4612 N.R.Tao et al.Acta Materialia 50 (2002)4603-4616 (a) 100nm b Fig.13.A plane-view HRTEM image of the nanostructure in the top surface layer. sized)were formed in the surface layer of the Fe sample during the SMA treatment.Based on the microstructure features observed in various sec- tions with different strains in the deformed surface layer,one may find that the following elemental 100nm processes are involved in the grain refinement pro- cess: Fig.11.Cross-sectional TEM images in the top surface layer showing:(a)lamellar nanocrystallites;(b)equiaxed nanocrys- 1.development of DDWs and DTs in original tallites with small angle misorientations. grains and in the refined cells (under further straining)as well; 2.transformation of DDWs and DTs into subboun- daries with small misorientations separating individual cells or subgrains; 3.evolution of subboundaries to highly misori- ented grain boundaries. The grain refinement mechanism can be sche- matically illustrated in Fig.14,in which each pro- cess will be discussed in terms of the experi- mental observations. 4.1.Development of DDWs and DTs 100nm 100nm In order to accommodate plastic strains in Fig.12.(a)A bright-field and (b)a dark-field cross-sectional polycrystalline materials,various dislocation TEM images showing nanocrystallites in the very top surface activities are normally motivated,including slid- layer. ing,accumulation,interaction,tangling,and spatial rearrangement.In the SMA treated Fe sample,dis- location activities lead to formation of DDWs and DTs in original grains of the surface layer.Devel-4612 N.R. Tao et al. / Acta Materialia 50 (2002) 4603–4616 Fig. 11. Cross-sectional TEM images in the top surface layer showing: (a) lamellar nanocrystallites; (b) equiaxed nanocrys￾tallites with small angle misorientations. Fig. 12. (a) A bright-field and (b) a dark-field cross-sectional TEM images showing nanocrystallites in the very top surface layer. Fig. 13. A plane-view HRTEM image of the nanostructure in the top surface layer. sized) were formed in the surface layer of the Fe sample during the SMA treatment. Based on the microstructure features observed in various sec￾tions with different strains in the deformed surface layer, one may find that the following elemental processes are involved in the grain refinement pro￾cess: 1. development of DDWs and DTs in original grains and in the refined cells (under further straining) as well; 2. transformation of DDWs and DTs into subboun￾daries with small misorientations separating individual cells or subgrains; 3. evolution of subboundaries to highly misori￾ented grain boundaries. The grain refinement mechanism can be sche￾matically illustrated in Fig. 14, in which each pro￾cess will be discussed in terms of the experi￾mental observations. 4.1. Development of DDWs and DTs In order to accommodate plastic strains in polycrystalline materials, various dislocation activities are normally motivated, including slid￾ing, accumulation, interaction, tangling, and spatial rearrangement. In the SMA treated Fe sample, dis￾location activities lead to formation of DDWs and DTs in original grains of the surface layer. Devel-