Effect of grain size on strain rate sensitivity of cryomilled Al–Mg alloy
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Al–Mg alloy powder was cryomilled to achieve a nanocrystalline (NC) structure having an average grain size of 50 nm with high thermal stability, and then consolidated by quasi-isostatic forging. The consolidation resulted in a bulk material with ultrafine grains of about 250 nm, and the material exhibited enhanced strength compared to conventionally processed Al–Mg alloy. The hardness of as-cryomilled powder, the forged ultrafine-grained (UFG) material, and the conventional coarse-grained (CG) alloy were measured by nanoindentation using various loading rates, and the results were compared with strain rate sensitivity (SRS) from uniaxial compression tests. Negative SRS was observed in the cryomilled NC powder and the forged UFG material, while the conventional alloy was relatively insensitive to strain rate. The dependence on loading rate was stronger in the NC powders than in the UFG material.
KeywordsStrain Rate Sensitivity Select Area Diffraction Pattern Dynamic Strain Aging Processing Control Agent Negative Strain Rate Sensitivity
Research was sponsored by U.S. Army Research Laboratory (ARL) and was accomplished under Cooperative Agreement W911NF-08-2-0028. The views and conclusions made in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of ARL or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation hereon. The authors are grateful to JEOL USA, Inc. for providing access to the SM-09010 cross-section polisher, to Scott Sitzman (Oxford Instruments America, Inc.) for his effort on EBSD analysis, to Prof. Andrea Hodge (University of Southern California) for her permission to use the nanoindenter, and to Prof. Kwang Ho Kim (National Core Research Center for Hybrid Materials Solution, Busan, Korea) for valuable technical discussions. The authors also gratefully acknowledge A. Piers Newbery previously of University of California, Davis, for his assistance.
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