Abstract: Recrystallization of rolled Zr single crystals is considered in comparison with analogous recrystallization processes in rolled coarse-grained iodide Zr.
Materials Science Forum Vol. 753 (2013) pp 275-278 Online available since 2013/Mar/26 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.753.275
RECRYSTALLIZATION OF COLD-ROLLED Zr SINGLE CRYSTALS Margarita Isaenkova, Yuriy Perlovich, Nikolay Krapivka, Vladimir Fesenko, Olga Krymskaya, Alina Sudakova National Research Nuclear University “Moscow Engineering Physics Institute”, Moscow, Russia Keywords: Zr single crystal, rolling, recrystallization, texture, strain hardening.
Abstract: Recrystallization of rolled Zr single crystals is considered in comparison with analogous recrystallization processes in rolled coarse-grained iodide Zr. The X-ray method of direct pole figures was used for texture studies of rolled and recrystallized samples. A distinguishing feature of α-Zr single crystals consists in formation of several new texture maxima due to twinning. Therefore rolled α-Zr single crystals at the very early deformation stage cease to be single crystals and formation of the rolling texture develops according to principles, typical for polycrystalline samples. Rolled Zr single crystals recrystallize by removal of twins and growth of crystallites with intermediate orientations, especially those corresponding to orientations of stable texture maxima. Experimental procedure The single crystal of pure Zr was obtained by means of solid-phase over-crystallization. Samples 7x10x3 mm in size were cut out of the cylindrical single crystal by the electro-erosion method. Samples were rolled at the laboratory rolling mill between plates of stainless steel up to the deformation of εmax= 80% with ~5% reductions per pass. The initial monocrystalline plate and rolled single crystals were subjected to X-ray texture analysis by the method of direct pole figures. Then rolled single crystals were annealed in vacuum vessel at RD 580oC for 1 h and after recrystallization their texture was studied a repeatedly. All X-ray studies used the X-ray diffractometer Bruker D8 DISCOVER with LynxEye position-sensitive detector. Partial pole figures PF(0001) and {1120} were constructed with angular radii 80o. The treatment of texture data included construction of diagrams [PF(0001)recr - PF(0001)roll], where difference between pole densities for recrystallization and rolling textures are calculated at each point (ψ, φ), so that regions of maximal texture changes are seen. Subtraction of pole densities was made only for those regions of PF(0001)roll where pole densities were strong, b exceeding a threshold value of 0.5. Besides the above-described experiments with Zr single crystals, analogous treatments and Xray measurements were applied to samples of rolled coarse-grained iodide Zr in parallel with the single crystals. Results The inner part of PF {1120} up to 80o for the monocrystalline plate, cut from the initial single crystal, is shown in Fig. 1-a. On the same PF, the position of basal axis (0001) was drawn with circles at the distance of 90o from projections of prismatic axes {1120} near the 80o-circle. The center of this PF coincides with the axial direction of the initial cylindrical single crystal, which by
Fig. 1. Twinning in initial Zr single crystals. RD – rolling direction.
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RD
RD
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1 0.5
c
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Fig.2. Recrystallization in rolled Zr single crystals (A, B) and in rolled iodide Zr (C, D): A, C - ε~50%; B, D - ε~80%; a – PF(0001)roll, b - PF(0001)recr; c – [PF(0001)recr - PF(0001)roll].
subsequent rolling is parallel to the normal to rolling plane. Changes in the PF(0001) of the single crystal as a result of rolling by ~5% are shown in Fig. 1-b. PF(0001) for single crystals, rolled by ~50% and ~80%, are presented in Figs. 2-Aa and 2-Ba, respectively; PF(0001) for same samples after their recrystallization annealing at 580oC – in Figs. 2Ab and 2-Bb; difference diagrams [PF(0001)recr - PF(0001)roll] for both deformation degrees, - in Figs. 2-Ac and 2-Bc. Similarly, rolling textures of iodide Zr after the same deformations are presented in Fig. 2-Ca and 2-Da, with recrystallization textures in Fig. 2-Cb and 2-Db, and difference of pole figures [PF(0001)recr - PF(0001)roll] in Fig. 2-Cc and 2-Dc. Fig. 3 shows reorientations of prismatic axes {1120} due to recrystallization in the rolled Zr single crystal (a) and of the coarse-grained iodide Zr (b), both rolled by ~80%. Fig. 4 characterizes changes in basal axis distributions along the equatorial section of PF(0001) by recrystallization of the same samples. {1120} RD
RD
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RD
3.5
2.5
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rolling ε=80%
a
annealing 580oC -1 h
rolling ε=80%
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annealing 580o C -1 h
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Fig. 3. Texture changes by recrystallization of rolled Zr single crystals (a) and rolled iodide Zr (b).
