A Microstructural Study of Grain Boundary Engineered Alloy 800H
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TRODUCTION
GRAIN boundary engineering (GBE) is a relatively new technique for improving the material properties of metals and alloys with low stacking-fault energy. The aim of GBE processing is to manipulate the grain boundary network in order to create greater resistance to grain boundary–influenced damage mechanisms. First proposed in 1984,[1] many GBE studies have shown success in improving desired properties in facecentered-cubic materials such as nickel alloys, copper, and brass through the use of GBE. A summary of such studies is contained in an overview by Randle.[2] GBE has been employed typically to combat the effects of intergranular corrosion,[3] cracking,[4] and creep,[5] although success has also been shown in areas of ductility,[6] weldability,[7] and microstructural stability.[8] In almost all cases, the GBE process itself involves a series of thermomechanical cycles. These cycles typically consist of a deformation step (such as plate rolling), followed by an annealing step, and are normally repeated between 3 and 7 times. The effect of the processing has generally been an increase in the fraction of low-R coincidence-site-lattice (CSL) grain boundaries with respect to the fraction of randomly oriented highangle boundaries (HABs). Early work suggested that these low-R grain boundaries (sometimes termed ‘‘special boundaries,’’ although this term has become somewhat ambiguous) have superior properties compared to random HABs due to their potential for structural order in the boundary plane.[9] The term ‘‘special boundary’’ generally refers to a boundary classed according to the CSL nomenclature as DANIEL J. DRABBLE, Graduate Student, and CATHERINE M. BISHOP and MILO V. KRAL, Associate Professors, are with the Department of Mechanical Engineering, University of Canterbury, Christchurch 8041, New Zealand. Contact e-mail: milo.kral@canterbury. ac.nz Manuscript submitted October 18, 2009. Article published online November 24, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
having R < 29, where R is the reciprocal of the fraction of coincident lattice sites across a grain boundary, should the two grains be theoretically superimposed. Hence, there is some periodicity across the boundary (i.e., it is geometrically ‘‘special’’), as opposed to a random HAB. Electron backscattered diffraction (EBSD) is typically employed to identify these low-R grain boundaries, and provides a measure of the effectiveness of GBE processing by simple statistics such as the fraction of total grain boundary length identified as low-R. EBSD orientation maps are built up by calculating the crystal orientation from diffraction patterns in a defined grid of points on a polished surface. These crystal orientation data are then used to determine grain boundary misorientation for each boundary segment. This misorientation is checked against specific misorientations unique to each R value, and thus, a R value can be assigned to a grain boundary based on the misorientation between adjoining crystals. However, the CSL model used in EBSD measurements
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