Orientation Distribution of Σ3 Grain Boundary Planes in Ni Before and After Grain Boundary Engineering

The distribution of grain boundary plane orientations in polycrystalline Ni has been measured before and after grain boundary engineering. The grain boundary engineered microstructure has a relatively higher concentration of Σ3 grain boundaries and, when compared to the initial structure, more of these boundaries have orientations that are inclined by more than 10° from the (111) orientation of the ideal coherent twin. Although the conventionally measured grain size is not affected by the grain boundary engineering process, the average size of the regions containing only Σ3 n grain boundaries increases by nearly a factor of two. The observations indicate that the increase in the relative population of Σ3 grain boundaries results both from the preferential elimination of random grain boundaries and the generation of new Σ3 grain boundaries which do not have (111) grain boundary plane orientations. Introduction Iterative thermomechanical processing is a well known method of manipulating the grain boundary character distribution (GBCD) of face centered cubic metals and alloys. When the repeated cycles consist of relatively small levels of deformation and relatively low temperature annealing, the relative areas of grain boundaries with a 60° misorientation around the [111] axis (referred to here as Σ3 grain boundaries, according to the coincident site lattice notation) increase while maintaining an approximately constant grain size and a random texture. When this processing improves the bulk properties of the material, it is referred to as 'grain boundary engineering' (GBE) [1]. For example, GBE has been used to produce Ni alloys with reduced rates of intergranular stress corrosion cracking [2]. Improvements in properties have been associated with increases in the populations of grain boundaries with the Σ3 and Σ3 n (where n is 2 or 3) misorientations and, therefore, the evolution of the misorientation distribution as a function of processing has been well characterized [3]. However, the orientations of the Σ3 boundaries introduced by GBE have not been extensively studied. A recent investigation of grain boundary engineered brass indicated that the new Σ3 boundaries are mostly coherent twins, where a coherent twin is defined as a pure twist type Σ3 interface comprised to two parallel (111) planes on either side of the grain boundary [4]. In this paper, we will use the term incoherent Σ3 to indicate any Σ3 grain boundary whose plane is inclined by more than 10° from the ideal coherent twin orientation. The purpose of this paper is to determine the characteristics (crystallographic orientation) of the Σ3 grain boundaries introduced by the grain boundary engineering process in Ni, and how they affect the structure of the grain boundary network. The results indicate that while the misorientation distributions created by grain boundary engineering are similar in brass and Ni, the orientations of the grain boundary planes, and their effect on the structure of the polycrystalline network are very different. Materials Science Forum Vols. 558-559 (2007) pp. 641-647 online at http://www.scientific.net © (2007) Trans Tech Publications, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 128.2.112.172-25/05/07,18:45:24) Experimental The polycrystalline Ni used in this study was obtained from Integran Technologies, Inc. The samples were provided in a reference state (before GBE) and in a final state following the GBE process. The two samples will be referred to as reference and GBE, respectively. The samples were prepared by first grinding the surface with SiC and diamond abrasives, then polishing using a vibratory chemomechanical process with a 0.05 μm SiO2 slurry, and finally electropolishing in a chilled 9:1 methanol:perchloric acid solution. Crystal orientation maps on planar sections were obtained using an electron backscatter diffraction (EBSD) mapping system integrated with a scanning electron microscope. Orientation maps were recorded at a 60° tilt with a 20 kV beam. The step sizes for the orientation mapping were between 2 μm and 4 μm, and were recorded over individual areas of 1 to 4 mm 2 . In total, 64.0 mm 2 of the reference sample were mapped (an area containing approximately 20,000 grains) and 67.3 mm 2 of the GBE sample were mapped (an area containing approximately 25,000 grains). The orientation data were then processed to remove spurious observations using a ‘grain dilation cleanup’ in the OIM software. A single orientation was then assigned to each grain by averaging all of the orientations belonging to a single grain. The OIM analysis software was then used to extract 86,335 boundary line traces from the reference sample and 115,231 traces from the GBE sample. The trace data are summarized in Table 1. The grain boundary plane distribution was determined from these traces using a procedure described previously [5,6]. Table 1. Numbers and lengths of grain boundaries in reference and GBE Ni N % L, μm L, % , μm L/A, μm -1 reference total 100 21.7x10 5 100 25.2 3.40x10 -2 random 73 12.5x10 5 58 20.0 1.95x10 -2 Σ3 27 9.23x10 5 42 38.9 1.44x10 -2 coherent Σ3 15 6.18x10 5 28 48.3 0.97x10 -2 incoherent Σ3 12 3.04x10 5 14 27.8 0.47x10 -2 GBE total 100 23.8x10 5 100 20.6 3.53x10 -2 random 47 8.79x10 5 37 16.3 1.31x10 -2 Σ3 53 15.0x10 5 63 24.5 2.22x10 -2 coherent Σ3 20 8.20x10 5 34 35.0 1.21x10 -2 incoherent Σ3 33 6.77x10 5 28 18.0 1.01x10 -2 N = Number, L = Length, A = Area Results The grain boundary segments in each sample are classified in the following way. If the misorientation is within Brandon’s criterion (8.7 °) of the ideal Σ3 misorientation, and the surface trace of the boundary is within 10° of orientation of the coherent twin, it is classified as a coherent twin. The tolerance of 10° was selected because this is commensurate with the resolution with which the five parameter grain boundary distribution data is discretized. If the misorientation is within Brandon’s criterion of the ideal Σ3 misorientation, but the surface trace of the boundary is more than 10° from the orientation of the coherent twin, then it is classified as an incoherent Σ3. It should be noted that some of the incoherent Σ3s have traces that are coincidentally within the 10° tolerance and will be incorrectly classified as coherent. This leads to a small overestimation of the coherent twin population and an underestimation of the incoherent Σ3 population. Recognizing the small error, we will henceforth consider all those boundaries with traces less than 10° from the ideal trace to be coherent twins. If the boundary does not meet the Brandon criterion, it is considered a random boundary. We recognize that some of the boundaries that are placed in the random category could also be classified according to their special characteristics (for example, the Σ9 and Σ27 grain boundaries), but for the purposes of this paper we differentiate only the Σ3 boundaries. Recrystallization and Grain Growth III 642