1504 HUANG et al.:NANOSTRUCTURED Cu larger CBs may further divide into smaller CBs. REFERENCES DTZs may transform into dislocations cells. 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Z..Powder Metall.,1998,41,11. 7.Weertman,J.R..Mater.Sci.Engng.1993.A166,161. 8.Sanders,P.G.,Eastman,J.A.and Weertman.J.R..Acta 5.CONCLUSIONS nater..,1997,45,4019. 9.Segal,V.M.,Mater.Sci.Engng.1995,A197,157 The RCS process effectively reduced the grain size 10.Valiev,R.Z.,Islamgaliev,R.K.and Alexandarov,I.V., of a high-purity copper bar from 765 um to about Prog.Mater.Sci.,2000,45,103. 500 nm,demonstrating the RCS as a promising new 11.Iwahashi,Y..Horita,Z..Nemoto,M.and Langdon,T.G.. Acta mater.,1998.46.3317. technique for producing bulk nanostructured metal 12.Ferrase,S.Segal,V.M.,Hartwig.K.T.and Goforth,R. materials.The change of strain path during the RCS E..Metall.Mater.Trans..1997.28A.1047. process generally enhances the effectiveness of 13.Ghosh,A.K.and Huang.W.,Investigations and Appli- grain refinement. cations of Severe Plastic Deformation,in:T.C.Lowe and The development of the microstructure during the R.Z.Valiev (Eds.).NATO Science Series.Series 3.High RCS process was characterized by TEM and Technology,Vol.80.Kluwer Academic,Boston,2000, p.29. HRTEM.Dislocations cell structures,IDCs,cell- 14.Chen,W.,Ferguson,D.and Ferguson,H.,Ultrafine blocks (CBs),dense-dislocation walls (DDWs),clus- Grained Materials,in:R.S.Mishra,S.L.Semiatin,C. tered-small-cell walls (CSCWs),UDWs,DTZs, Suryanrayana.N.N.Thadhani and T.C.Lowe (Eds.). subgrains,low-angle GBs and high-angle GBs were TMS,Warrendale,PA,2000,p.235. 15.Zhu,Y.T.,Jiang,H.,Huang,J.and Lowe,T.C.,Metall. observed.The UDWs,DTZs and IDCs are new Mater.Trans.A (submitted for publication). microstructural features not observed in rolling- 16.Hansen,N.,Mater.Sci.Technol.,1990,6,1039. induced Cu or Al.The dislocation is mostly 60 type 17.Bay,B..Hansen,N..Hughes,D.A.and Kuhlmann- and it tends to pile-up along the (111}glide planes Wilsdore,D..Acta mater..1992,40,205. to form DDWs,CSCWs,CBs,etc. 18.Hughes,D.A.and Hansen,N.,Acta mater.,1997,45. 3871. Most dislocations are 60 type.Screw dislocations 19.Bay,B.,Hansen,N.and Kuhlmann-Wilsdorf,D.,Mater. and Frank dislocations are also frequently observed. Sci.Engng,1989,A113,385. DDWs contain high-density dislocations,interstitial 20.Kuhlmann-Wilsdorf,D.and Hansen,N.,Scr.Metall. loops and vacancy loops.The dislocation density is Mater.,1991,25,1557. as high as 3x1017m-2.Subgrain boundaries formed 21.Hansen,N.,Scr.Metall.Mater.,1992,27,1447 22.Liu,Q.and Hansen,N.,Scr.Metall.Mater.,1995,32, by DDWs are in non-equilibrium state. 1289. This work for the first time observed the existence 23.Ananthan,V.S.,Leffers,T.and Hansen,N.,Scr.Metall. non-equilibrium GBs.However,equilibrium GBs are Mater.,1991.25.137. also observed.Therefore,equilibrium and non-equi- 24.Hansen,N.and Huang,X.,Acta mater.,1998,46,1827 librium GBs coexist in RCS-processed Cu.Further 25.Hansen,N.and Hughes,D.A.,Phrys.Status Solidi B,1995 149.155. study is needed to find out what affects the equilib- 26.Hansen,N.and Juul Jensen,D..Philos.Trans.R.Soc. rium state of GBs. Lomd.,1999.A357.1447. The grain refinement and microstructural evol- 27.Liu,Q.,Juul Jensen,D.and Hansen,N.,Acta mater.,1998. ution during RCS is as follows:at low strains, 46.5819. grains is first divided into CBs,which contain dis- 28.Valiev,R.Z..Yu Gertsman,V.and Kaibyshev,O.A., Phys.Status Solidi A,1986,97,11. location cells.DTZs may also develop inside CBs. 29.Valiev,R.Z.,Kaibyshev,O.A.and Khnnanov.Sh.Kh.. With increasing RCS strains,CBs may further sub- Phys.Status Solidi A,1979,52,447. divide into smaller CBs and DTZs may transform 30.Jiang.H.,Zhu,Y.T.,Butt,D.P.,Alexandrov,I.V.and into dislocations cells.Subgrains will develop from Lowe,T.C.,Mater.Sci.Engng,2000,A290,128. 31.Furukawa,M..Iwahashi,Y..Horita,Z.,Nemoto,M., both CBs and dislocations cells.The latter become Tsenev.N.K..Valiev,R.Z.and Langdon,T.G.,Acta subgrains when the misorientation across their maer.,1997,45,4751. boundaries are so large that they develop their own 32.Islamgaliev,R.K.,Chmelik,F.and Kuzel,R.,Mater.Sci. unique slip systems.The misorientation across Engng,1997,A234-236.335. subgrain boundaries increases with further RCS 33.Horita,Z..Smith,D.J.,Nemoto,M.,Valiev,R.Z.and strain,and eventually becomes large enough to Langdon,T.G..J.Mater.Res..1998,13,446. 34.Horita,Z.Smith,D.J..Furukawa,M..Nemoto,M.,Val- transform the subgrain boundaries into low-angle iev,R.Z.and Langdon,T.G..J.Mater.Res..1996.11, GBs or high-angle GBs. 1880.1504 HUANG et al.: NANOSTRUCTURED Cu larger CBs may further divide into smaller CBs. DTZs may transform into dislocations cells. Subgrains will develop from both CBs and dislo￾cation cells. The misorientation across subgrain boundaries increases with further RCS strain, and eventually becomes large enough to transform the subgrain boundaries into low-angle GBs or high￾angle GBs. Note that the change of strain path gener￾ally enhances the effectiveness of grain refinement [53]. Other factors that affect the efficiency of grain refinement include crystal structure, orientation and deformation mode [53]. 5. CONCLUSIONS The RCS process effectively reduced the grain size of a high-purity copper bar from 765 µm to about 500 nm, demonstrating the RCS as a promising new technique for producing bulk nanostructured metal materials. The change of strain path during the RCS process generally enhances the effectiveness of grain refinement. The development of the microstructure during the RCS process was characterized by TEM and HRTEM. Dislocations cell structures, IDCs, cell￾blocks (CBs), dense-dislocation walls (DDWs), clus￾tered-small-cell walls (CSCWs), UDWs, DTZs, subgrains, low-angle GBs and high-angle GBs were observed. The UDWs, DTZs and IDCs are new microstructural features not observed in rolling￾induced Cu or Al. The dislocation is mostly 60° type and it tends to pile-up along the {111} glide planes to form DDWs, CSCWs, CBs, etc. Most dislocations are 60° type. Screw dislocations and Frank dislocations are also frequently observed. DDWs contain high-density dislocations, interstitial loops and vacancy loops. The dislocation density is as high as 3×1017 m2 . Subgrain boundaries formed by DDWs are in non-equilibrium state. This work for the first time observed the existence non-equilibrium GBs. However, equilibrium GBs are also observed. Therefore, equilibrium and non-equi￾librium GBs coexist in RCS-processed Cu. Further study is needed to find out what affects the equilib￾rium state of GBs. The grain refinement and microstructural evol￾ution during RCS is as follows: at low strains, grains is first divided into CBs, which contain dis￾location cells. DTZs may also develop inside CBs. With increasing RCS strains, CBs may further sub￾divide into smaller CBs and DTZs may transform into dislocations cells. Subgrains will develop from both CBs and dislocations cells. The latter become subgrains when the misorientation across their boundaries are so large that they develop their own unique slip systems. The misorientation across subgrain boundaries increases with further RCS strain, and eventually becomes large enough to transform the subgrain boundaries into low-angle GBs or high-angle GBs. REFERENCES 1. Gleiter, H., Prog. Mater. Sci., 1989, 33, 223. 2. Koch, C. C. and Cho, Y. S., Nanostruc. Mater., 1992, 1, 207. 3. Rigney, D. A., Annu. Rev. Mater. Sci., 1988, 18, 141. 4. Alexandrov, I. V., Zhu, Y. T., Lowe, T. C., Islamgaliev, R. K. and Valiev, R. Z., Metall. Mater. 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