Kovove Mater. 53 2015 251–258 DOI: 10.4149/km 2015 4 251 251 Microstructure decomposition and unique mechanical properties in an ultrafine-grained Al-Zn alloy processed by high-pressure torsion A. Baris 1 , N. Q. Chinh 1, *, R. Z. Valiev 2,3 , T. G. Langdon 4,5 1 Department of Materials Physics, E¨ otv¨ os Loránd University, H-1117 Budapest, Pázmány Péter Sétány 1/A, Hungary 2 Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, 12 K. Marx str., Ufa 450000, Russia 3 Research Laboratory for Mechanics of New Nanomaterials, Saint Petersburg State Polytechnical University, Saint Petersburg 195251, Russia 4 Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, U.S.A. 5 Materials Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, U.K. Received 15 February 2015, received in revised form 23 May 2015, accepted 23 May 2015 Abstract An ultrafine-grained (UFG) Al-30wt.%Zn alloy was processed by high-pressure torsion (HPT) and then the mechanical and microstructural properties were investigated using depth- -sensing indentations (DSI), focused ion beam (FIB), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). Emphasis was placed on the microstructure changes due to HPT processing as well as the effects of grain boundaries and the unusually high strain rate sensitivity. The deformation characteristics are explained by the formation of a Zn-rich phase which wets the Al/Al grain boundaries and enhances the role of grain boundary sliding in this UFG alloy. The occurrence of intensive grain boundary sliding in this UFG alloy at room temperature was also demonstrated by deforming micro-pillars. It is shown that, as a result of grain boundary sliding, the plastic deformation process of the UFG samples remains stable even at the micro-scale without the intermittent flow and detrimental strain avalanches which are an inherent feature of micro-size conventional crystals. This result illustrates the advantage of using UFG materials for effective applications in micro-devices. K e y w o r d s: grain boundary sliding, indentation, micro-pillars, strain rate sensitivity, ul- trafine grains 1. Introduction The Al-Zn alloys are probably the best known of all aluminum-based alloys. In this system there are some typical compositions such as the eutectic Al-95wt.%Zn, the eutectoid Al-78wt.%Zn and solid solutions having Zn contents lower than 31.6 wt.%: all of these materials have been extensively stud- ied [1–8]. Recently, the supersaturated solid solution Al-30wt.%Zn alloy, which is an important basic mater- ial in the aluminum industry, was processed by severe plastic deformation (SPD) using high-pressure tor- *Corresponding author: tel: +36-1-3722845; e-mail address: chinh@metal.elte.hu sion (HPT) [8, 9]. As a consequence of the severe de- formation, an ultrafine-grained microstructure was ob- tained in this alloy and the material exhibited a super- ductility at room temperature with unusually high elongations up to 150 % and relatively high strain rate sensitivities [4, 5]. It is well known that the ultrafine- grained (UFG) materials produced by SPD generally exhibit only limited tensile ductilities of about 5–10 % and this is correlated with the extremely low strain rate sensitivity (SRS) of about 0.01–0.03 for these ma- terials [10]. It is also well established that UFG mater- ials generally have reasonably saturated microstruc-