Grain boundary character evolution during grain growth in a Zr alloy

Grain boundary character in samples of Zr701 annealed at two different temperatures has been investigated in terms of lattice misorientation. The main difference between the two samples was the extent of grain growth post-recrystallization. The textures were typical for the material. Differences between the texture-based misorientation distribution function (T-MDF) and the microstructure-based MDF (M-MDF) revealed significant preferences for certain grain boundary types, notably those with <11-20> rotation axes. Introduction In low alloyed zirconium, the texture change along annealing mainly occurs during the grain growth stage [1-3], at which the grain boundaries (GBs) play an uppermost important role : grain growth proceeds by GB migration under a driving force which is linked with the minimization of the interface contribution to the global free enthalpy. Understanding the reasons for the texture evolution requires then a better knowledge of the GB types and properties. Most of the studies on GB structure and properties in metals concern cubic materials [4]. Very few data are available for hexagonal metals. In this work, the grain boundary populations in Zr, as characterized by the misorientation distribution functions, are compared for two distinct stages of the grain growth process in order to detect if particular boundaries may be responsible for the texture evolution or more generally may appear as "special boundaries". Experimental procedures. Sample preparation. The as-received material was a 3 mm thick sheet of annealed Zr701 (composition given in Table 1). It is an almost-single phase material (HCP), including nevertheless about 0.1 vol.% of intermetallic particles (Zr combined with Fe, Cr and Ni). The sheet was first cold rolled to 80% thickness and then samples were annealed for 15 min at 600°C and 750°C under Ar inert atmosphere. This resulted in equiaxed microstructures with an average grain size of 5.5 and 17.3 μm, respectively. The samples were prepared for EBSD measurements by mechanical polishing (in-plane or cross-sections) and final electrolytic polishing (20% perchloric acid80% acetic acid, 3 C, 17 V, 15 sec). Materials Science Forum Vols. 558-559 (2007) pp. 863-868 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: 194.57.144.26-24/05/07,10:33:04) Sn Si Fe Cr Ni O <30 <20 271 68 31 310 Table 1. Chemical composition of the studied Zr701 alloy (weight ppm, balance Zr) EBSD work. EBSD was carried out using a HKL-Technology system (Channel 5 software) coupled to a Jeol JSM 6500F FEG-SEM. Orientation maps were acquired with a measurement step size that was approximately 0.1 of the mean grain size, which provides reasonable spatial resolution of the microstructure [5]. The measurements were performed at different locations in the samples to maximize the statistical quality of the data sets. The raw maps were filtered to remove spikes, systematic mis-indexing data points [6,7] and the non-indexed pixels (only non-clustered ones). Grains were then detected as being areas of at least 4 pixels wide and with less than 5° misorientation between adjacent pixels. Boundaries between these grains are analyzed in this paper. Table 2 gives details of the two EBSD data sets. Sample annealed for 15 min at : 600 750 °C Average grain size (equiv. circle diameter) : 5.5 17.3 [μm] Step size for EBSD map acquisition : 0.5 1.5 [μm] Total area of EBSD maps : 2.57 6.23 [mm 2 ] Number of grains (> 4 pix. ; ≥ 5°) : 83358 20488 Table 2. Details of the EBSD data sets Characterization of grain boundary misorientations. Among the five macroscopic parameters characterizing a grain boundary, the three related to the misorientation can easily be obtained from EBSD data. Most of the commercial EBSD software packages permit calculation of the misorientation angle distribution, and plotting of the misorientation axes in the crystal reference frame (in the form of inverse pole figures), as well as projection in a sample frame (pole figures). The two other parameters describing the GB plane, which is a 3D piece of information, are more difficult to determine and require specific additional analysis [8,9]. In this work, the microstructure-based misorientation distribution function (M-MDF) was obtained by making a list of the misorientations corresponding to each of the boundaries, represented by quaternions chosen in the fundamental domain, and weighted by the boundary length (normalized so that the sum of the weights over the misorientation-space fundamental-domain is 1). From this list, sub-populations can be extracted, for example based on a specified misorientation axis. In this case, the axis was defined with a 10° tolerance. Since the MDF is closely linked to the ODF, much can be learned from a comparison of M-MDFs ("real" ones) with texture-based MDFs (T-MDF). The T-MDF was calculated by drawing pairs of orientations at random from the EBSD grain list, and assigning a weight, proportional to the product of the two grain volume fractions, to the resulting misorientation. T-MDFs were normalized in the same way as M-MDFs. If no local configuration, i.e. grain boundary type, is favoured (as a result of a low GB energy for example), the M-MD and the T-MD are similar. Differences between those two distributions, on the other hand, can be interpreted as the signature of special or preferred boundaries. Results. Textures. The Orientation Distribution Functions (ODFs) were calculated from the grain list {mean grain orientation gk ; weight wk = grain area} by using the series expansion method with generalized spherical harmonic functions. Euler angles {φ1, Φ, φ2 ; [°]}, as defined by Bunge [10] were used to describe crystal orientations, the crystal Cartesian frame {X,Y,Z} being chosen as X = [10-10], Y = [11-20] and Z = [0001]. The textures of both samples are shown in Fig. 1. Recrystallization and Grain Growth III 864