Texture Evolution during Deep drawing of Mo sheet Kwang Kyun Park 1, J.H. Cho 1, Heung Nam Han 2, Hui-Choon Lee 3 and Kyu Hwan Oh 1

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1 Key Engineering Materials Vols (003) pp c 003 Trans Tech Publications, Switzerland Texture Evolution during Deep drawing of Mo sheet Kwang Kyun Park 1, J.H. Cho 1, Heung Nam Han, Hui-Choon Lee 3 and Kyu Hwan Oh 1 1 School of Materials Science and Engineering, Seoul National University, Seoul, Korea Technical Research Laboratories, POSCO, Pohang, 7-785, Korea 3 Sapphire Technology Co.,Ltd, Seoul, , Korea Keywords: molybdenum, EBSD, X-ray, deep drawing Abstract. Texture evolution of the deep drawing of molybdenum sheet was investigated with EBSD (Electron Back Scattered Diffraction), X-ray diffraction techniques and orientation distribution function (ODF) analysis. The texture during deep drawing is hardly changed due to the strong of initial texture. Specimens symmetry of 45 from rolling direction have monoclinic symmetry during deep drawing process. EBSD can be used for the quantitative determination of macroscopic texture as like X-ray diffraction techniques if a sufficient number of grains are measured. Grain sizes show the similar values during deep drawing process but their values along 45 direction from rolling direction at flange and wall are tend to increase. It is associated with the 45 earing of deep drawing processing. The correlation function values were 0.58 for RD flange and 0.36 for RD wall and suggest that high Taylor factor regions have low pattern quality and low Taylor factor regions have high pattern quality. Introduction Owing to the excellent high temperature strength, oxidation- and corrosion-resistance, the metal molybdenum in sheet form is finding wide applications as electrodes, heating elements and ideal substrate materials in many industrial and technological fields, such as electronics, electric power, nuclear energy and even aerospace engineering. Usually for using commercial industry, the physical characteristics of molybdenum can be tailored to suit particular applications with other metals. This is done using specific machining or metalworking procedures. The sheets of molybdenum are crossrolled in order to further enhance its properties. Cross-rolling involves rolling the original sheet both parallel and perpendicular to its length, producing a specific texture defined as Rotated cube component [001]<110> [1]. And it was reported that the formability of molybdenum sheets may be improved by cross-rolling as a result of influence on the final state of textures and plastic anisotropy [,3]. Yoo et al. introduced that the tensile deformation characteristics of single-crystalline molybdenum sheet containing low level of Ca and Mg, which was prepared by secondary recrystallization, were investigated at 300K and compared with that of the crystal containing a low level of oxygen (10ppm) in view of slip geometry and fracture behavior [4]. Most of the published work concentrates on the effects of cross-rolling and deformations on the properties of molybdenum sheets although the deep drawing test is very useful for evaluating the planar anisotropy of metallic sheets. Recently EBSD has been a useful tool to link optical and scanning microscopy to X-ray diffraction for bulk texture and transmission electron microscopy [5]. Using this technique, we can get an orientation information together with position information and determine the crystallographic textures [6-7], which can give the statistical data of texture, grain boundary and so on. Therefore in this study, texture evolutions of molybdenum sheets for the deep drawing were investigated based on the calculated orientation distribution functions (ODFs) with analyzing by EBSD. Effects of deformation on texture evolution, the comparison of ODFs obtained from pole figure (PF) X-ray and EBSD which have macro- and microscopic texture, respectively, grain size and the statistical analysis of microstructures related with orientation and image quality were also discussed during deep drawing process of molybdenum sheet.

