Texture and Microstructure of Cold Rolled and Recrystallized Pure Niobium

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1 Materials Science Forum Vols (2007) pp Online available since 2007/Mar/15 at (2007) Trans Tech Publications, Switzerland doi: / Texture and Microstructure of Cold Rolled and Recrystallized Pure Niobium Abreu, H. F. G. 1,a, Tavares, S.S.M 2,b, Carvalho, S.S. 1,c, Eduardo, T.H.T. 1,d, Bruno, A.D.S. 1,e, Prado da Silva,M.H. 3,f 1 Universidade Federal do Ceará - Campus do Pici, s/n - Bloco 714 LACAM - CEP Fortaleza, CE, Brasil; 2 Universidade Federal Fluminense, TEM/PGMEC, Rio de Janeiro, RJ, Brasil. 3 Centro Brasileiro de Pesquisas Físicas, Praia Vermelha, Rio de Janeiro, RJ- Brasil. a hamilton@ ufc.br b ssmtavares@terra.com.br c sheylavasconcellos@yahoo.com.br d thiagohanderson@gmail.com e antonia_daniele@yahoo.com.br f mhprado@cbpf.br Keywords: niobium, texture, recrystallization Abstract Crystallographic macrotexture of pure niobium cold rolled to 30, 60, 80 and 90% reduction was analyzed by X-ray diffraction and compared with low carbon steel texture. Annealed samples from 800 o C, to 1200 o C were investigated by X-ray diffraction and electron back scattering diffraction (EBSD). The texture of cold rolled polycrystalline niobium is characterized by a component {001} < 110 > that increases in intensity with the cold work percentage. After annealing, the component {001} < 110 > spreads out about 20 o. Introduction Niobium (Nb) has a bcc crystal structure, a melting point of 2,468 o C and the lowest density and the best workability among refractory metals [1, 2]. The main application of niobium is as alloying element to steel. However, another important application is in the superconducting cavities. In this case, high purity niobium sheets are cold rolled to thin sheets, annealed and deep drawn. The knowledge of crystallographic texture for deep drawn operations is very important. The major world supplier of ferro-niobium and niobium oxide is Companhia Brasileira de Metalurgia e Mineração (CBMM) from Brazil and most of the raw material used in the practical superconducting alloys comes from this source [3]. Niobium alloy mill products can be fabricated into various complex shapes by almost all of the common metal forming processes, such as closed die forging, spinning, hydroforming, welding, etc. The relatively low density of niobium alloys, combined with their ease of fabrication, frequently favors niobium alloys as compared to other refractory metals such as molybdenum, tantalum or tungsten [3]. Pure unalloyed niobium is extremely ductile, even in the cast condition. Ingots can be cold worked extensively (in excess of 95%) without annealing. However, improved grain size uniformity can be achieved with periodic recrystallization annealing after multiple cold working steps. Annealing temperature and the amount of cold working also have a combined effect on the resulting grain structure. Pure niobium has the recrystallization temperature at approximately 700 o C. Commercially pure niobium recrystallizes around 900 o C [3]. Sadin et. al. studied preferred orientation in rolled, coarse-grained niobium and reported the strong heterogeneity and dependence on both deformation and annealing processes [4]. This work analyses texture evolution with cold rolling, texture changes with annealing and microstructure after annealing. Materials and Methods Pure hot rolled niobium sheets were cold rolled to 30, 60, 80 and 90% from the initial thickness in successive steps in a laboratory rolling mill. The annealing process was performed in a 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, (ID: /09/09,15:51:22)

2 Materials Science Forum Vols high vacuum furnace from 800 to 1200 o C for one hour. A Philips XPRO X-ray diffractometer with Cu-Kα radiation was used to measure three incomplete pole figures (110), (200) and (211) with maximum tilt of 80 o on the central layer of the sheet. Orientation distribution functions (ODF) were calculated for these three pole figures using a series expansion method up to L=22 with the software POPLA. An Oxford Crystal 300 EBSD system attached to a Philips XL-30 scanning electron microscope was used for microtexture and recrystallization studies. Vickers microhardness tests were carried out in a Shimadzu hardness tester and the parameters in all tests were: 1 kg load and 15 s penetration time. Results and Discussion Fig. 1 presents Bunge ODF sections for ϕ 2 =45 o for samples in the as received condition and cold rolled 30%, 60%, 80% and 90%. The deformation texture increases ODF intensities with the increasing of plastic deformation. The 30% cold rolled sample is characterized by two fibres (111) -γ-fibre- and (001) -α-fibre. Increasing deformation to 60% in thickness the intensities (111)[121] and (001)[110] become more intense. Continuing the deformation process to 80% in thickness, the (001)[110] component becomes more intense and (111)[121] moves to (111)[011]. The 90% deformed sample presents a very intense (001)[110] texture component and in the {111} plane the (111)[011] moves to a position between (111)[0 11] and (111)[123]. Comparing this cold rolled niobium texture with cold rolled low carbon steel, is possible to note some differences. According to Hölsccher et. al. [5], in cold rolled low carbon steel, the α-fibre increases continuously with deformation. He also stated that the γ-fibre {111}<112> is dominant up to rolling degree of 70% and {111}<110> for higher rolling degrees. In the deformation range between 70 and 80% strong shear band formation takes place in the {111}<112> grains which is probably responsible for the weak increase of texture. Cold rolled niobium did not present an increase of the γ-fibre. The (001)[110] component is dominant component, which increases with deformation. The annealing of cold rolled niobium changes texture. Fig. 2 shows ϕ 2 =45 o Bunge ODF sections for samples cold rolled 80% in thickness and annealed for one hour at 800 o C, 1000 o C and 1200 o C. Comparing this set of ODF sections with ODF presented in fig. 1 is possible to verify the decrease of intensity in component (001)[110] and also in the γ-fibre. It is also noticeable in fig. 2 a and c that the component (001)[110] spread out about 20 o. In low carbon steels submitted to low degrees of rolling reduction the recrystallization destroys the whole texture. For high degrees of deformation annealing leads to grow orientations like {011}<100>. In some other cases, annealing can result in the increase of γ-fibre intensity [5].

