Crystallography of tempered iron-nitrogen martensite: The roles of (011) twin boundaries

Abstract

Abstract The decomposition of iron—nitrogen martensite (α′) containing 5.4 at.% N, was analysed by both conventional and high-resolution transmission electron microscopy of specimens annealed at 333, 373 and 403 K. As a result of the anneals, α′′ precipitates (the α′′ unit cell has ideally a composition indicated by Fe16N2) were obtained in a ferrite (α) matrix (‘first stage of tempering’). Because of the determined specific orientation relationship between α′′ and α′-α, unambiguous proof for extensive (011)-type twinning in the original martensite was obtained. The crystallography at and near the original (011)α′ twin boundary, after the decomposition α′ ← α′′ + α was scrutinized. After decomposition three types of interface originated from the original (011)α′ twin boundary: α′′T1—α′′T2, αT1-αT2 and α′′T2-αT2, where T1 and T2 denote the original α′ crystals constituting the twin. It followed that the local lattice orientations of α′′ and α at or near the T1—T2 interface were dominated by the intrinsic p...