Grain refinement in AZ91 magnesium alloy during thermomechanical processing N.V. Ravi Kumar a,1 , J.J. Blandin a, *, C. Desrayaud b , F. Montheillet b , M. Sue ´ry a a Institut National Polytechnique de Grenoble (INPG), Ge ´nie Physique et Me ´canique des Mate ´riaux (GPM2), U.M.R. C.N.R.S. 5010, F.R. C.N.R.S. 2145, ENSPG, BP 46, 38402 Saint-Martin d’He `res Cedex, France b Ecole des Mines de Saint-Etienne, Centre S.M.S., Plasticite ´, Endommagement et Corrosion des Mate ´riaux, U.R.A. C.N.R.S. 1884, F.R. C.N.R.S. 2145, 153 cours Fauriel, 42000 Saint-Etienne, France Received 22 January 2003; received in revised form 10 April 2003 Abstract Microstructural changes during high-temperature extrusion and torsion of an AZ91 alloy (Mg  /9Al /1Zn, wt.%) were investigated. In the experimental domain studied, dynamic recrystallisation (DRX) occurs and the effect of temperature and strain rate on the resulting recrystallised grain size was investigated. Complete recrystallisation in torsion is associated with the development of a stress plateau after softening from the peak stress, which is systematically observed in the first steps of straining. The resulting grain size can be related to the value of the peak stress. It appears that the precipitation of the Mg 17 Al 12 phase does not affect significantly the torsion behaviour of the alloy in the experimental domain investigated here. This study supports the idea that very fine-scale microstructures (i.e. with a mean grain size smaller than 5 mm) can be easily produced by DRX during high- temperature extrusion of the AZ91 alloy. # 2003 Elsevier B.V. All rights reserved. Keywords: Magnesium alloy; Recrystallisation; Extrusion; Torsion; Grain size 1. Introduction Magnesium alloys are attractive for structural com- ponents in transportation applications due to their high specific properties. Increasing volumes of magnesium are being used in automotive industry to satisfy the demands for fuel efficient and low emission vehicles [1]. Due to their relatively poor workability, the main fabrication route of magnesium alloy parts remains die-casting. Nevertheless, wrought magnesium alloys are increasingly used to produce components with superior mechanical properties. It is well known that mechanical properties of polycrystals at room temperature can be improved through grain refinement, as illustrated by the Hall  /Petch relationship. When microstructures with grain sizes smaller than 10 mm are produced, super- plastic properties may also be obtained [2  /4]. Such fine grains are frequently produced by thermomechanical processing but in the case of Mg alloys, such procedures remain poorly documented, despite some recent studies [5,6]. At room temperature, magnesium and its alloys deform essentially by basal slip and twinning, which limits their formability. Consequently, thermomechani- cal processing is frequently carried out at high tempera- ture. It is generally considered that additional (prismatic and pyramidal) slip systems contribute significantly to deformation only at temperatures higher than 300 8C (i.e. when the associated critical resolved shear stresses are comparable). Nevertheless, it has been also sug- gested that twinning may still be activeeven at high temperature [7]. The deformation mechanisms of pure magnesium [8] and Mg  /0.8%Al [9] were investigated in detail for temperatures between 200 and 520 8C and for strain rates lower than 10 3 s 1 . In the main part of the * Corresponding author. Tel.:  /33-4-76-826341; fax:  /33-4-76- 826382. E-mail address: jean-jacques.blandin@inpg.fr (J.J. Blandin). 1 Present address: Institut fu ¨r Metallkunde und Metallphysik, RWTH Aachen, 52056 Aachen, Germany. Materials and Engineering A359 (2003) 150  /157 www.elsevier.com/locate/msea 0921-5093/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5093(03)00334-4