Thermal stability in bulk cryomilled ultrafine-grained 5083 Al alloy |
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Authors: | Indranil Roy Manish Chauhan Farghalli A Mohamed Enrique J Lavernia |
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Affiliation: | (1) the Department of Chemical Engineering and Materials Science, University of California, 92697 Irvine, CA;(2) the Department of Chemical Engineering and Materials Science, University of California, 95616 Davis, CA |
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Abstract: | Thermal stability in bulk ultrafine-grained (UFG) 5083 Al that was processed by gas atomization followed by cryomilling, consolidation,
and extrusion, and that exhibited an average grain size of 305 nm, was investigated in the temperature range of 473 to 673
K (0.55 to 0.79 T
m
, where T
m
is the melting temperature of the material) for different annealing times. Appreciable grain growth was observed at temperatures
> 573 K, whereas there was limited grain growth at temperatures < 573 K even after long annealing times. The values of the
grain growth exponent, n, deduced from the grain growth data were higher than the value of 2 predicted from elementary grain growth theories. The
discrepancy was attributed to the operation of strong pinning forces on boundaries during the annealing treatment. An examination
of the microstructure of the alloy suggests that the origin of the pinning forces is most likely related to the presence of
dispersion particles, which are mostly introduced during cryomilling. Two-grain growth regimes were identified: the low-temperature
region (<573 K) and the high-temperature region (>573 K). For temperatures lower than 573 K, the activation energy of 25 ±
5 kJ/mol was determined. It is suggested that this low activation energy represents the energy for the reordering of grain
boundaries in the UFG material. For temperatures higher than 573 K, an activation energy of 124 ± 5 kJ/mol was measured. This
value of activation energy, 124 ± 5 kJ/mol, lies between that for grain boundary diffusion and lattice diffusion in analogous
aluminum polycrystalline systems. The results show that the strength and ductility of bulk UFG 5083 Al, as obtained from tensile
tests, correlate well with substructural changes introduced in the alloy by the annealing treatment. |
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Keywords: | |
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