674 F.Yan et al.:J.Mater.Sci.Technol.,2011,27(8),673-679 20 161 15 d=91 nm 10 d =473 nm h 200 400 600 800 10001200 Boundary spacing /nm 20 8=3.8° 40 5 10 15 Misorientation angle deg. Fig.1 A representative TEM image of dislocation structure in LN-DPD Cu (a),the corresponding statistic distribution of boundary spacing (b)and the boundary misorientation angles (c).The arrow shows the loading direction of high angle boundaries >99%.Details of the DPD been observed in the LN-DPD Culll.The quantita- treatment can be found elsewherell.In the present tive characterizations of these microstructures will be investigation,the pure Cu was subjected to DPD at followed by an analysis of the strengthening mecha- cryogenic temperature to a strain of 2.1. nisms. The deformed microstructures of LN-DPD Cu were observed by JEOL 2010 TEM in the plane that is 3.1 DS-region parallel to the loading direction,i.e.longitudinal sec- tion.The boundary spacing was measured directly The DS-region is composed of extended disloca- from the micrographs and the boundary misorienta- tion boundaries that are nearly perpendicular to the tion angles were determined by CBED in the following loading direction,interconnecting dislocation bound- ways:(1)obtaining the Kikuchi diffraction patterns of aries and isolated dislocations presented in the vol- crystallites adjacent to a boundary,(2)calculating the umes between the boundaries,Fig.1(a).Such mi- orientation matrix of each crystallite,(3)calculating crostructural features resemble those observed in Cu the rotation matrix or angle/axis pair by considering deformed at low strain rates at room temperatures(71. the crystallographic symmetry of cubic structure(to- However,they are less recovered as reflected by the tally 576 rotation matrixes or angle/axis pairs),(4)se- higher interior dislocation density and poorly-defined lecting the minimum rotation angle as the misorienta- interconnecting dislocation boundaries.The bound- tion angle across the boundary and(5)repeating these ary spacing was determined by measuring the inter- operations,allowing large number of boundaries to be ception length along a line perpendicular (dr)and analyzed.Detailed information of this technique can parallel(dL)to the extended dislocation boundaries. be found elsewherel5,61. dr gives a narrow distribution from 30 to 300 nm, whereas dL shows a wide one from 150 to 1200 nm. 3.Results and Discussion Fig.1(b).The average value of dr and dL is 91 and 473 nm,respectively,which pins an one-dimensional Three types of microstructures relevant to dislo- nanostructure with an aspect ratio,i.e.dL/dr=5.2. cation slip (denoted as DS-region),nano-scale twin- The misorientation angles across these boundaries, ning (NT-region)and shear banding (SB-region)have Fig.1(c),are in the range from 0.1 to 12 with an674 F. Yan et al.: J. Mater. Sci. Technol., 2011, 27(8), 673–679 Fig. 1 A representative TEM image of dislocation structure in LN-DPD Cu (a), the corresponding statistic distribution of boundary spacing (b) and the boundary misorientation angles (c). The arrow shows the loading direction of high angle boundaries >99%. Details of the DPD treatment can be found elsewhere[1]. In the present investigation, the pure Cu was subjected to DPD at cryogenic temperature to a strain of 2.1. The deformed microstructures of LN-DPD Cu were observed by JEOL 2010 TEM in the plane that is parallel to the loading direction, i.e. longitudinal sec￾tion. The boundary spacing was measured directly from the micrographs and the boundary misorienta￾tion angles were determined by CBED in the following ways: (1) obtaining the Kikuchi diffraction patterns of crystallites adjacent to a boundary, (2) calculating the orientation matrix of each crystallite, (3) calculating the rotation matrix or angle/axis pair by considering the crystallographic symmetry of cubic structure (to￾tally 576 rotation matrixes or angle/axis pairs), (4) se￾lecting the minimum rotation angle as the misorienta￾tion angle across the boundary and (5) repeating these operations, allowing large number of boundaries to be analyzed. Detailed information of this technique can be found elsewhere[5,6]. 3. Results and Discussion Three types of microstructures relevant to dislo￾cation slip (denoted as DS-region), nano-scale twin￾ning (NT-region) and shear banding (SB-region) have been observed in the LN-DPD Cu[1]. The quantita￾tive characterizations of these microstructures will be followed by an analysis of the strengthening mecha￾nisms. 3.1 DS-region The DS-region is composed of extended disloca￾tion boundaries that are nearly perpendicular to the loading direction, interconnecting dislocation bound￾aries and isolated dislocations presented in the vol￾umes between the boundaries, Fig. 1(a). Such mi￾crostructural features resemble those observed in Cu deformed at low strain rates at room temperatures[7]. However, they are less recovered as reflected by the higher interior dislocation density and poorly-defined interconnecting dislocation boundaries. The bound￾ary spacing was determined by measuring the inter￾ception length along a line perpendicular (dT) and parallel (dL) to the extended dislocation boundaries. dT gives a narrow distribution from 30 to 300 nm, whereas dL shows a wide one from 150 to 1200 nm, Fig. 1(b). The average value of dT and dL is 91 and 473 nm, respectively, which pins an one-dimensional nanostructure with an aspect ratio, i.e. dL/dT=5.2. The misorientation angles across these boundaries, Fig. 1(c), are in the range from 0.1◦ to 12◦ with an