Phase Transformations in Iron-Nitrogen Martensites; Role of Elastic Strain Energies

Abstract

A survey is presented of the successive stages of structural change experienced by Fe-N martensitie 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.c. α nitride (Fe 16 N 2 ) precipitation: (iv) above ∼200°C, dissolution of α nitride and precipitation of γ nitride (Fe 4 N); (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 a α pinale precipitates initially (in the martensite matrix) as perfect Fe 16 N 2 , whereas in an end stage of precipitation (then in a matrix impoverished in nitrogen, i.e. approaching ferrite) imperfect Fe 16 N 2-x develops, i.e. structural vacancies on the N sublattice occur. These observations are understood fully as the outcome of the interplay of three elastie stram-energy contributions: (a) static iron-atom displacement energy: (b) nitrogen-atom ordering energy: (c) partrele/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 clastic (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 redistribution 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 α-type precipitates develop but nitrogen clusters occur.