Crystallographic structure of vanadium carbide precipitates in a model Fe–C–V steel

Abstract

The crystallographic structure of vanadium carbide precipitates in iron is investigated using High Resolution Transmission Electron Microscopy (HRTEM) and conventional Selected Area Diffraction (SAD) analysis. After a two-step precipitation treatment (10 hours at 700°C and 10 days at 800°C) and different annealing treatments (from 870 to 920°C) performed on an ultra-pure Fe–V–C model alloy, carbides exhibit unambiguously the ordered monoclinic form V₆C₅. The often reported V₄C₃ structure, that refers to the pioneering work by Baker and Nutting is not encountered in the present investigation. Reasons for this contradiction are discussed, and the conclusion is drawn that no literature data are available to unambiguously support the existence of precipitates with the V₄C₃ structure.

Acknowledgements

The authors are grateful to the CLYME (Consortium Lyonnais de Microscopie Electronique) for access to the JEOL 2010F microscope. This work was financially supported by Ascometal and thanks are due to P. Dierickx (CREAS) for fruitful discussion.

Notes

†According to previous work on a similar steel, carbides as small as 3 nm have been successfully extracted 9.

†The iron austenitic matrix at 800°C turns into a ferrito-perlitic microstructure after slow air cooling.

†According to the basic assumptions that (i) the coarsening is limited by vanadium diffusion, and (ii) the characteristic diffusion distance ranges as , where D is the diffusion coefficient at a given temperature T ( for V in ferrite, and for V in austenite γ-Fe 1, with R equal to 8.32 J/K), 100 days at 700°C is ‘equivalent’ to 10 days at 800°C.

‡According to electrolytic dissolution and plasma spectroscopy, 80% of the vanadium is precipitated in that state 8.

†The initial Baker–Nutting 10 orientation relationship between precipitates and the ferritic matrix at 700°C (see figure 2) has been lost during the subsequent coarsening treatment at 800°C in the austenitic domain.

‡More details on the M₆C₅ ordered phases are given in the appendix.

†This montage summarizes some of the ⟨110⟩_fcc and ⟨112⟩_fcc reciprocal lattice sections of the various forms of V₆C₅ as indexed in the disordered cubic structure. For the sake of brevity, only the most significant zone axes are shown. In the case of the monoclinic phase, it must be emphasized that the 032 _M and diffraction patterns (respectively [101]_fcc and [011]_fcc) do not exhibit any superlattice reflections, which may correspond to what is observed in figure 2 (as discussed in section 5).