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1 Journal of Korean Powder Metallurgy Institute DOI: /KPMI š p x œ w Al jy p sƒ xá a Á Á½x * swœ w œw, a x HYSCO» Microstructure and Mechanical Behavior of Ultrafine Grained Bulk Al Processed by High Pressure Torsion of the Al Powders Soo-Hyun Joo, Seung Chae Yoon a, Chong Soo Lee, and Hyong Seop Kim* Department of Materials Science and Engineering, POSTECH (Pohang University of Science and Technology), Pohang, , Korea a Automotive Steel Research & Development Team, Hyundai HYSCO, Dangjin-gun, , Korea (Received January 18, 2010, Revised February 3, 2010, 2009; Accepted February 17, 2010) Abstract Bulk nanostructured metallic materials are generally synthesized by bottom-up processing which starts from powders for assembling bulk materials. In this study, the bottom-up powder metallurgy and High Pressure Torsion (HPT) approaches were combined to achieve both full density and grain refinement at the same time. After the HPT process at 473K, the disk samples reached a steady state condition when the microstructure and properties no longer evolve, and equilibrium boundaries with high angle grain boundaries (HAGBs) were dominant. The well dispersed alumina particles played important role of obstacles to dislocation glide and to grain growth, and thus, reduced the grain size at elevated temperature. The small grain size with HAGBs resulted in high strength and good ductility. Keywords : Severe plastic deformation, High pressure torsion, High angle grain boundaries, Tensile behavior 1. ù l j» x w p ùkü ƒ š, ù w/» ƒÿ š. w w q š, ù w t š [1-4]. ù» w w j w Bottom-up œ w š [5-7].» w y, ù š», w», x v w. š šxy x w,, ƒœ, / œ š w w t [8-10]. ù k w j ƒ w, šxy œ y e y ƒœ œ w ƒ. (Severe Plastic Deformation: SPD) œ mw šxyƒ ww x sƒ ƒ w [7, 9, 10]. œ *Corresponding Author : [Tel : ; hskim@postech.ac.kr] 52

2 š p x œ w Al jy p sƒ 53 wù š p (High Pressure Torsion: HPT) x œ x mw š ƒ ƒ ³ w ù, ƒ y š [11]. š (99.99%) Al HPT œ mw j»ƒ ~500 nm, ƒ š [11].» j w x w j» w ƒ, w HPT œ w. œ w, w œ z». w, Al w HPT œ z j š. Al 473K HPT œ mw šxy, ù j wš,» š wš w. 2. x 473K w w Al(CP, commercially pure) w HPT œ ww. 1 HPT œ. w w 0.25 mm ¾ x y š, w w e w 6GPa ƒw. 10 z, w ƒ 1rpm 6GPa k w z wš, 10 z óü z x w. HPTœ z j j» xw» w j Sand paper 0.25 µm w t ñ w, Ÿ wx mw ü w»œ d w. f 300 g 10, j ƒ Future-Tech FM-700 l l» w d w. p sƒ w Dog-bone r HPT j 2.5 mm ù Wire cutting w w š, r Gauge length 1.25 mm š s 1 mm. ƒ r Ì HPT j Ì. 1 x» x s š, x w r(gauge length=1.25 mm) yw x ARAMIS 5M mw d. ARAMIS 5M x r 3 t x l mw d w Vision strain gauge system. j» s(misorientation distribution) p EBSD(Electron Backscatter Diffraction) n x (Transmission Electron Microscope) w w. x r»» mw HPT j Ì 150 µm» ƒœw z, ƒœ x y w» w 0.25 µm w mw. EBSD r» HPT j, Fig. 1. Schematic illustration of HPT facility and operation with a pressure.

