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 degrees C and -40 degrees C, transformation of part of the retained austenite to martensite; (ii) below similar to 100 degrees 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 similar to 200 degrees C, for the predominant part of the nitrogen, formation of local enrichments of nitrogen exhibiting ordering of nitrogen, i.e. alpha " nitride (Fe16N2) precipitation: (iv) above similar to 200 degrees C, dissolution of alpha " nitride and precipitation of gamma nitride (Fe4N): (v) above similar to 240 degrees C, decomposition of retained austenite. The attention is especially devoted to the interstitial redistribution leading to alpha " nitride formation. It is shown that alpha " nitride precipitates initially (in the martensite matrix) as perfect Fe16N2, whereas in an end stage of precipitation (then in a matrix impoverished 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. The influence of alloying elements, as nickel, on the interstitial is distribution expresses itself in the matrix-interstitial interaction which is assessed qualitatively by considering the electronegativities of the elements. Thus it is understood that in FeNi-N martensite no alpha "-type precipitates develop but nitrogen clusters occur.