A new Bcc-Fcc orientation relationship observed between ferrite and austenite in solidification structures of steels
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HE crystallographic orientation relationship (OR) between two phases with different crystal structures is an important parameter affecting microstructure-property relationships in such processes as precipitation from solid solution, the martensitic transformation, epitaxial growth, and solidification. An OR between two phases is often found in solidification microstructures where the relationship is established either during solidification or during subsequent solid-state transformation on cooling. For example, the weld or cast microstructures of austenitic stainless steels frequently contain ⬃2 to 10 vol pct delta-ferrite (␦ ) in the austenite (␥ ) matrix. Both alloy composition and solidification/cooling conditions dictate whether or not an OR is established between delta-ferrite and austenite in the resultant two-phase microstructure. Although extensive studies have been published on the solidification and microstructural evolution in austenitic stainless steel welds and castings,[1–11] it is not always straightforward to determine when the two phases were formed, or if an OR existed between delta-ferrite and austenite, when it was established. This is due in part to the complex nature and wide range of ferrite morphologies that can exist even within a single solidification structure.[12] The structure of delta-ferrite is bcc, and that of austenite is fcc. Historically, a small number (five) of ORs has been reported to exist between bcc and fcc lattices. The three best known are the Bain,[13] the Kurdjumov and Sachs (K–S),[14] and the Nishiyama and Wasserman (N–W)[15,16] ORs. The Greninger and Troiano (G–T)[17] and the Pitsch[18] ORs are less well known. It is common practice to describe an OR by a set of planes that are parallel in the two lattices and a set of directions within those planes that are parallel. This T.J. HEADLEY, Distinguished Member of Technical Staff, is with the Materials Characterization Department, Sandia National Laboratories, Albuquerque, NM 87185-1411. J.A. BROOKS, Principal Member of Technical Staff, is with the Engineered Materials Department, Sandia National Laboratories, Livermore, CA 94551-9402. Manuscript submitted July 10, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
information is given in Table I for the five known bccfcc ORs. The Bain OR was proposed first in 1924 to explain the martensite* transformation in carbon steels in terms of a *Martensite, with a small tetragonal distortion, is found experimentally to obey the same ORs as bcc lattices and is grouped with the bcc system in this description.
simple set of orthogonal strains that would transform the austenite lattice directly to the martensite lattice. It is now well known that the Bain distortion alone is not sufficient to describe the martensitic transformation in carbon steels,[19] and the Bain OR is not observed for martensite in these alloys. The Bain OR has been observed for ordered Fe3Pt martensites and Fe3Al-C martensite in high-aluminum steel.[20] It has not been reported for ferrite-austenite microstructures
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