Grain Boundary Behavior in an Ultrafine-Grained Aluminum Alloy
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(2) Does cyclic loading play a role in such grain boundary effects?
Grain Boundary Behavior in an Ultrafine-Grained Aluminum Alloy
The evolution of grain boundary during cold deformation of metals was extensively studied by Hansen[2] from an experimental viewpoint. At high strains, ‘‘microbands’’ (MBs), which are a few tenths of a micron wide and several microns in length, form within the grains.[2] Situated between the MBs are cell structures delineated by ‘‘dense dislocation walls’’ (DDWs).[2] In fact, some of the MBs themselves are composed of small pancake-shaped cells with orientation difference across the band extending to 10 to 15 deg.[2] For other MBs, the orientation mismatch, however, is much lower in value.[2] Such a subdivision of grains by MBs and DDWs into smaller cell structures was rationalized in terms of the low energy dislocation structure theory of work hardening.[3] In the percolation-based strain hardening model of Kocks and Mecking,[4] geometrical storage of dislocation loops leads to athermal strain hardening. Intersection of these loops with moving dislocations results in the formation of ‘‘tangles’’ and ‘‘cell walls.’’ The internal stresses within tangles and cell walls are further stabilized by dislocation-assisted plastic relaxation on secondary slip systems along with static and dynamic recovery. For UFG metals with grain size in the length scale similar to or smaller than the deformation damage structure itself, the validity of the preceding models, however, is uncertain. Moreover, the work-hardening behavior and strain rate sensitivities are also markedly different from microcrystalline alloys. Consequently newer models for deformation in UFG and nanocrystalline metals were proposed.[5,6] However, these new models are aimed at predicting the strength or ductility of UFG metals and are not extended to explain the microstructural evolution. To address the preceding knowledge gap and the questions mentioned earlier, the change in LAGBs during monotonic testing of UFG Al alloys was further examined using orientation imaging microscopy (OIM). The experimental results for LAGB behavior in UFG alloys during monotonic and cyclic loading are then rationalized from a thermodynamic viewpoint. A two-phase UFG Al alloy was produced by friction stir processing a solutionized AA7075 (ALCOA, Davenport, IA) rolled sheet, 3.2-mm thick. The processed material was heat treated at 423 K (150 C) to produce a stabilized UFG Al alloy henceforth called UFG-423K. Minitensile specimens of UFG-423K with 2.3-mm gage length, 1-mm thickness, and 1.16-mm width were machined. The tensile specimens were subsequently mechanically polished to 0.05-lm silica surface finish. The samples were then tested for their monotonic tensile behavior using a computer-controlled tensile testing machine. An interrupted tensile test was also performed where the sample was characterized using OIM (area of 10 9 10 lm2) to obtain the corresponding distribution of LAGBs at each interruption. The corresponding OIM maps in Euler angle colorin
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