4608 N.R.Tao et al.Acta Materialia 50 (2002)4603-4616 some regions the dislocation density is rather high.High-density dislocations arraying in tangles were observed,as in Fig.5(b).In tangles dislocations are randomly arranged without preferable sliding orientations. 3.2.2.Grain subdivision and subgrains As the depth decreases.deformation strains and strain rate increase.In the micro-sized regime (section(iii),40-60 um deep),most original grains (a) 1μm (b) 200nm are found to be subdivided into micro-sized cells (or 'blocks'),of which the shapes are either Fig.5.Cross-sectional TEM images in the matrix of the roughly equiaxed or lamellar in cross-sectional treated sample with plastic deformation evidences of DDWs TEM observations. ((a)numbers signifying misorientations across DDWs),and DTs((b)in which white arrows indicate location of high dislo- Fig.6 shows these regular-shaped cation density regimes). (parallelogram)cells separated by two sets of inter- secting DDWs in the {110)planes.These inter- secting DDWs seem to 'cut'the original coarse section (iv)),in which three typical deformation- grain into refined (micro-sized)blocks,across induced microstructure features were identified: which small misorientations (less than 1)are observed. 1.Dislocation lines (DLs):Homogeneously dis- tributed DLs are observed in Fe {110)planes and other sliding planes (such as (112)and (123)),depending upon the orientations of the grains.The density of DLs increases with a decrease of depth from the top surface. 2.Dense dislocation walls(DDWs):They are fre- quently seen inside some grains in this section. as shown in Fig.5(a).These DDWs (along Fe {110)planes)are parallel to each other separ- ated with a uniform spacing.It is noticed that the spacing between parallel DDWs varies (from a fraction of micron to a few microns) from grain to grain,depending upon grain orien- tations.DDWs are believed to result from dislo- cation accumulation and rearrangement for min- imizing the total energy state.It is worth noting that small misorientations across DDWs are detected,usually smaller than 1(as indicated in Fig.5(a)).Meanwhile,dislocation lines along the (112)planes are also visible,which inter- sect with DDWs in the {110)planes.Inter- secting DDWs are occasionally observed in 1μm some grains in which DDWs were developed simultaneously in different slip planes. Fig.6.A TEM image showing intersecting DDWs developed 3.Dislocation tangles (DTs):In some grains,the in two sets of(110)planes that cut the original grain into paral- dislocation distribution is not uniform and in lelogram cells.4608 N.R. Tao et al. / Acta Materialia 50 (2002) 4603–4616 Fig. 5. Cross-sectional TEM images in the matrix of the treated sample with plastic deformation evidences of DDWs ((a) numbers signifying misorientations across DDWs), and DTs ((b) in which white arrows indicate location of high dislo￾cation density regimes). section (iv)), in which three typical deformation￾induced microstructure features were identified: 1. Dislocation lines (DLs): Homogeneously dis￾tributed DLs are observed in Fe {110} planes and other sliding planes (such as {112} and {123}), depending upon the orientations of the grains. The density of DLs increases with a decrease of depth from the top surface. 2. Dense dislocation walls (DDWs): They are fre￾quently seen inside some grains in this section, as shown in Fig. 5(a). These DDWs (along Fe {110} planes) are parallel to each other separ￾ated with a uniform spacing. It is noticed that the spacing between parallel DDWs varies (from a fraction of micron to a few microns) from grain to grain, depending upon grain orien￾tations. DDWs are believed to result from dislo￾cation accumulation and rearrangement for min￾imizing the total energy state. It is worth noting that small misorientations across DDWs are detected, usually smaller than 1° (as indicated in Fig. 5(a)). Meanwhile, dislocation lines along the {112} planes are also visible, which inter￾sect with DDWs in the {110} planes. Inter￾secting DDWs are occasionally observed in some grains in which DDWs were developed simultaneously in different slip planes. 3. Dislocation tangles (DTs): In some grains, the dislocation distribution is not uniform and in some regions the dislocation density is rather high. High-density dislocations arraying in tangles were observed, as in Fig. 5(b). In tangles dislocations are randomly arranged without preferable sliding orientations. 3.2.2. Grain subdivision and subgrains As the depth decreases, deformation strains and strain rate increase. In the micro-sized regime (section (iii), 40–60 µm deep), most original grains are found to be subdivided into micro-sized cells (or ‘blocks’), of which the shapes are either roughly equiaxed or lamellar in cross-sectional TEM observations. Fig. 6 shows these regular-shaped (parallelogram) cells separated by two sets of inter￾secting DDWs in the {110} planes. These inter￾secting DDWs seem to ‘cut’ the original coarse grain into refined (micro-sized) blocks, across which small misorientations (less than 1°) are observed. Fig. 6. A TEM image showing intersecting DDWs developed in two sets of {110} planes that cut the original grain into paral￾lelogram cells