The decomposition of α' (martensite) which contained 5.4at%N was analysed by using both conventional and high-resolution transmission electron microscopy of specimens which had been annealed at 333, 373 or 403K. As a result of annealing, α" precipitates were obtained in an α (ferrite) matrix. Because of the specific orientational relationship between α" and α'-α, unambiguous proof was obtained for extensive (011)-type twinning in the original martensite. After decomposition, 3 types of interface originated from the original (011)α' twin boundary: α"T1-α"T2, αT1-αT2, α"T2-αT2, where T1 and T2 denoted the original α' crystals which constituted the twin. It followed that the local lattice orientations of α" and α at, or near to, the T1/T2 interface were dominated by the intrinsic precipitation process in untwinned martensite. This 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.

Crystallography of Tempered Iron-Nitrogen Martensite - the Roles of (011) Twin Boundaries. K.Han, H.W.Zandbergen, A.Böttger, E.J.Mittemeijer: Philosophical Magazine A, 2000, 80[2], 311-36