Electron crystallography study of tempered iron-nitrogen martensite and structure refinement of precipitated α″-Fe 16N 2

Abstract

At the first stage of the tempering of iron-nitrogen martensitic steel, martensite α′ decomposes into tetragonal nitride α″-Fe 16N 2 and low nitrogen martensite α′ 1. The α″ crystals are plate-like in shape with wide (001)α″ surface and thin in the [001]α″ direction, while α′ 1 is mosaic. They are in parallel orientation, and make a lamellar texture. Rarely, a new nitride α″′-Fe 16N is formed coherently with α″. It is a base-centered orthorhombic crystal and is formed from α″ by removing nitrogen atoms in every two (110)α″ planes. The crystal structure of α″-Fe 16N 2 has been refined on the basis of single crystal electron diffraction intensity measurements using imaging plates and dynamical intensity calculations. In the space group 14/ mmm, Fe atoms are at the 8 h position with x = 0.220, at 4 e with z = 0.303 and at 4 d, while N atoms are at 2 a. The refined structure may contribute to the interpretation of the giant saturation magnetization of α″-Fe 16N 2.