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

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, α_T1-α_T2 and α′′_T2-α_T2, 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 precipitation process as observed in untwinned martensite, which led to particular clockwise and anticlockwise rotations of the α′′ and α lattices, with respect to the parent α′, in T1 and vice versa in T2. While the α_T1-α_T2 and α_T1-α_T2 interfaces remained largely at the T1-T2 interface, the α′′_T1-α_T2 interface was tilted away from this interface to minimize the α′′-α misfit strain energy. The misfit at the α_T1-α_T2 and α′′_T1-α_T2 interfaces was largely accommodated elastically (occasionally misfit dislocations were also present). The misfit at the α′′_T1-α′′_T2 interface was accommodated by a change in orientation of the (001)_α′′ lattice planes in both α′′_T1 and α′′_T2 near the T1-T2 interface.