3 3438 THERMEC 2006 {001} < 110 > (a) (b) (c) (d) (e) Figure 1- Bunge ODF sections for samples as received (a) and cold rolled 30% (b), 60% (c), 80% (d) and 90% (e) in thickness. {001} < 110 > a b c Figure 2- Bunge ϕ 2 =45 o ODF sections for samples cold rolled 80% and heat treated at 800 o C (a), 1000 o C (b) and 1200 o C (c). EBSD can be regarded as a reference method for measuring grain size. It not only enables each grain to be located, but it also enables this to be accomplished according to the level of the misorientation angle between two grains. Fig. 3 a, b, c and d are EBSD images showing grain maps for samples cold rolled 80% and heat treated at 800 o C (a), 1000 o C (b) 1100 o C (c) and 1200 o C (d). According to Fig.3a, at 800 o C grain sizes are small and present relative homogeneity, suggesting full recrystallization. Annealing between 1000 o C and 1200 o C, Fig. 3b, c and d, show a non homogeneous grain distribution, suggesting a process of grain growth. Improved grain size uniformity however, could not be achieved. According to Wajcik et. al. this could be achieved with periodic recrystallization annealing after multiple cold working steps. Annealing temperature and the amount of cold working also have a combined effect on the resultant grain structure and mechanical properties in pure niobium [3]. Fig. 4 presents hardness for samples cold rolled and annealed at temperatures from 800 and 900 o C. The amount of cold work affects hardness not only in the cold rolled state but also after recrystallization. The values of hardness for samples annealed at 800 o C and 900 o C are indicating a recrystallization process..

4 Materials Science Forum Vols a b c d Figure 3- EBSD image of grain distribution in samples annealed one hour at 800 o C (a), 1000 o C (b), 1100 o C and 1200 o C (d) CR A800 A900 Hardness (HV) % of cold rolling Figure 4- Hardness for samples cold rolled and annealed.

5 3440 THERMEC 2006 (111)[110] (110)[001] Figure 5- Aspect of partial recrystallization in a sample cold rolled 30% and annealed at 800 o C. Fig. 5 presents an EBSD image of a sample cold rolled to 30% and annealed for one hour at 800 o C. The large grains in the top and bottom of the figure have orientation (111)[110] characteristic of bcc deformed structure. The central strip shows a large amount of new grains being formed. The orientation of these new grains is characterized by a sharp <100> parallel to rolling direction and the plan of rolling parallel to {110} with a spread of 15 o. b c a Figure 6- Orientation mapping (a) and inverse pole figures for a 90% cold rolled sample after annealing at 800 o C for 1 hour, sample normal (b) and rolling direction (c). Sandin et. al. studied the recrystallization behavior of cold rolled niobium. They also found a cube and close to cube texture along grain boundaries in sample cold rolled and partially recrystallized. According to these authors, these grains exerted a minor influence in the final recrystallization texture of both grains [6, 7]. Fig. 6 shows the orientation image mapping of a 90% cold rolled sample after annealing at 800 o C for 1 hour and inverse pole figures for sample normal and rolling direction. The texture is {001}<101> with a deviation of 10 o and confirms results obtained by X-ray diffraction.

6 Materials Science Forum Vols Conclusion Preferred orientations are developed in cold-reduced niobium sheet during the rolling process. The cold rolled preferred orientation is predominantly {100}<110> that increases with the amount of deformation. The {111}<uvw> (γ-fibre) typical of steels is weaker and do not increase with deformation. A recrystallized microstructure was obtained in samples cold rolled 80% and annealed at 800 o C. Annealing temperatures higher than 1000 o C showed grain growth. The <100> orientation was observed in samples partially recrystallized but this family of components does not appear after recrystallization. Both x-ray diffraction and EBSD indicate that after recrystallization preferred orientation is {100}<110>. Summary It was studied texture of pure niobium cold rolled and annealed after cold rolling. The study used x-ray diffraction and EBSD. Some important aspects of recrystallization were also discussed. Acknowledgements Authors are grateful to CBMM for supplying the niobium, and CNPq and ANP for scholarships. References 1- Laverick, C., Niobium Demand and Superconductor Applications: an Overview. Journal of the Less-Common Metals, 139 (1988) Hgrmann M., Production of High Thermal Conductivity Niobium on a Technical Scale for High Frequency Superconductivity, Journal of the Less-Common Metals, 139 (1988) Wojcik, C.C., Wah, C., Thermomechanical Processing and Properties of Niobium Alloys, Proceedings of the International Symposium Niobium 2001 Orlando, Florida, USA. December 2-5, Sandim H.R.Z., Raabe D. Microtexture investigation of oriented gradients and grain subdivision in rolled coarse-grained niobium, Raabe, edoc Server, Max-Planck-Society. 5- Hölscher M., Raabe D. and Lücke K., Rolling and recrystallization textures of bcc steels. Steel research 62, (1991) No Raabe D., Zhao Z., Park S.-J and Roters F., Theory of orientation gradients in plasticallynstrained crystals, Acta Materialia 50 (2002) Sandim H.R.Z., Lins J.F.C., Pinto A.L., Padilha A.F., Recrystallization behavior of a cold rolled niobium bicrystal, Materials Science and Engineering, A354, 2003,

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