Ultrafine-Grained Aluminum Processed by a Combination of Hot Isostatic Pressing and Dynamic Plastic Deformation: Microst
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INTRODUCTION
BULK ultrafine-grained (UFG) metals with grain sizes in the submicron range have been at the center of intense study in recent years because of their unique mechanical and physical properties.[1] Within the framework of producing light and mechanically resistant structures, UFG aluminum and their alloys have been produced routinely by various methods, including quasistatic severe plastic deformation (SPD). The most important SPD procedures are high-pressure torsion (HPT),[2] equal-channel angular pressing (ECAP),[3] accumulative roll bonding (ARB),[4] multidirectional forging (MDF),[5] and the combinations of these processes, such as ECAP+HPT.[6] In most SPD methods, the reduction of the grain size occurs via the formation of subgrains or cells bounded by low-angle grain boundaries (LAGBs), which subsequently transform into high-angle grain boundaries (HAGBs) with subsequent straining.[7] In the ECAP method, the rate of transformation from LAGBs to HAGBs depends on the G. DIRRAS, B. BACROIX, and S. RAMTANI, Professors, T. CHAUVEAU, Research Engineer, and Q. BUI, PhD, are with the LSPM, UPR CNRS 3407, Universite´ Paris 13, 93430 Villetaneuse, France. Contact e-mail: [email protected] A. ABDUL-LATIF, Professor, is with the Laboratoire d’Inge´nierie des Syste`mes Me´caniques et des Mate´riaux, 93407 St Ouen Cedex, France. J. GUBICZA, } PhD Student, are with the Associate Professor, and Z. HEGEDUS, Department of Materials Physics, Eo¨tvo¨s Lora´nd University, Budapest H-1518, Hungary. Manuscript submitted January 21, 2011. Article published online December 21, 2011 1312—VOLUME 43A, APRIL 2012
processing route. For example, equiaxed UFG grains with high fractions of HAGBs formed already after four ECAP passes applying route BC, whereas in the case of route A, many more passes were needed to reach the same state.[3] Recently, SPD methods applied at high strain rates and at cryogenic or ambient temperatures have also been used to produce UFG microstructures in facecentered cubic (fcc) materials such as Cu and Ni.[8] These procedures are referred to as dynamic (severe) plastic deformation (DPD) methods. It was reported that during DPD at cryogenic temperatures, grain refinement was promoted by an additional mechanism of nucleation of twin bundles, which are subsequently fragmented by the passage of adiabatic shear bands.[9] Under a subsequent quasi-static uniaxial tensile or compression deformation at room temperature (RT), the high dislocation density (1015 to 1016 m2) formed at cryogenic temperatures yields flow stress stagnation and eventually strain softening as a result of recovery. In such a case, plastic instability might occur and result in limited tensile ductility.[10,11] Powder metallurgy (PM) methods such as hot isostatic pressing (HIP) and spark plasma sintering (SPS) can produce bulk UFG metals and alloys starting from nanopowders.[12–16] The as-processed UFG materials have high fractions of HAGBs and random crystallographic texture.[14,17] However, some inherent weaknesses of the consolidat
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