Phase transformations in iron-nitrogen martensites; role of elastic strain energies

A survey is presented of the successive stages of structural change experienced by Fe-N martensitic specimens upon tempering: (i) between -160 C and -40 C, transformation of part of the retained austenite to martensite: (ii) below ∝100 C segregation of a small amount of nitrogen at lattice defects and transfer of a small amount of nitrogen from a,b-type to c-type octahedral interstices: (iii) below ∝200 C, for the predominant part of the nitrogen, formation of local enrichments of nitrogen exhibiting ordering of nitrogen, i.e. α'' nitride (Fe16N2) precipitation: (iv) above ∝200 C, dissolution of α'' nitride and precipitation of γ nitride (Fe4N): (v) above ∝240 C, decomposition of retained austenite. The attention is especially devoted to the interstitial redistribution leading to α'' nitride formation. It is shown that α'' nitride precipitates initially (in the martensite matrix) as perfect Fe16N2, whereas in an end stage of precipitation (then in a matrix improved in nitrogen, i.e. approaching ferrite) imperfect Fe16N2-x develops, i.e. structural vacancies on the N sublattice occur. These observations are understood fully as the outcome of the interplay of three elastic strain-energy contributions: (a) static iron-atom displacement energy: (b) nitrogen-atom ordering energy: (c) particle/matrix misfit-strain energy. The occurrence of interstitial enrichments exhibiting interstitial ordering for Fe-N martensite and the occurring of interstitial enrichments without such ordering (called clusters) for Fe-C martensite is explained as the net result of elastic (same for N-N and C-C) and chemical (opposite for N-N and C-C) interstitial-interstitial interactions