Fine structure of tempered Fe-Cr-N martensite

Abstract

The structure of Fe-5.9%Cr-0.5%N, quenched and tempered in the range 25–550°C, has been investigated by electron microscopy, electron and X-ray diffraction, and measurement of microhardness. The observations are interpreted in terms of a continuous precipitation series of structural intermediates between the solid solution and equilibrium fcc CrN. At low temperatures the atomic arrangement of the finely dispersed precipitates or zones is similar to that in α″-Fe₁₆N₂, whereas at higher temperatures the zones consist of Cr-N monolayer plates on the cube planes of the matrix. At all stages of tempering the precipitation gives rise to ‘tweed’ contrast in electron micrographs and associated electron diffraction streaking, which is shown to arise from an array of tetragonal strain centres arranged such that the total strain energy is minimized.