110 and 140 MPa in cases of the Nb and Ti added alloys, which were higher than that of the previously reported B2-NiAl strengthened steels and commercial heat-resistant steels. By using the formation of nanoscale G-phase precipitates, a new ferritic steel family with very high strength and creep resistance has been proposed. Further alloy optimization is in progress.