2 568 Engineering Plasticity from Macroscale to Nanoscale Experimental Procedure The chemical composition of molybdenum sheet in this study is 99.9 wt% Mo. This sheet made from Plansee company in Austria was compacted and sintered with Mo powder and hot rolled to.0mm thickness. The hot rolled Mo sheet is cold-rolled to 0.4 mm thickness. And the cold-rolled sheet was deep-drawn with mm of blank radius (r b ), 100mm of punch radius (r p ), 13 ton of blank holding force and 40 mm to 70 mm of dome height. The specimens for EBSD observation were cut out from the deep drawn material as shown in photo.1, and after a mechanical polishing, electro-polished in 30% perchloric acid at 10 C. The ODFs of each specimen were analyzed using an EBSD and X-ray. The EBSD measurement was done by INCA crystal, which is installed on JEOL 6500F FEG-SEM. In PF X-ray, the three incomplete pole figures, {110}, {00}, {11}, were measured in the range of the polar angle α from 0 to 70 with Co K α1 radiation in the Seifert D3000 with goniometer to analyze the textures of the specimens quantitatively. Both of obtained ODFs were calculated by WIMV method introduced by Matthies[8] from measured pole figures. All measurement have been performed in the center of molybdenum sheet. Rolling direction (RD) and transeverse direction (TD) are taken into account with orthorhombic symmetry of specimens. But the 45 angle part from rolling direction where is between RD and TD during deep drawing has monoclinic symmetry caused by ironing resulted from shear deformation gradient. In calculating ODFs from measured PFs the specimen is considered monoclinic symmetry and crystals is considered cubic symmetry, 0, 0, 0 ϕ range. The fluctuations of material microstructures can be described by so-called second-order characteristics such as the variance of the volume of a microsturctural component or phase. When the mean or the first moment, E, of the volume, V, of a component Ξ restricted to a spatial window W is given by ΕV ( Ξ W), then the variance, var, of the volume of Ξ W is varv ( Ξ W) = ΕV ( Ξ W) -[ ΕV ( Ξ W)] (1) where ΕV ( Ξ W) is the second moment of volume of the Ξ-phase. If f (x) and f (y) are the probabilities of random variables, x and y, the covariance of the pair ( f ( x), f (y)) is given, cov ( f ( x), ) = Ε[( f ( x) Εf ( x))( Εf ( y))] () The normalized covariance function is called the correlation function, and takes values between 1 and +1. ρ = cov ( f ( x), ) f ( x) σ σ (3) f ( x) where, σ f (x) and σ are the standard deviations of f (x) and f (y), respectively. A value of 1.0 or +1.0 indicates perfect linear prediction between x and y, whereas a value of zero indicates no linear predictive value [1]. This correlation function was used to check whether or not the <100> grains of the deep drawn molybdenum sheet exhibited a higher image quality than <111> grains.

3 Key Engineering Materials Vols a) TD RD b) TD RD Photo.1 Drawn cup by deep drawing operation and measuring specimen positions. a) punch stroke : 40mm (1:RD flange, :45 flange, 3:TD flange, 4:RD wall, 5:45 wall, 6:TD wall, 7:bottom), b) punch stroke : 70mm Earing Height (mm) Experimental Results Angle from RD (Degree) Fig.1 Experimentally measured height profiles for the drawn cup. Results and Discussion Texture : Photo.1 shows the drawn cup by deep drawing operation and measuring specimen positions. In theoretical values of r and r on the basis of the Taylor model, the desired properties of good drawability and minimum earing are associated with {111}<uvw> type textures in low carbon steels [9]; e.g. Rotated cube {001}<110> and Goss {110}<001> textures are undesirable. But Cold-rolled molybdenum sheet in this study has relatively lower γ-fiber intensity than α-fiber. So 45 earing is observed during deep drawing as shown in Photo.1. Fig.1 shows the experimentally measured height profiles for the drawn cup. Fig. shows the ϕ =45 ODF sections obtained from EBSD of deep drawing sheet which is mapped in 100 x 100µm. It is relatively mapped over larger area than fig.4 which is mapped over 100 x 100µm of area. An initial texture (as-received) of cold-rolled molybdenum sheet was typical for the common cold-rolled Mo sheets as shown in