G-phase strengthened iron alloys by design

Abstract

Density functional theory (DFT) calculations were used to model G-phase precipitates of formula X6M16Si7 where X is Cr, Hf, Mn, Mo, Nb, Ta, Ti, V, W and Zr and M is either Fe or Ni. It was found that the occupancy of the d-orbital is correlated to the formation enthalpies of each structure. Past thermal expansion coefficient data was used to predict the lattice misfit between each G-phase and body centred cubic (BCC) Fe. All except Hf and Zr containing G-phases were predicted to have zero misfit between 581−843 K. Of the Ni containing G-phases, Mn6Ni16Si7 was predicted to have the most similar elastic properties to BCC Fe. DFT calculations of the substitution energies of Al, Cr Cu, Fe, Ge, Hf, Mo, Nb, P, Ta, Ti, V, Zr, and vacancies onto the Mn6Ni16Si7 G-phase from BCC Fe were performed. It was predicted that Cu, P and vacancies favour G-phase substitution. Suppression of the G-phase is predicted when Si content is reduced by half, at which point the BCC phase is favoured. It is hypothesised that including Zr to form a (Mn,Zr)6Ni16Si7 precipitate will allow for higher ageing temperature and expediate nucleation in an Fe alloy. Thermocalc was used to predict that a mixture of FebalCr9Ni4Si2(Mn0.6Zr0.4)1.2 (at.%) will produce a G-phase strengthened Fe alloy with potential for a good balance of strength, ductility and oxidation/corrosion resistance at room temperature. This alloy composition was experimentally determined to precipitate the G-phase in ≤24 h with cube-on-cube orientation to the BCC Fe matrix.