Early-stage widmanstätten growth of the γ phase in a duplex steel

The present work studies the Widmanstätten γ-phase morphology and crystallographic orientation relationships with ferrite during its initial-stage growth in a duplex steel. Transmission electron microscopy (TEM) and convergent-beam electron diffraction (CBED) analyses have been applied to determine the orientation relationships with high precision. When the secondary γ phase and the α phase have special orientation relationships close to the K-S (Kurdjumov-Sachs) relation, the preferential growth involves γ phase, protruding into α phase and this leads, finally, to the formation of Widmanstätten needles. The needles grow nearly along the invariant-line direction, but they do not satisfy exactly the K-S relation, and a slight angular deviation (θ_[110]α) is always present. This deviation is explained by a compromise between the well-known invariant-line condition, which offers the easiest way for directional growth of the γ phase, and a good atomic matching of the close-packed (111)_γ and (110)_α planes of both phases. A matching minimum at θ_[110]α =2.3 deg is obtained when only three pairs of the nearest-neighbor atoms are considered in an atomic misfit calculation.