Influence of Processing Severity During Equal-Channel Angular Pressing on the Microstructure of an Al-Zn-Mg-Cu Alloy
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THE need for lightweight airframes has led to the development of very high-strength alloys used as plates, sheets, and extrusions. The highest room-temperature strengths attained in wrought aluminum alloy products correspond to the aluminum–zinc–magnesium–copper alloys.[1,2] Among the 7xxx series alloys, the Al 7075 has been widely used in commercial applications because it exhibits high strength and can be heat treated to a variety of microstructures.[3] However, additional homogenization and refinement of such microstructures by severe plastic deformation (SPD) processing is beneficial to improve the mechanical properties at ambient temperature and to obtain a rapid superplastic forming at relatively low temperatures, providing that the developed ultrafine grains are stable at such temperatures.[4] Equal-channel angular pressing (ECAP) is a very interesting method for modifying microstructure, producing ultrafine-grained (UFG) materials.[5–11] In an idealized description of ECAP, deformation takes place by simple shear confined to a narrow zone at the plane of intersection of die channels, as illustrated in Figure 1, without any change in cross-sectional area after passing through the intersection of the die channels. This process is, therefore, amenable to repetition.[12] The repetitive pressings also provide an opportunity for CARMEN M. CEPEDA-JIME´NEZ and JUAN M. GARCI´AINFANTA, Postdoctoral Researchers, OSCAR A. RUANO, Professor, and FERNANDO CARREN˜O, Staff Researcher, are with the Department of Physical Metallurgy, CENIM, CSIC, 28040 Madrid, Spain. Contact e-mail: [email protected] EDGAR F. RAUCH and JEAN J. BLANDIN, Professors, are with the Universite´ de Grenoble/CNRS, SIMAP Laboratory, Grenoble INP, 38402 Saint Martin d’He`res, France. Manuscript submitted August 9, 2011. Article published online May 26, 2012 4224—VOLUME 43A, NOVEMBER 2012
initiating new slip systems by rotating the sample about the longitudinal axis between consecutive passes.[13] It has been shown experimentally that, when using an ECAP die with an angle of 90 deg between the two parts of the channel, optimum processing is achieved using route BC, which is referred to a rotation through 90 deg in the same direction between consecutive passes. This route leads most rapidly both to an array of reasonably equiaxed grains and to a high fraction of grain boundaries having high angles of misorientation.[14] The process of ECAP was proven[15–21] to be an effective tool for both increasing the strength and toughness. In addition, if the ultrafine grain sizes are retained to elevated temperature where diffusion becomes quite rapid, there is a possibility of achieving a superplastic forming capability even at high strain rates.[22] A careful inspection of the literature shows that the strategy followed in the majority of SPD-processed Al alloys to stabilize the microstructure at high temperature is to add small amounts of elements such as Sc and Zr.[23–25] A fine dispersion of Al3Sc and/or Al3Zr formed during casting acts as an inhibitor of gr
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