3 54 xá Á Á½x š 2.5mm, 5mm 3» re z, w mw xd w. EBSD d POSTECH NCNT(National Center for Nano Technology) 3D Total Analysis(Dual FIB: Helios Nanolab) e Hikari EBSD detector w. HPT œ Al j j»ƒ w d, m w ~80 nm w yw 20 nm w µm 2 e ƒ d [12]. EBSD» w Confidence index(ci) 0.1 Clean-up š, EBSD ƒ w w ƒ 2 w w o [13]. n x r EBSD r w, j 2.5 mm 3 mm x r. 3mm x r Focused Ion Beam(FIB)» w r z, NCNT JEOL JEM- 2100F 200 KeV TEM d w. 3. HPT œ r x w : Shear strain γ=2πrn/t,» N z, t mx r š r z (HPT j ) w. HPT œ z 10 w Shear strain 393 SPD œ m 5~10 w [14-15]. 2 HPT œ z j Ÿwx ü»œ k. HPT œ w 6 GPa Al w {». f HPT j j l 3 w. f j 1mm ¾ ƒƒ 70 Hv 85 Hv ƒw w. z Fig. 2. Optical microscope image of the HPT processed bulk Al disk. Fig. 3. Vickers microhardness results of bulk Al disk from the disk center to the edge with respect to the distance. 1mm 5mm p w y w. f ƒ j w w Harai et al.[16] w.» ( x ) j (Center) yƒ, x j ƒ (Edge) û ƒ. w, HPT œ 10 z mw j x ƒw, ³ w Steady state k w. w, j j ü ³ w w, j. HPT jƒ Steady state k ( ³ k) yw y w» w EBSD d w. j Journal of Korean Powder Metallurgy Institute

4 š p x œ w Al jy p sƒ 55 Fig. 4. Orientation images and image quality maps for the Al powder HPT sample. (a) Center, (b) Middle and (c) Edge Fig. 5. Normalized grain size distribution. (Center), š 2.5 mm(middle), 5 mm (Edge) EBSD d w 4 ùkü. d 3 ³ w š, s³ j» l 290nm, 310nm š 295 nm d. 5 ƒ j»(d) s³ j»(<d>) ù ³y s (d/ <d>) ùkü, s³ j» 2 j ³ w. w w s (Misorientation distribution) ùkü 6, s³ ƒ l 21.7 c, 20.7 o š o 18.9 ƒ., Bottom-up Al w 473K HPT œ j w, ü HAGBs(High Angle Grain Boundaries: misorientation angle>15 )ƒ o. 7 HPT j 2.5 mm n x bright field image SAED ql. TEM s³ j» ~300 nm, EBSD e w. TEM 473K HPT œ z j sx k (Equilibrium boundary). SAED ql z ƒ ƒ w r x ùkù ƒ sx k(equilibrium boundary) w. 8 ARAMIS 5M x d Stress-Strain f q r x s l ùkü. HPT j j Stress- Strain f 255 MPa q 25%, ARMIS l ƒ r Necking x.

5 56 xá Á Á½x Fig. 6. Misorientation angle distribution of the Al powder HPT sample in number fraction. (a) Center, (b) Middle and (c) Edge Fig. 7. TEM image with SAED patterns. 4. š w w Al w 473K HPT œ, j j w. HPT j l d f j 3 (Center, Middle, Edge) EBSD w, HPT 10 z œ z HPT jƒ ³ w Steady state kƒ y w. EBSD d, j» ³ w. w Al w Bottom-up j w 300 nm, SPD œ j» w : 473K 12 passes ECAE œ z 0.9 µm Fig. 8. Tensile test results. (a) Stress-Strain curve (b) Initial specimen image (c) Major strain overlay of Al powder and (d) Strain contour and distribution of Al powder Journal of Korean Powder Metallurgy Institute

