Trans. Nonferrous Met. Soc. China 24(2014) 3848−3857 Microstructure evolution and dislocation configurations in nanostructured Al−Mg alloys processed by high pressure torsion Man-ping LIU 1 , Ting-hui JIANG 1 , Xue-feng XIE 1 , Qiang LIU 1 , Xue-feng LI 1 , Hans J. ROVEN 2,3 1. School of Materials Science and Engineering, Jiangsu Province Key Laboratory of Materials Tribology, Jiangsu University, Zhenjiang 212013, China; 2. Department of Materials Science and Engineering, Norwegian University of Science and Technology, Trondheim 7491, Norway; 3. Center for Advanced Materials, Qatar University, P.O. Box 2713, Doha, Qatar Received 17 October 2013; accepted 5 November 2014 Abstract: Microstructure evolution and dislocation configurations in nanostructured Al–Mg alloys processed by high pressure torsion (HPT) were analyzed by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The results show that the grains less than 100 nm have sharp grain boundaries (GBs) and are completely free of dislocations. In contrast, a high density of dislocation as high as 10 17 m -2 exists within the grains larger than 200 nm and these larger grains are usually separated into subgrains and dislocation cells. The dislocations are 60° full dislocations with Burgers vectors of 1/2〈110〉 and most of them appear as dipoles and loops. The microtwins and stacking faults (SFs) formed by the Shockley partials from the dissociation of both the 60° mixed dislocation and 0° screw dislocation in ultrafine grains were simultaneously observed by HRTEM in the HPT Al–Mg alloys. These results suggest that partial dislocation emissions, as well as the activation of partial dislocations could also become a deformation mechanism in ultrafine-grained aluminum during severe plastic deformation. The grain refinement mechanism associated with the very high local dislocation density, the dislocation cells and the non-equilibrium GBs, as well as the SFs and microtwins in the HPT Al−Mg alloys were proposed. Key words: Al−Mg aluminum alloy; severe plastic deformation; high pressure torsion; dislocation configurations; grain refinement; deformation mechanism 1 Introduction Over the last two decades, severe plastic deformation (SPD) techniques have been widely used to produce bulk nanostructured materials with unusual properties that are very attractive for various structural and functional applications [1−5]. It is well known that Al−Mg alloys belong to an important class of non-heat treatable alloys (the 5xxx series) [6]. Recent works have shown that Mg addition in SPD aluminium alloys enhances their properties such as the work hardening rate, dynamic strain aging effect, thermal stability, dislocation generation, grain refinement and thus the strength and ductility [7,8]. In addition, Al−Mg alloys are often used for studying the well-known serrated yielding or Portevin-Le Châtelier (PLC) effect and the associated plastic instabilities evident during tensile flow [9]. Therefore, the binary SPD Al–Mg alloys have attracted substantial interest, both for fundamental research and technological development. High pressure torsion (HPT) is one of the most promising SPD techniques because it has the potential to produce nanostructures with grain size less than 100 nm [10]. However, the genesis of the structural features in HPT-processed Al–Mg alloys is not yet fully understood. These features are quite complex and the presence of microtwins and stacking faults (SFs), non-equilibrium Foundation item: Project (BK2012715) supported by the Basic Research Program (Natural Science Foundation) of Jiangsu Province, China; Project (14KJA430002) supported by the Key University Science Research Project of Jiangsu Province, China; Project (50971087) supported by the National Natural Science Foundation of China, China; Projects (11JDG070, 11JDG140) supported by the Senior Talent Research Foundation of Jiangsu University, China; Project (hsm1301) supported by the Foundation of the Jiangsu Province Key Laboratory of High-end Structural Materials, China; Project (Kjsmcx2011004) supported by the Foundation of the Jiangsu Province Key Laboratory of Materials Tribology, China Corresponding author: Man-ping LIU; Tel: +86-511-88780192; E-mail: manping-liu@263.net; manpingliu@ujs.edu.cn DOI: 10.1016/S1003-6326(14)63542-1