Cementite-type (DO 11 ) orthorhombic phase in tempered Fe–Mn–N alloys

Abstract

Although the ‘equilibrium’ precipitate which forms in Fe–4·22Mn–0·7N martensite during prolonged tempering at 450°C was previously reported to be the fct η-manganese nitride (Mn₃N₂), this claim is now known to be based on inadequate X-ray and electron diffraction evidence. As briefly reported elsewhere, more exhaustive investigation has shown that the martensitic matrix progressively decomposes by a discontinuous or cellular reaction in which the principal nitrogen-rich phase exhibits the DO₁₁ orthorhombic structure of cementite. This M₃N-type phase precipitates after an interaction between the manganese and nitrogen atoms has occurred within the α-solid solution causing intense secondary hardening on tempering for 30 min at this temperature.