fig. (a). In general, the rolling texture of some BCC metals, such as iron and low carbon steel, consists of two prominent orientation spreads as an orientation fiber close to =55 and ϕ =45, so-called γ-fiber and the α-fiber close to =0 and ϕ =45 in Euler space. The former is a fairly complete fiber texture with the <111> fiber axis parallel to the sheet normal; the major components in this fiber texture have <110>, <11> and <13> aligned with the rolling direction. The other is a partial fiber texture with a <110> fiber axis parallel to the rolling direction and intensity maxima at {001}<110>, {11}<110> and {111}<110> [9]. The initial texture in this study has developed with Rotate cube {001}<110>, A {11}<110> and Z {111}<110> but in this state α-fiber has well developed but not γ-fiber though A {11}<110> as one of the partial γ-fiber has developed. The deformation state of flange part has experienced compressive and tensile stress during deep drawing process. In RD direction of flange, Z {111}<110> component is shifted and γ-fiber has a little developed than initial state as shown in fig. (c). Thereafter deep drawing process on RD wall,

4 570 Engineering Plasticity from Macroscale to Nanoscale a Level 1 5 Max. 9.0 Level 1 5 Max. 8.4 b c d e Level 1 5 Max. 7.7 Level 1 5 Max. 8. Level 1 5 Max. 7.1 f g h Level 1 5 Max. 6.0 Level 1 5 Max. 9.0 Level Max. 10. Fig. ϕ =45 ODF sections obtained from EBSD of (a) as-received sheet, (b) bottom part of deep drawing, (c) RD flange, (d) 45 flange, (e) TD flange, (f) RD wall, (g) 45 wall and (h) TD wall a Level 1 5 Max. 9.7 b Level Max. 10. c d e Level Max Level 1 5 Max. 9.1 Level 1 5 Max. 5.0 f g h Level 1 5 Max. 5. Level 1 5 Max. 5.3 Level Max. 4. Fig.3 ϕ =45 ODF sections obtained from PF X-ray of (a) as-received sheet, (b) bottom part of deep drawing, (c) RD flange, (d) 45 flange, (e) TD flange, (f) RD wall, (g) 45 wall and (h) TD the texture is hardly changed but intensity was changed as shown in fig. (f) although wall part has experienced compression, bending and tension in addition to flange deformation. Texture of flange and wall part at TD direction and bottom in deep drawing cup were typical with the texture of coldrolled sheets having the α-fiber and γ-fiber. All of the flange and wall part texture of RD and TD during deep drawing shows the nearly same texture component as initial state texture due to having strong texture in initial state by cold-rolling. However there are some difference of texture in both of RD and TD in deep drawn cup. It is caused that texture component during deep drawing evolves in a different way due to the deformation direction related with grain shape.

5 Key Engineering Materials Vols a b c Level 1 5 Max. 8.9 Level 1 5 Max. 5.8 Level 1 Max. 3.4 Fig.4 ϕ =45 ODF sections obtained from local area of EBSD of (a) RD flange, (b) TD flange, (c) RD wall Lücke and his collaborators have reported Equivalent Circle Diameter that the orientation density along the α fiber 60 increases fairly uniformly with strain up to 50 ~70% reduction in rolling of carbon steel but with further rolling A {11}<110> and Z {111}<110> become more prominent. The γ fiber is relatively uniform at 0 reductions up to 80% but thereafter the Z {111}<110> component strengthens 10 0 [10,11]. Thus in BCC metals, if the initial texture is strong the texture during deep Sample Fig.5 Grain size characteristics of Mo deep drawing. drawing is hardly changed although flange and wall part is experienced with the compressive, tensile stress state and plane strain state, respectively. RD and TD specimen except 45 angle from rolling direction have orthorhombic symmetry. ODFs of 45 angle from rolling direction have monoclinic symmetry and show the 10 rotation about ND axis due to the deformation characteristic of 45 angle as shown in fig. (d) and (g). Also, for obtaining representative of ODF, it was compared with the ODF measured from EBSD and X-ray as shown in fig. and fig.3. Both of ODFs are very similar to all of specimens. But local ODF data measured with an insufficient number of grains can be only obtained the partial ODF as shown in fig.4. Fig.4 shows the ϕ =45 ODF sections obtained from local area of EBSD which is mapped over 