Analysis of Misorientation Relationships Between Austenite Parents and Twins
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ON rapid cooling from the austenite phase field, austenite grains in steels with a carbon content of 0.6 pct or below decompose into martensite with 24 crystallographic variants of a lath morphology.[1,2] At higher concentrations of carbon, a mixture of lath-like and plate-like martensite morphologies are observed.[3–5] The martensitic transformation is athermal and oftentimes goes to full completion, where little to no parent austenite remains, before the material reaches room temperature. Microstructure characterization must therefore be performed solely on the transformed product, and the microstructure that existed in the A.F. BRUST is with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. S.R. NIEZGODA is with the Department of Materials Science and Engineering, The Ohio State University, Department of Mechanical and Aerospace Engineering, The Ohio State University, Columbus, OH 43210. Contact e-mail: [email protected] V.A. YARDLEY is with the Eutextikon Computational Materials Consulting LLC, 33 Eastgate St., Stafford, ST16 2LZ, UK. E.J. PAYTON is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, Dayton, OH 45433. Manuscript submitted April 06, 2018. Article published online November 21, 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A
austenite phase field must be inferred from observations of the martensitic microstructure. Parent austenite grain structure plays a key role in the performance and properties of the transformed microstructure, such as the ductile to brittle fracture occurrence based on increasing prior austenite grain (PAG) diameter[6,7] and the classification of creep and cavitation sites.[8,9] A transformed austenite grain consists of blocks of laths of two paired crystal variant orientations grouped into packets with a shared habit plane. Both packet and block boundaries are important hindrances to plastic deformation and crack propagation in steels.[10–12] Therefore, the size and morphology of the prior austenite grains play a significant role in the mechanical properties of the transformed martensite. Furthermore, the prior austenite structure also contributes to the performance of the material through mechanisms such as impurity segregation at prior austenite grain boundaries, leading to temper embrittlement.[13,14] Thus, the reconstruction of the austenite microstructure from the observable martensite is not only highly desired but also necessary for optimizing material processing and performance.
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In iron alloys, it is often assumed that the exhibited orientation relationship is close to one of the two ‘‘named’’ orientation relationships. When approximately 12 variants are observed, the Nishiyama–Wasserman (NW) orientation relationship is referenced[15,16]: f111gc ==ð011Þa0 ;
c ==a0 :
½1
When 24 variants are observed, the Kurdjumov–Sachs (KS) orientation relationship is often cited[17]: f111gc ==ð011Þa0 ;
c ==a0 :
½2
In the actual orientation relationships, however, the pa
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