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Discussion First of all, obtained experimental results contain much new data, concerning the role of deformation twinning in recrystallization processes in α-Zr. As a result of rolling by only ~5%, (Fig. 1-b): the basal axis of single crystal reorients by ~35o due to twinning on planes {1121}and due to twinning on planes {1012} it reorients by ~95o. This reorientation occurs in α-Zr during rolling according to predictions of works [1-2], where diagrams were constructed showing operating deformation mechanisms for grains with different positions of the basal axis relative to the rolling direction (RD), transversal direction (TD) and normal direction (ND). Further rolling of the single crystal by ~50% produced strengthening of the twinning texture maxima due to further twinning and additionally a shift due to basal slip [3] (Fig. 2-Aa). The next rolling up to 80% (Fig. 2-Ba) creates in the deformed single crystal the usual rolled α-Zr two-component texture, Recrystallization textures of rolled single crystals disclose a sharp fall of pole density within central texture maxima, due to twinning (Fig. 2-Ad, 2-Bd). Hence, twinned crystallites are characterized with low strain hardening and contain relatively small numbers of recrystallization nuclei, growing at the expense of neighbors. This principle of recrystallization in α-Zr proves to be of the general significance, since all PF(0001) after recrystallization show a decrease of the pole density within the central part where, according to texture formation models, only deformation twinning makes a contribution [1]. It is clear, that the tendency for primary recrystallization is connected with strain hardening of corresponding crystallites of the deformed matrix [4]. Therefore disappearance of some crystallites in PF of rolled samples by recrystallization indicates that these crystallites were swallowed up by neighbors having higher strain hardening and stronger tendency to recrystallization. It is essential, that in the case of rolled single crystals the fall of pole density within the central part of PF(0001) is accompanied by its increase at regions, close to location of stable maxima of the α-Zr rolling texture where, according to diagrams [1-2], boundaries between regions of different deformation mechanisms are located (Fig. 2-Ac, 2-Bc). A supposition can be made, that within fragments of single crystals, deformed initially by twinning, the formed substructure is characterized by decreased lattice distortion. This would explain the main features of difference diagrams: a fall of pole density at the center of PF(0001) and its growth near locations of stable texture maxima. Similar changes of the pole density distribution as a result of rectystallization are seen in difference diagrams for recrystallized rolled iodide Zr (Fig. 2-Cc, 2-Dc). The most widespread opinion, concerning recrystallization of α-Zr, consists in assertion that the main mode of its lattice reorientation is a 30o-rotation about basal axes [5-6]. But the real situation is more complicated; in particular during recrystallization of α-Zr, its basal axes rotate as well [7]. Indeed, the rolled Zr single crystal, deformed even up to 80% by twinning and predominant basal slip (Fig. 2-Ba) does not show a noticeable rotation of prismatic axes after 580o-annealing (Fig. 3-a, 3-b), while iodide Zr, rolled up to 80% (Fig. 2-Da), manifests a sharp reorientation of prismatic axes after recrystallization (Fig. 3-c, 3-d) by means of 30o-rotation about basal axes, coinciding with the right texture maximum in Fig. 2-Da. Obviously, rotation around basal axes only takes place in those cases when recrystallizing α-Zr grains had been deformed with the participation of prismatic slip, which, according to diagrams of [1-2], develops in a rather wide zone in the stereographic projection, adjacent to TD. The increase of the pole density seen within stable texture maxima or near their outer boundaries indicates that at these regions, substructure elements of rolled samples are characterized by increased strain hardening, resulting in great numbers of recrystallization nuclei. Namely here, grains are localized, deformed by participation of two or three different slip systems, i.e. prismatic, basal and pyramidal ones [3]. Stability of final components in the rolling texture of α-Zr is conditioned by mutually balanced operation of different slip systems. A distinguishing feature of the α-Zr single crystal, rolled in the given initial asymmetric orientation, consists in formation of several new texture maxima due to twinning by a number of systems at the first stage of deformation. These new maxima respond to the substructure, consisting of fine-dispersed twins with alternating orientations. In the following stage of rolling, slip systems activate and these neighboring twins of different families interact with each other, so that the
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corresponding increase of strain hardening within PF zones located between separate twin maxima, occurs and after recrystallization it can be revealed by an increase of pole density in difference diagrams. The most difference arises between crystallites, produced by twinning, and those, connected with simultaneous operation of basal and prismatic slip, resulting in formation of stable texture maxima. It is here, therefore, that the lattice distortion and the residual strain hardening are maximum. As a consequence, recrystallization nuclei at these boundaries are most numerous, so that in difference diagrams the pole density increases most here. Due to operation of twinning, an α-Zr single crystal in the very early deformation stage ceases to be a single crystal and formation of the rolling texture there develops according to principles typical for polycrystalline samples. Summary 1. Regions of the α-Zr matrix, deformed with predominant participation of twinning, are characterized by a decreased tendency to recrystallization. 2. Recrystallization of α-Zr, connected with rotation of its crystalline lattice by 30o about basal axes, takes place only in the case where prismatic slip participates in plastic deformation. 3. The greatest number of recrystallization nuclei forms in α-Zr grains plastically deformed by operation of different slip systems, as in crystallites corresponding to stable orientations of the rolling texture. References [1] Hobson D.O. Textures in deformed zirconium single crystals – Trans. Met. Soc. AIME, 1968, v. 242, p. 1105-1110. [2] Matcegorin I., Rusakov A., Evstyukhin A. Analysis of the texture formation mechanism in α-Zr by application of computer modeling.- In: Metallurgy and Metal Science of Pure Metals. No 14. Ed. V. Emelyanov and A. Evstyukhin. Atomizdat, Moscow, 1980, pp. 39-52 (in Russian). [3] Isaenkova M., Perlovich Yu. Kinetics and mechanisms of texture formation in α-Zr under rolling. – Physics of Metals and Metal Science, 1987, v. 64, 1, pp. 107-112 (in Russian). [4] Perlovich Yu., Bunge H.J., Isaenkova M. Inhomogeneous distribution of residual deformation effects in textured BCC metals - Textures and Microstructures, 29 (1997), 241-266. [5] F. Hessner (ed.), Recrystallization Metallic Materials [in Russian], Metallurgiya, Moscow (1982). -352 p. [6] Kocks U.F., Tome C.N., Wenk H.-R. Texture and Anisotropy. Preferred orientation in polycrystals and their effect on materials properties. - Cambridge University Press, 1998. – 676 p. [7] Isaenkova M., Perlovich Yu. Regularities of recrystallization in sheets and tubes of Zr-alloys. In: Microstructural and Crystallographic Aspects of Recrystallization. Ed.N.Hansen et al. Riso National Lab., Roskilde, Denmark, 1995, p. 371-376.
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