Microstructure and tensile properties of compacted, mechanically alloyed, nanocrystalline Fe-AI
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INTRODUCTION
NUMEROUStechniques have been developed to produce nanocrystalline materials: e.g., vapor condensation, sol-gel, high-energy ball milling, etc. Many promising statements have been made about the potential mechanical properties of nanocrystalline materials, however, to date, limited experimental data have been reported. Much of the difficulty in obtaining macroscopic data is due to either the limited quantity of nanostructured material available for analysis or to inadequate retention of the nanostructure during processing. To date, the majority of mechanical properties reported on nanostructured compacts have been density, hardness, and a few compaction tensile strengths. (A tensile technique has recently been reported for testing compact disks which may reduce the quantity of material needed for testingYJ) Many of the reported material properties have been obtained from samples with less than full density. Recently, several consolidation techniques (cold/warm compaction, explosive compaction, hot isostatic pressing (HIP), etc.) have been partially successful in production of nearly fulldense nanostructure compacts. Each of these compaction techniques has advantages and limitations. (a) One of the more promising consolidation techniques is cold or warm compaction of vapor-condensed nanosized particles: 2,3j Nearly full-dense nanocrystalline compacts have been made of nanopowders at temperatures below 600 ~ However, consolidation of nanoparticles suffers from the limited amount of materials that can be produced by vapor condensation and the difficulty in maintaining a noncontaminated nanoparticle surface prior to compaction. (b) Consolidation of mechanically alloyed, nanocrystalline
J. RAWERS, Materials Scientist, G. SLAVENS, Research Chemist, D. GOVIER, Analytical Specialist, C. DOC-AN, Senior Analyst, and R. DOAN, Research Assistant, are with the United States Department of Energy, Albany Research Center, Albany, OR 97321. Manuscript submitted January 22, 1996.
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powders by explosive compaction also produces fulldense compacts:4,5]However, explosively formed compacts often contain fine cracks that greatly restricted evaluation of the compact's mechanical properties. (c) The HIP consolidation of nanocrystalline powders produces full-dense nanocrystalline compacts, c61However, the elevated temperatures used during consolidation often result in significant grain growth. In addition, the configuration of the final compacts cannot be controlled and often is irregularly shaped, thus greatly restricting evaluation of mechanical properties. (d) Hot pressing of mechanically processed, nanograin powders at temperatures near 0.5TMp (where MP = melting point) has been shown to produce nearly fulldense compacts while retaining much of the nanostructure: 7.8~ Hot-pressed samples can be prepared in size and shape for evaluation of macroscopic properties. One limitation in consolidation of nanostructured materials is their rapid grain growth at elevated temperatures. Grain-size the
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