Structures and Mechanical Properties of ECAP Processed 7075 Al Alloy upon Natural Aging and T651 Treatment
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Structures and Mechanical Properties of ECAP Processed 7075 Al Alloy upon Natural Aging and T651 Treatment Yonghao Zhao1, Xiaozhou Liao1, Ruslan Z. Valiev2 and Yuntian T. Zhu1 1 Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM-87545, U.S.A. 2 Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, 12 K. Marx Street, 450000 Ufa, Russian Federation ABSTRACT Equal-channel angular pressing (ECAP) processed ultrafine grained (UFG) and coarse grained (CG) 7075 Al alloys were treated by natural aging and T651 temper (annealed at 120 ºC for 48 h in Ar atmosphere), respectively. Mechanical tests showed that for the UFG sample, the natural aging resulted in the highest strength (the ultimate tensile strength is 720 MPa). In contrast, for the CG sample, the T651 treatment resulted in a higher strength (the ultimate strength is 590 MPa) than the natural aging (530 MPa). Microstructural analyses indicated that the enhanced strength of the T651 treated CG sample was mainly caused by high densities of GP zones and metastable η′ precipitates. The enhanced strength of the naturally aged UFG sample was mainly caused by the high densities of G-P zones and dislocations. Upon T651 treatment, the dislocation density of the UFG sample deceased significantly, overcompensating the precipitation strengthening. INTRODUCTION In the last decade, equal channel angular pressing (ECAP) technique has been widely used to produce bulk ultrafine grained (UFG) metal and alloys, such as Al and its alloys, Cu, Ni, Ti and its alloys, and steels. During an ECAP process, the sample is deformed by being pressed through a die containing two channels, equal in cross section and intersecting at a certain angle. The bulk UFG materials are thought to have considerable potential for industrial applications due to their high strength with good ductility, superplasticity at moderate temperatures and high strain rates, etc. [1]. For Al alloys, most ECAP effort has focused on work strengthening Al-Mg alloys [2-5]. Much less attention has been paid to the precipitate hardening Al-Zn-Mg 7000 series alloys [6,7], which show the highest strength of all commercial Al alloys and are widely used for structural applications in military and civil aircraft as well as sporting goods. Conventional cold working, which usually improves the strength of metals and alloys, has been found to be ineffective in improving the strength of 7000 series Al alloys [8]. It is of great interest to investigate if the strengthening effect observed in ECAP-processed UFG materials can be added to the precipitation hardening effect in the 7000 series Al alloys. If these two strengthening effects can be made additive, it will be possible to significantly improve the strength of 7000 Al alloys, making them much more attractive in high strength structural applications. In the literature, the highest strength of the 7000 Al alloys was obtained by a T651 temper (annealing at 120 °C for 24 h after solution treatment). T651 treatm
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