Journal of Alloys and Compounds 486 (2009) 103–108 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom The role of Zr-rich cores in strength differential effect in an extruded Mg–Zn–Zr alloy M. Shahzad ∗ , L. Wagner Institute of Materials Science and Engineering, Clausthal University of Technology, Agricolastr. 6, 38678 Clausthal-Zellerfeld, Germany article info Article history: Received 25 February 2009 Received in revised form 14 June 2009 Accepted 15 June 2009 Available online 26 June 2009 Keywords: Magnesium alloy ZK60 Extrusion DRX Strength differential effect abstract This work examines the effects of extrusion parameters namely ratio and temperature on recrystallization behavior of a Mg–Zn–Zr alloy, and their consequent effects on anisotropy in the mechanical properties. Upon extrusion, the characteristic Zr-rich cores that do not recrystallize form the so-called “soft stringers” which are deformed bands elongated in the extrusion direction and curled around the extrusion axis. At higher extrusion ratio, there is more twinning contribution and the DRX response is improved, making the recryatallized grains finer and increasing the proportion of recrystallized Zr-rich cores. A basal texture after extrusion and the directional activation of tensile twinning cause anisotropy in the mechanical properties. In addition, the microstructural features such as large unrecrystallized regions and coarse crystallized grains also contribute in the strength differential effect. Further slip in the strain-hardened unrecrystallized grains is inhibited while twin activation under favorable orientation becomes easier in the coarse recrystallized grains. A higher proportion of large unrecrystallized and coarse crystallized gains in the case of lower extrusion ratio result in a much higher strength differential effect (∼100 MPa) in comparison to the one caused by the crystallographic texture only (∼25 MPa). © 2009 Elsevier B.V. All rights reserved. 1. Introduction Extrusion is an established industrial forming process to produce metallic, ceramic or polymeric profiles of constant cross- sections. Magnesium alloys undergo dynamic recrystallization during extrusion and the recrystallized grains are much finer than the one in feedstock cast alloys. Consequently, as-extruded alloys show higher strength and ductility than the as-cast alloys. How- ever, Mg alloys suffer from poor formability and unfavorable texture after extrusion. Because of their close packed hexagonal crystal lattice and an axial ratio (c/a ratio) of 1.624, the most active slip system in Mg alloys is the (0 0 0 1) 〈11 ¯ 20〉 basal slip. Since the crys- tallographic direction 〈a〉=〈11 ¯ 20〉 is the shortest one in Mg-CPH lattice, other slip systems i.e. prismatic and pyramidal, also slip in the same direction. Therefore, after a uni-directional deformation such as extrusion, most of the grains have their basal plane nor- mal at 90 ◦ to the extrusion direction. Such an orientation causes anisotropy in the mechanical properties, which depends not only on loading direction but also on the sign of applied stress. In addi- tion to elemental alloying which affects the c/a ratio of the alloy [1] and thus the activation of various slip systems and deformation twinning, texture development in Mg alloys is affected by the pro- ∗ Corresponding author. Tel.: +49 5323 722760; fax: +49 5323 722766. E-mail address: Muhammad.shahzad@tu-clausthal.de (M. Shahzad). cessing parameters such as strain rate and temperature; in essence, it depends on the relative ease of slip and twinning [2]. Cast Mg alloys suffer from inhomogeneous coarse grain sizes. In case of high pressure die casting, the grain sizes are generally fine because of the significantly high cooling rates. But during other casting processes such as sand casting, permanent mold casting and direct chill (DC) casting, the cooling rates are low and a grain refin- ing agent is required to get a uniform fine microstructure [3]. The present study is based on a DC-cast Mg–6% Zn–0.5% Zr (ZK60) alloy which uses the exceptional grain refining ability of Zr to refine the microstructure. The refining action of Zr is based on the peritec- tic reaction between solid Zr particles added to the melt and the molten Mg. The initial solid solution formed around Zr particles has much higher Zr concentration than its immediate neighbor- hood and these regions are usually referred as Zr-rich cores [4]. An earlier study by the same authors has shown that microstruc- ture of the alloy in DC-cast condition contains Zr-rich cores within the grains where the concentration of Zr is over 1 wt.% [5]. Other microstructural features of the alloy are Zn-rich rings at the grain boundaries, some Mg–Zn eutectic constituents and the Zn–Zr inter- metalics [5]. In addition to grain refinement, Zr addition enhances the homogeneity of the microstructure by making the grains round, and reduces the amount of eutectic at the grain boundaries, so that more Zn can go in the solid solution and contribute in strength [4]. During plastic deformation, the Zr-rich cores and the intermetallic particles strongly affect the recrystallization behavior of the ZK60 alloy and the DRX is suppressed [4,6]. 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.06.123