100 x 100 µm of area. To get statistically reliable data, larger area or multiple mapping is required but its area is depended on grain size of specimen. We can confirm that EBSD can be used for the quantitative determination of macroscopic texture if a sufficient number of grains is measured. Equivalent Circle Diameter (µm) As-received RD-flange 45-flange TD-flange RD-wall 45-wall TD-wall Bottom Grain size : Fig.5 shows the change of grain size during deep drawing. The equivalent circle diameter (ECD) of initial state is 38.5µm and flange and wall part of RD, TD and bottom has the similar values as 35~40µm, e.g. as deep drawing processes its grain size has hardly changed. But 45 direction from rolling at flange and wall has different values. It can be understood that 45 of flange part during deep drawing has relatively smaller r than RD and TD and brings about 4-fold earing. Therefore 45 earing of flange part has applied both relatively smaller deformation and tensile stress along the circumference direction toward RD and TD flange when flange part of RD and TD is applied both compressive stress along the circumference direction and tensile stress toward drawing direction. Statistical analysis of microstructures : The image quality maps of deep drawing molybdenum are shown in fig.6 and suggests that <100> regions are associated with high image quality (IQ). In order to quantify this observation, a correlation function was calculated with the two variables, orientation (or Taylor factor) and image quality. This approach is based on the fact that high Taylor factor regions have high stored energy and exhibit a low pattern quality, whereas low Taylor factor

6 57 Engineering Plasticity from Macroscale to Nanoscale a) b) 60 µ m Fig.6 Image pattern quality (PQ) maps of Mo deep drawing from RD flange (a) and RD wall (b) indicating white color and gray color is high PQ and low PQ, respectively. regions have low stored energy and higher pattern quality. Using the orientations, Taylor factor was calculated, and then the image quality of the orientations was combined as shown in eq.3. The resulting value was 0.58 for RD flange and 0.36 for RD wall. This suggests that high Taylor factor regions have low pattern quality and low Taylor factor regions have high pattern quality in keeping with the qualitative observation made previously. Summary 1. Texture during deep drawing of molybdenum sheet is hardly changed when the initial texture (as-received) is strong.. Specimens of RD and TD have orthorhombic symmetry, but specimens of 45 angle from rolling direction have monoclinic symmetry during deep drawing process. 3. EBSD can be used for the quantitative determination of macroscopic texture if a sufficient number of grains are measured. 4. Grain sizes show the similar values during deep drawing process but their value of 45 angle from rolling direction at flange and wall is tend to increase. It is associated with the 45 earing of deep drawing processing. 5. The correlation function values are 0.58 for RD flange and 0.36 for RD wall. The results suggest that high Taylor factor regions have low pattern quality and low Taylor factor regions have high pattern quality. References [1] Technical note, HKL Technology, Determining the processing history of Mo sheet, (001) [] Y.S.Liu and P.Van Houtte : Int. J Ref. Met. & Hard Mat. 19 (001), p [3] Tuominen SM, Davison RM : JOM-J MET 8(1) (1976) A5 [4] Myoung Ki Yoo, Yutaka Hiraoka and Ju Choi : Scripta Metall. 33(9) (1995), p [5] Friedl F, Zimmerman E, Klinkenberg C, Pircher H. Examples of microstructure analysis by orientation imaging microscopy. Steel Res 70 (199) p.54-9 [6] S.I.Wright, B.L.Adams, Metall. Trans. 3A (199) p. 759 [7] R.A.Schwarzer, Micron 3 (1997) 49 [8] S.Matthies and G.W.Vinel : Phys. Status Solid., 11 (198), p.111 [9] F.J. Humphreys and M. Hatherly : Recrystallization and related annealing phenomena. Pergamon (1995) [10] von Schlippenbach, U., Emren, F. and Lücke, K. : Acta Metall. 34 (1986), p.189 [11] Emren, F., von Schlippenbach, U. and Lücke, K. : Acta Metall. 34 (1986), p.105 [1] J.Ohser and F. Müchklich: Statistical analysis of Microstructures in Materials Science, John Wiley & Sons, Ltd., 000, Chap 5.