Short Communication The microstructure, hardness and tensile properties of a new super high strength aluminum alloy with Zr addition S.H. Seyed Ebrahimi * , M. Emamy, N. Pourkia, H.R. Lashgari Center of Excellence for High Performance Materials, School of Metallurgy and Materials, University of Tehran, Tehran, Iran article info Article history: Received 10 January 2010 Accepted 4 April 2010 Available online 10 April 2010 abstract This research is based on the addition of zirconium to a new super high strength Al–12.24Zn–3.25 Mg– 2.46Cu alloy. The results showed that Zr addition decreases average grain size (approximately 20%), forms rosette-like microstructure and introduces proper distribution of the second phases. In addition, due to the presence of Al 3 Zr particles in Zr-refined specimen, the area fraction of recrystallized regions decreased by 65% as compared with unrefined specimen. Furthermore, tensile strength, yield strength and particularly elongation values of the extruded refined sample were enhanced nearly 34%, 25% and 1850%, respectively, when compared with not-extruded unrefined one. Microstructural observations and fractographic examinations of the fractured surfaces of several extruded specimens indicated dim- ple-like mechanism while the presence of coarse eutectic constituent was responsible for brittle mode of fracture in not-extruded specimens. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Wrought Al–Zn–Mg–Cu alloys are considered as high strength Al alloys which are widely used in aerospace field [1,2]. These al- loys have a tensile strength above the strongest of the 6000 series alloys and attain the highest strength of all Al alloys, exceeding normal structural steel. In order to improve mechanical properties of the alloys, higher concentrations of alloying elements are added. However these alloys suffer mainly from segregation of the alloy- ing elements and this segregation is detrimental for the mechani- cal properties of the alloys. In addition to higher concentration of alloying elements, desired grain size and interparticle distance are important to achieve outstanding properties in the aluminum alloys [3]. On the other hand, extrudability index of the alloy is re- stricted when higher amounts of alloying elements are added [4]. Therefore, an appropriate extrusion process for Al–Zn–Mg–Cu al- loys needs to a fully homogenized billet with a minimum segrega- tion. Thus, it is necessary to get effective control on the microstructure during production processing which can be per- formed by using a grain refiner [5]. However, the amounts of grain refiner agents are added to aluminum should be controlled. Form- ing particles which cannot act as effective nucleation sites are re- jected by advancing a-Al dendrite network and accumulate in interdendritic regions as an inclusion [6,7]. The aim of this study is to obtain a more homogenized micro- structure in order to improve hot extruded tensile properties of a super high strength aluminum alloy by the addition of zirconium in age-hardened Al wrought alloys (7XXX series). For comparison, hardness values of the specimens at different conditions and ten- sile properties of not-extruded but heat treated (T6) unrefined and Zr-refined specimens were also studied. 2. Experimental procedure The aluminum alloy used in the present work had the composi- tion (wt.%) of Al–12.24Zn–3.25 Mg–2.46Cu–0.16Fe–0.03Si (0.3 zir- conium). The amount of zirconium has been achieved as an optimum content with regard to the structural changes resulted from different amounts of Zr added on the alloy in the previous work [7]. Iron and silicon impurities were associated with the pri- mary aluminum that was used to prepare the alloy. At the beginning Al–Zn–Mg–Cu ingots were made by the use of commercial pure metals (Al, Zn, Mg and Cu), according to the above-mentioned composition. The produced ingots were re- melted at 750 °C by using an electrical resistance furnace (with accurate temperature measuring system, ±5 °C). Degassing was conducted by submerging dry C 2 Cl 6 containing broken tablets (0.3 wt.% of the molten alloy). Then preheated Al–5Zr master alloy (to provide 0.3 wt.% Zr in casting) was added to the melt. After a period of 5 min, the melt was cast into a cylindrical iron steel mould with dimensions of Ø in–out (34–40) mm  45 mm. Speci- mens were homogenized at 450 °C for 24 h and machined to pro- duce 29.9 mm diameter cylindrical billets for subsequent extrusion processing. Extrusion was performed in a direct mode on a 200 tons hydraulic press having a container diameter of 30 mm. Billets were preheated at 450 °C for 30 min in an electrical resistance furnace 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.04.006 * Corresponding author. Tel.: +98 21 61114143; fax: +98 21 61114083. E-mail address: shsebrahimi@ut.ac.ir (S.H.S. Ebrahimi). Materials and Design 31 (2010) 4450–4456 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes