F.Yan et al.:J.Mater.Sci.Technol.,2011,27(8),673-679 675 Table 1 Structural parameters of three types of microstructures in the LN-DPD Cu Structure dr/nm dr/nm 0av/deg. JHAB/% p/105m-2 DS-region 91 473 3.8 0 5.6 NT-region 26 60 100 17 SB-region 46 149 7.4 7 16 average of 3.80 (0av),implying that all the newly- formed boundaries are low angle dislocation bound- 6 (a) -CRI☒ aries.This is highly different from the nanostructures ECAPIS ECAP formed by traditional severe plastic deformation at ←RT-DPDI19 large strains,where the density of high angle bound- ◆LN-DPD湖 ★RT-DPD [Present data] aries can be ~70%16.8.91. 2 These boundary parameters allow the dislocation density to be roughly estimated,based on the assump- (b) ·-ECAP1 tion that dislocations are mainly presented in the low ECAPI1 angle dislocation boundaries,whereas the dislocation ARBI20 -Compressionl22 density in the volumes between boundaries is rela- Con tively low (1014 m-2)[i0]: 200 LN-DPD Present data] p≥ 1.5SLABOLAB (c) b (1) 40 where SLAB and LAB are the surface area per unit volume and the average misorientation angle of low 20 ◆CR2 angle dislocation boundaries,that are misoriented AP -ARBI2iT <l5o(SLAB=f九AB(h+孟))，andbis Burgers ★ LNDPD (Prescm t vector of Cu(0.256 nm).By inserting the structural (d) parameters given in Table 1,the dislocation density is approximated as 5.6x1015 m-2.Although this calcu- lation underestimates the dislocation density due to neglecting the abundant dislocations that do not con- tribute to the misorientation rise,the value is higher *LN-DPD [Present data] by a factor of 5-6 compared with the counterparts 6 10 deformed via cold rolling(CR)12],equal channel an- gular pressing (ECAP)[13.14]and RT-DPD(DPD at room temperature)(15],Fig.2(a).The relative low Fig.2 Comparison of characteristic parameters of dis- dislocation density of LN-DPD Cu in literature 15 location structures in Cu processed by different may be due to the lower strain(1.8)than the present approaches:(a)the dislocation density;(b)the investigation (2.1). average boundary spacing;(c)the average bound- ary misorientation angle;and (d)the fraction of The significantly high dislocation density of LN- high angle boundaries.Data obtained by EBSD DPD Cu can be partially attributed to the high strain and CBED are marked by and **respectively rate,which facilitates the accumulation of disloca- tions,because the strain rate (is proportional to the velocity (v)and the density (pm)of mobile disloca- is the significantly low fraction of high angle bound- tions:Additionally,low temperature in- aries (fHAB)and the smaller average misorientation hibits the annihilation of dislocations by cross-slip and angle (0),Fig.2(c)and (d).This is partially as- climbl171.Consequently,LN-DPD Cu shows higher cribed to the quantitative characterization techniques dislocation density.This may explain the smaller being used.When electron backscatter diffraction spacing between the dislocation boundaries,as com- (EBSD)was used to quantify the microstructures. pared with ECAP,accumulative rolled bonding the boundaries with the misorientation angle <2 (ARB)120.21]and compressionl1.22]deformed counter- are excluded for statistics due to the uncertaintyl231 parts,Fig.2(b).As the boundary spacing was deter- On the contrary,these boundaries are included dur- mined by the moving distance of dislocations before ing CBED analysis due to its high angular resolu- being captured to form dislocation boundaries,i.e.the tion >0.15.61.Consequently,EBSD analysis gener- free path of mobile dislocations.Given the strain am- ally results in higher fHAB and eav than CBED.The plitude(s),the free path(A)of mobile dislocations is large values for the ARB Cul211,CR Cul24]and ECAP inversely proportional to the density:=/pmb6. Cul9l in Fig.2 are obtained by EBSD,whereas the Another important characteristic of LN-DPD Cu smaller data for compression[22 are obtained fromF. Yan et al.: J. Mater. Sci. Technol., 2011, 27(8), 673–679 675 Table 1 Structural parameters of three types of microstructures in the LN-DPD Cu Structure dT/nm dL/nm θav/deg. fHAB/% ρ/ 1015m−2 DS-region 91 473 3.8 0 5.6 NT-region 26 – ∼60 100 17 SB-region 46 149 7.4 7 16 average of 3.8◦ (θav), implying that all the newly￾formed boundaries are low angle dislocation bound￾aries. This is highly different from the nanostructures formed by traditional severe plastic deformation at large strains, where the density of high angle bound￾aries can be ∼70%[6,8,9]. These boundary parameters allow the dislocation density to be roughly estimated, based on the assump￾tion that dislocations are mainly presented in the low angle dislocation boundaries, whereas the dislocation density in the volumes between boundaries is rela￾tively low (1014 m−2)[10]: ρ = 1.5SLABθLAB b (1) where SLAB and θLAB are the surface area per unit volume and the average misorientation angle of low angle dislocation boundaries, that are misoriented <15◦ (SLAB = fLAB( 1 dT + π 2dL )[11]), and b is Burgers vector of Cu (0.256 nm). By inserting the structural parameters given in Table 1, the dislocation density is approximated as 5.6×1015 m−2. Although this calcu￾lation underestimates the dislocation density due to neglecting the abundant dislocations that do not con￾tribute to the misorientation rise, the value is higher by a factor of 5–6 compared with the counterparts deformed via cold rolling (CR)[12], equal channel an￾gular pressing (ECAP)[13,14] and RT-DPD (DPD at room temperature)[15], Fig. 2(a). The relative low dislocation density of LN-DPD Cu in literature [15] may be due to the lower strain (1.8) than the present investigation (2.1). The significantly high dislocation density of LN￾DPD Cu can be partially attributed to the high strain rate, which facilitates the accumulation of disloca￾tions, because the strain rate ( ˙ε) is proportional to the velocity (v) and the density (ρm) of mobile disloca￾tions: ˙ε=ρmbν[16]. Additionally, low temperature in￾hibits the annihilation of dislocations by cross-slip and climb[17]. Consequently, LN-DPD Cu shows higher dislocation density. This may explain the smaller spacing between the dislocation boundaries, as com￾pared with ECAP[18,19], accumulative rolled bonding (ARB)[20,21] and compression[1,22] deformed counter￾parts, Fig. 2(b). As the boundary spacing was deter￾mined by the moving distance of dislocations before being captured to form dislocation boundaries, i.e. the free path of mobile dislocations. Given the strain am￾plitude (ε), the free path (λ) of mobile dislocations is inversely proportional to the density: λ = ε/ρmb[16]. Another important characteristic of LN-DPD Cu Fig. 2 Comparison of characteristic parameters of dis￾location structures in Cu processed by different approaches: (a) the dislocation density; (b) the average boundary spacing; (c) the average bound￾ary misorientation angle; and (d) the fraction of high angle boundaries. Data obtained by EBSD and CBED are marked by ∗ and ∗∗, respectively is the significantly low fraction of high angle bound￾aries (fHAB) and the smaller average misorientation angle (θ), Fig. 2(c) and (d). This is partially as￾cribed to the quantitative characterization techniques being used. When electron backscatter diffraction (EBSD) was used to quantify the microstructures, the boundaries with the misorientation angle <2◦ are excluded for statistics due to the uncertainty[23]. On the contrary, these boundaries are included dur￾ing CBED analysis due to its high angular resolu￾tion >0.1◦[5,6]. Consequently, EBSD analysis gener￾ally results in higher fHAB and θav than CBED. The large values for the ARB Cu[21], CR Cu[24] and ECAP Cu[19] in Fig. 2 are obtained by EBSD, whereas the smaller data for compression[22] are obtained from