Dislocation structure and deformation in iron processed by equal-channel-angular pressing
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Dislocation Structure and Deformation in Iron Processed by Equal-Channel-Angular Pressing BING Q. HAN, ENRIQUE J. LAVERNIA, and FARGHALLI A. MOHAMED The evolution of dislocation structure in pure Fe during equal-channel-angular pressing (ECAP) is investigated. Also, the effect of the formation of this dislocation structure on deformation and fracture behavior is examined. The results show that intensive dislocation cell blocks are present after one pass and even more after subsequent pressings. The low-energy dislocation structures (LEDS) may have changed into the high-energy dislocation structures (HEDS) in the final several pressings. The high-density array of dislocations plays a significant role in strengthening. The HEDS may cause the materials to lose work-hardening ability and show a cleavage morphology of the fracture surface. A proper subsequent annealing treatment will lead to the evolution of HEDS to LEDS while maintaining little grain growth. This change in the nature of dislocation structures allows ultrafinegrained materials to achieve an excellent combination of high strength and high ductility.
I. INTRODUCTION
EQUAL-CHANNEL angular pressing (ECAP) is a processing technique that is capable of producing ultrafinegrained (UFG) materials with grain sizes of 200 to 500 nm.[1] During ECAP, the material is pressed through a die that has two channels containing the same cross section with an angle of 90 deg or higher. In each pass, large shear deformation is introduced into materials. This processing technique possesses some advantages over several other processing techniques, including the consolidation of mechanically alloyed (MA) powders, heavy cold work or high-pressure torsion. Consolidation of MA powders is an effective approach to manufacturing nanostructured or UFG materials. However, bulk MA alloys contain residual porosity and impurities that will influence mechanical performance.[2,3] While extensive cold deformation can also be used to produce large amounts of UFG materials, such materials often contain a cellular substructure in which grain boundaries (GBs) have low-angle misorientations, causing anisotropy of mechanical properties.[4,5] The high-pressure torsion technique is one of the most effective processing techniques for producing nanostructured materials (grain sizes 100 nm).[6,7] However, the amount of pressed materials is generally very small. Moreover, this approach requires equipment that is capable of generating large pressures, which can be a technological obstacle for extensive application in industry. Therefore, it is difficult to scale this approach up for mass production for grain refinement. Comparatively, ECAP can be used to produce large amounts of porosityfree and impurity-free bulk UFG materials with GBs having high-angle misorientations. Early investigations using ECAP processing extensively focused on aluminum or copper alloys.[1] Very recently, BING Q. HAN, Assistant Researcher, Department of Chemical Engineering and Materials Science, an
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