Analysis of EBSD Grain Size Measurements Using Microstructure Simulations and a Customizable Pattern Matching Library fo
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INTRODUCTION
MANY mechanical, electrical, and magnetic properties of engineering materials are intrinsically related to the grain size in the microstructure. The Hall–Petch relationship, which empirically correlates the grain size with yield strength, is a classic example of this dependence.[1,2] Grain size plays a significant role in many key phenomena in materials processing and performance, including creep response,[3,4] flow stress during forming operations,[5] phase transformations,[6] and propagation of fatigue cracks.[7] Mechanical and functional properties are often improved by grain size refinement.[8–12] Hence, reliable and accurate methods to measure grain size are essential in engineering of new materials
Y.A. COUTINHO is with the Kazuo Inamori School of Engineering, The New York State College of Ceramics at Alfred University, 2 Pine St., Alfred, NY 14802, and also with the CAPES Foundation, Ministry of Education of Brazil, Brasilia, DF 70040-202, Brazil. S.C.K. ROONEY is with Ellwood Materials Technologies, 700 Moravia St., New Castle, PA 16101. E.J. PAYTON is with the Materials and Manufacturing Directorate, Air Force Research Laboratory, 2230 Tenth St., Wright-Patterson AFB, OH 45433. Contact e-mail: [email protected] Manuscript submitted May 10, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
technologies as well as understanding reproducibility and variability in materials processing. Grain boundaries are conventionally revealed by electrochemical, thermal, and/or chemical etching on a polished cross-section of material. The etched surfaces are then imaged using optical microscopy or scanning electron microscopy (SEM)-based techniques. In both cases, images are nowadays commonly recorded in a digital format. An SEM-based technique that is becoming increasingly common is electron backscatter diffraction (EBSD), where an electron beam is rastered across the surface of a tilted specimen and a high-speed camera captures images of the diffraction patterns that form when the backscattered electrons hit a phosphor screen. EBSD data are collected serially on a square or hexagonal grid and can be plotted as a map of crystal orientation or phase. The principal benefit of EBSD grain size measurement over imaging of etched surfaces is the potential for reduced ambiguity about what observed feature constitutes a grain boundary, as a grain boundary in EBSD data can be identified in most cases by a sudden change in crystal orientation. Since EBSD mapping and grid-based digital measurements are essentially equivalent to a systematic point count, grain areas are a natural choice of units for grain size characterization in these techniques. The grain microstructure is typically segmented in the spatially resolved EBSD orientation data using a percolation approach. The calculation is initiated by assigning a
measurement point in one corner of the scan as a member of grain #1. The crystal misorientation between the seed point and its neighbors is checked, and it is assumed that the adjacent pixel is a member of a diff
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