Crystallographically-Necessary Constraints at Triple Junctions in Grain Boundary Engineered Alloys
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Crystallographically-Necessary Constraints at Triple Junctions in Grain Boundary Engineered Alloys Valerie Randle Materials Research Centre, School of Engineering, University of Wales Swansea, Swansea SA2 8PP, UK. ABSTRACT Crystallographic characteristics of a large number of Σ3 boundaries have been investigated in a nickel superalloy. The results indicated that {111}, vicinal-to-{111} (within 10° of {111} planes) and not-{111} Σ3s were all present in the sample population. At Σ3/Σ3/Σ9 junctions a combination of one {111} and one vicinal-to-{111} Σ3 was more likely to occur than two {111} or two not-{111} Σ3s, and an explanation is proposed. An analysis of the interface planes for the not-{111} Σ3s indicated that more than half the boundaries in this category could not have {211}{211}, {774}{855}, {111}{511}, {001}{221} or {110}{411} planes.
INTRODUCTION Interfaces classified as Σ3 in coincidence site lattice notation are of technological relevance, especially to grain boundary engineering. Σ3s can be coherent twins (i.e. on {111}), or have other rational or irrational boundary planes. The properties of each group are different. Furthermore it has been found that there are groups of Σ3s which are ‘vicinal’ to {111}, i.e. a small deviation from the {111} reference structure is accommodated by steps on the boundary plane [1]. Several investigations have consistently shown that a number of Σ3 boundaries are in fact either vicinal to the coherent twin reference structure or vicinal to another reference structure such as an asymmetrical tilt configuration [e.g. 2]. The different behaviours of the various Σ3 boundary types complicate interpretation of data, particularly if only misorientation is measured. Nowadays most grain boundary misorientation measurements are acquired by orientation mapping using electron backscatter diffraction (EBSD) in an SEM [3]. The principal disadvantage is that only part of the grain boundary crystallography, the misorientation and not the interface plane, is measured automatically. Moreover the standard EBSD data processing algorithms classify all Σ3 boundaries together. Several workers are now directing research initiatives towards extracting more in-depth information from orientation mapping investigations of the grain boundary network. A recent significant advance in measurement technology allows the misorientation and grain boundary plane of all boundary segments to be assessed and output statistically [4]. Data acquired so far has suggested that boundaries should be considered special if they are terminated by low-index planes. Another strand, which is the topic of this paper, concentrates on measurement of local grain boundary plane crystallography. Recently the method for determination of boundary planes has been refined by the use of a single-surface trace analysis procedure, which has the advantage of greatly reducing the large experimental effort and eliminating the serial sectioning errors involved in boundary plane determination [5,6]. This method has been used to identi
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