Characterization of Martensite Orientation Relationships in Steels and Ferrous Alloys from EBSD Data Using Bayesian Infe
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NTRODUCTION
IN steels with a carbon content of 0.6 pct or below, the austenite phase (c) transforms athermally to the metastable martensite phase (a0 ) when the cooling rate from the austenite phase field is sufficiently rapid. For many steel compositions, the transformation goes to completion such that only the martensite phase is observable at room temperature. With little to no remaining austenite, the morphology and orientation of the parent austenite grains can only be inferred from observations of martensite. The transformed microstructure exhibits a hierarchical arrangement of up to 24 crystallographic variants of the original austenite grain orientations.[1,2] Blocks are composed of pairings of the variant orientations that
A.F. BRUST is with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. E.J. PAYTON is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, Dayton, OH 45433. V. SINHA is with the Air Force Research Laboratory, Materials and Manufacturing Directorate and also with UES, INC., Dayton, OH 45433. V.A. YARDLEY is with Impression Technologies, Ltd., Coventry CV5 9PT, UK. S.R. NIEZGODA is with the Department of Materials Science and Engineering, The Ohio State University and also with the Department of Mechanical and Aerospace Engineering, The Ohio State University, Columbus, OH 43210. Contact e-mail: [email protected] Manuscript submitted April 14, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
nominally share the same austenite c-axis, and are grouped together into packets that nominally share a habit plane.[1,3] Crystallography of the face-centered cubic transformations to body-centered cubic or body-centered tetragonal crystal structure dictates that there are four possible habit planes. While 24 variants are possible for each austenite orientation, not all must be exhibited within a grain and observation of the microstructure in cross-section results in the possibility that only a subset of the variants existing for a given grain is observed. A single prior austenite grain (PAG) grain can form up to four specific packets, each with a specific habit plane. Up to three blocks can be contained within a single packet, with each block consisting of a pair of crystallographically similar variant orientations separated by a low-angle misorientation. The prior austenite grain structure is known to affect some properties, and therefore the performance in service of steel used in the quenched and tempered conditions. These include a ductile to brittle fracture transition,[4,5] the identification of creep and cavitation sites,[6,7] and temper embrittlement due to impurity segregation at PAG boundaries.[8,9] Traditionally, the assumed orientation relationship (OR) has been ascribed to one of two commonly referenced orientation relationships. The observance of 24 crystallographic variants leads to the citation of the most popular orientation relationship in steels, the Kurdjumov–Sachs (KS) orientation relationship:[10] f111g ==ð011
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