Transformation Texture of Allotriomorphic Ferrite in Steel Dae Woo Kim, R. S. Qin, H.K.D.H Badeshia Graduate Institute of Ferrous Technology (GIFT) Pohang University of Science and Technology, Korea Method When a ferrite nucleus forms at an austenite grain surface, it will adopt a low-energy orientation relationship with at least one of the parent grains it is in contact with. It may by chance be possible for it to have a low-energy orientation with both the adjacent austenite grains. However, the probablity with which this dual orientation can happen has never been theoretically explained. A mathematical model based on crystallography is used to calculate the probabilities as a function of the crystallographic texture of the austenite, which then transforms into allotriomorphic ferrite by a reconstructive mechanism. Characterizing austenite and ferrite grains in 3dimensional space Tetrakaidekahedron (S J γ1) : Reference Grain (S J γ i) = (S J γ 1)(G1 J γ i) ,where i=2~1700 Limitation of right handed rotation angle = 0˚ ~ 45 ˚ Simulation of Crystallographic Orientation (1700 Austenite Grains) (a) (b) (αi JKS γ1) : K-S Relationship (αi J γ’1) = (αi JKS γ1)(γ i J γ1) place here a frequency distribution of dual orientation probability versus the strength of the austenite texture (degrees about axis) 0.579356 0.542586 0.102537 0.014470 0.133650 -0.78898 -0.55200 0.572979 0.086936 Simulation of Crystallographic Orientation (Ferrite Grains) = (c) (d) (αKS J αrandom) = (αi J γ’1)(γ’i JKS αi ) = (αi JKS γi) )(γ i J γ’i)(γ’i JKS αi) Axis – Angle Pair Calculations place here a diagram of an allot. at a gamma boundary Conclusions Origin of ferrite texture is non random parent crystallographic orientation Ferrite texture increased as austenite becomes more sharply textured Minimum percentage for dual orientation about 40% when austenite grains have relative orientations <5° Probability of dual orientation is independent of type of austenite texture (a) 100 - pole figure of randomly oriented austenite (b) 100 - pole figure of ferrite (c) Stereographic projection of Bain regions (11˚ circles) (d) 100bcc pole figure of intragranular ferrite * * R. Uemori, Scripta Materialia 48(2003) 371 Allotriomorphic ferrite can grow across austenite grain boundaries Ferrite texture on crystallographic orientation of parent phase Interested texture (Cube, Goss, Copper) shows almost the same tendency in pole figure Blue points : KS relationship by (αi JKS γi) Pink points : Random orientation by (αi J γ’i) Some variants are close to KS orientation → Some possibility for dual orientation relationship in fully random austenite matrix