6 š p x œ w Al jy p sƒ 57 [17], 8 passes ECAE œ z 473 K 1 anealling 0.47 µm[18]. Al v» SPD œ j r w. x ùkù š (473K) w Bottomup j ƒ ƒ w, Al j w œ ùkù w [11]. w œ ƒ j Al t w ù d œ xk w, w y w š [11]. š (473K) ù y w dw. w Al(CP, commercially pure) 548K 573K (Abnormal grain growth) ù š š [18]. j» ³y s (d/<d>) mw HPT œ 473K w Al(CP, commercially pure) ù y w. w w, EBSD TEM d š ƒ (HAGBs) sx (Equilibrium) k., w SPD œ ü (Internal stress) j w sx(non-equilibrium) kƒ, v z sx(equilibrium) k š ƒ w [19]. w œ z k, š ƒ ƒ sx k ƒ 473K HPT œ z. w EBSD d s TEM d z ƒ ƒ w r x ù SAED ql mw. SPD œ z w w, ƒ f [20-21]. š Al HPT œ mw j j r ƒ, HPT œ mw š ƒ ƒ w» ü. 5. š (473K) š p œ mw Al šxy y. 10z z, HPT j ³ w Steady state k w. j j ~300 nm y, Al t ùƒ œ xk ww»» j w œ w š. y š ƒ sx(equilibrium) ƒ, œ z sx(non-equilibrium) ƒ. w & š ƒ p w, HPT j w p. š x [1] H. S. Kim and Y. Estrin: Appl. Physics Lett., 79 (2001) [2] R. Z. Valiev and I. V. Alexandrov: Ann. Chim. Sci. Mat., 27 (2002) 3. [3] H. Gleiter: Nanostruct. Mater., 6 (1995) 3. [4] R. Z. Valiev, R. K. Islamgaliev and I. V. Alexandrov: Prog. Mater. Sci., 45 (2000) 103. [5] S. C. Yoon, S. J. Hong, M. H. Seo, Y. G. Jeong and H. S. Kim: J. Kor. Powder Metall. Inst., 11 (2004) 233. [6] H. S. Kim and D. N. Lee: Mater. Trans., 45 (2004) [7] J. Robertson, J. T. Im, I. Karaman, K. T. Hartwig and I. E. Anderson: J. Non-Cryst. Solids., 317 (2003) 114. [8] H. S. Kim: Mater. Sci. Eng., A251 (1999) 100. [9] S. C. Yoon and H. S. Kim: Mater. Sci. Forum., (2006) 221. [10] Y. G. Jeong, M. H. Seo, S. C. Yoon, S. I. Hong and H. S. Kim: J. Metastable Nanocryst. Mater., (2005) 383. [11] T. Tokunaga, K. Kaneko and Z. Horita: Mater. Sci. Eng., A490 (2008) 300. [12] A. A. Gazder, W. Q. Cao, C. H. J Davies and E. V. Pereloma: Mater. Sci. Eng., A497 (2008) 341. [13] F. J. Humphreys: Scripta. Mater., 51 (2004) 771. [14] H. S. Kim: Mater. Sci. Eng., A315 (2001) 122. [15] B. S. Moon, H. S. Kim and S. I. Hong: Scripta. Mater.,

7 58 xá Á Á½x 46 (2002) 131. [16] Y. Harai, Y. Ito and Z. Horita: Scripta. Mater., 58 (2008) 469. [17] Y. Y. Wang, P. L. Sun, P. W. Kao and C. P. Chang: Scripta. Mater., 50 (2004) 613. [18] C. Y. Yu, P. L. Sun, P. W. Kao and C. P. Chang: Mater. Sci. Eng., A366 (2004) 310. [19] R. Z. Valiev, N. A. Krasilnikov and N. K. Tsenev: Mater. Sci. Eng., A137 (1991) 35. [20] H. S. Kim and Y. Estrin: Appl. Phys. Lett., 79 (2001) [21] Y. H Zhao, T. Topping, J. F. Bingert, J. J. Thornton, A. M. Dangelewicz, Y. Li, W. Liu, Y. T. Zhu, Y. Z. Zhou and E. L. Lavernia: Adv. Mater., 20 (2008) Journal of Korean Powder Metallurgy Institute