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INTRODUCTION

THERE is a great need for strong and light metals in the automobile and aircraft industries because of the lack of fuel energy, and high-strength aluminum (Al) alloys are extensively used in these fields because of their high strength-to-weight ratio.[1,2] For example, 5xxx and 6xxx series Al alloys are used for panels, and 7xxx series Al alloys (Al-Zn-Mg(-Cu)) as high-strength Al alloys are widely used in aerospace and other transportation applications and even potentially replace steel.[2] An important strengthening mechanism in 7xxx series Al alloys is precipitation hardening.[3] The usual precipitation sequence can be summarized as: (supersaturated) solid solution fi Guinier–Preston (GP) zones (coherent Mg, Zn-rich solute clusters) fi metastable g¢ (semi-coherent MgZn1-2) fi stable g (incoherent MgZn2 with hexagonal structure).[4–8] The morphology and size of YAN DU, WANGTU HUO, and WEI ZHANG are with the Northwest Institute for Nonferrous Metal Research, Xi’an 710016, P.R. China. Contact e-mail: [email protected] JIANPING XU is with the Northwest Institute for Nonferrous Metal Research and also with the School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, P.R. China. Manuscript submitted June 15, 2020; accepted September 17, 2020.

METALLURGICAL AND MATERIALS TRANSACTIONS A

different types of precipitates including GP zone, g¢ and g were studied, and the effects of deformation and grain size on the precipitation kinetics have also been discussed in recent years.[9–12] However, most of the existing investigations focused on the simplified uniform structure such as precipitation behavior in coarse-grained (CG) 7xxx series Al alloys during natural aging[4,5] and artificial aging,[6–8] and microstructural evolution including precipitation behavior in ultrafine-grained (UFG) 7xxx series Al alloys by various processing such as cryogenic temperature large-strain extrusion machining (CT LSEM)[13] and high-pressure torsion (HPT).[10] Zhang et al.[10] reported that after HPT homogeneous UFG 7075 Al alloy with grain sizes < 200 nm could be obtained and grain refinement, grain boundary segregations and precipitations were closely related to HPT processing parameters such as strain and temperature. Yin et al.[13] studied the microstructural and mechanical property evolutions of UFG 7075 Al alloy processed by CT LSEM and found that grain refinement ( Cu. Moreover, the effective diffusion coefficient is related to dislocation density as high dislocation density can act as sources for pipe diffusion and increase the diffusion rate beyond the normal bulk volume diffusion mechanisms.[30] The activation energies for high dislocation density-induced pipe diffusion and grain boundary diffusion are 0.6 and 0.5 times the activation energy for bulk diffusion, respectively,[33,34] normally resulting in GBPs with larger size than GIPs. Our previous study[27] also found segregation of Zn at grain boundaries leading to the formation of g-MgZn2. For a depth > 150 lm, relative low strain and strain rates result in D