Development of high-strength aluminum alloys by mesoscopic structure control
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Development of High-Strength Aluminum Alloys by Mesoscopic Structure Control KOZO OSAMURA, OSAMU KUBOTA, PARSONS PROMSTIT, HIROSHI OKUDA, SHOJIRO OCHIAI, KAZUO FUJII, JUN KUSUI, TAKAMASA YOKOTE, and KOHEI KUBO The mechanical properties of A1ZnMgCu quaternary alloys are characterized in terms of precipitation hardening, where Guinier-Preston zones and/or 7/'MgZn2 phase of nanometer scale act as pinning centers for preventing dislocation motion. Improvement in mechanical properties will be achieved by controlling the morphology of those metastable fine precipitates. The atomizing technique allows a product of rapidly solidified powder that contains solutes in excess of the equilibrium and also has segregation-free microstructure even in a very high solute alloy. The successive mechanical and heat treatments provide a variety of mesoscopic structure changes leading to a remarkable improvement in properties. Thus far, several attempts have been made to improve the mechanical properties of A1ZnMgCu powder metallurgy (PM) processed alloys as reviewed by Pickens, u] citing the highest tensile strength of 879 MPa for the PM-processed alloy. Recently, our group has succeeded in developing extremely high-strength aluminum alloys with tensile strengths exceeding 900 MPa and elongation greater than 1 pct by controlling the mesoscopic structure. The chemical composition of the alloys used is summarized in Table I. The base alloy was A1-9 mass pct Zn-3 mass pct Mg-1.5 mass pct Cu. The extra elements, Mn, Ag, and Zr, were added systematically. The powder prepared by air atomizing was pressed into a rodlike shape KOZO OSAMURA, Professor, and HIROSHI OKUDA, Research Associate, Department of Materials Science and Engineering, OSAMU KUBOTA and PARSONS PROMSTIT, Graduate Students, Faculty of Engineering, and SHOJIRO OCHIAI, Professor, Mesoscopic Material Research Center, are with Kyoto University, Kyoto, Japan. KAZUO FUJII and KOHEI KUBO, Researchers, and JUN KUSUI and TAKAMASA YOKOTE, Senior Researchers, are with the Research and Development Laboratory, Toyo Aluminum K.K., Japan. Manuscript submitted July 6, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
by cold isostatic pressing (CIP). The rod was first preheated in an argon atmosphere at 773 K for 3.6 ks and then extruded at 773 K to an extrusion ratio of I0. It was confuTned that the specimen preheated at 773 K after CIP treatment included homogeneously distributed precipitates, but we observed scarcely any inhomogeneously remelted local region. This standard fabrication condition here is called Ar(10). When the rod was hotpressed under vacuum at 773 K and hot-extruded to an extrusion ratio of 20, it was termed HP(20). The products were machined to dumbbell-type shape and heattreated to T6 condition, for which the specimens were solution-treated at 763 K for 7.2 ks, then water-quenched and aged at 393 K for 86.4 ks, where the specimen was dropped into oil bath of 393 K from room temperature. The heatup rate thus achieved was the same for all present experiments. A part of spec
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