Microstructural evolution of an ultrafine-grained cryomilled Al 5083 alloy during thermomechanical processing
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Bing Q. Han and Enrique J. Lavernia Department of Chemical Engineering and Materials Science, University of California, Davis, Davis, California 95616 (Received 14 December 2004; accepted 19 April 2005)
The microstructural changes in cryomilled and consolidated Al 5083 following compression testing at several temperatures are described. Prior to testing, the material had an average grain size of approximately 138 nm and exhibited a duplex microstructure, containing coarse grains between 500 and 2000 nm. After uniaxial compressive deformation at temperatures between 423 and 573 K (0.49–0.66 Tm), the average grain size increased to between 200 and 300 nm, consistent with the average grain size of extrusions formed from the same material at similar temperatures. The similarity in grain size distribution following uniaxial compression or extrusion despite differences in total strain and stress state imposed by each indicates that much of the deformation in the extrusion process occurs in coarse-grained regions.
I. INTRODUCTION
The promise of nanocrystalline and ultrafine-grained materials for use in structural applications can be attributed to their enhanced strength relative to conventional materials of similar composition.1 Processing of bulk nanostructured materials has typically followed one of two routes. In the first, nanoparticles formed by such techniques as inert-gas condensation2 or mechanical milling3 are consolidated into bulk forms. In the second approach, bulk materials have their microstructures refined by means of severe plastic deformation, such as equal channel angular pressing (ECAP) or high-pressure torsion.4 In either case, one may expect that additional treatment or processing is necessary to achieve a final usable bulk form, whether simply to change its shape for a particular application, to remove porosity, or to manipulate mechanical properties or workability through annealing or heat treating. Because grain size plays an important role in defining the strength of nanostructured materials, it is important to monitor changes in the microstructure during processing. A thermally stable nanostructured microstructure has been reported on powders produced by mechanical milling of metallic powders within a liquid nitrogen slurry, a technique known as cryomilling.5 Nanocrystalline grain a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0261 J. Mater. Res., Vol. 20, No. 8, Aug 2005
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sizes have been maintained after annealing cryomilled Ni6 and Fe powders7 at more than half the melting point (0.5Tm). Cryomilled Fe–Al7, Al,8 and Al–Mg9,10 alloy powders have retained nanocrystalline dimensions to homologous temperatures exceeding 0.7Tm. At the same time, after consolidation and extrusion, bulk cryomilled materials exhibit heterogeneous microstructures with average grain sizes in the ultrafine grain size range of 100 to 500 nm.5,11–16 While the microstructural changes in cryomilled nanocrystalline powders s
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