ULTRAFINE-GRAINED MATERIALS Enhanced superplasticity of magnesium alloy AZ31 obtained through equal-channel angular pressing with back-pressure R. Lapovok Æ Y. Estrin Æ M. V. Popov Æ S. Rundell Æ T. Williams Received: 6 March 2008 / Accepted: 29 April 2008 / Published online: 17 July 2008 Ó Springer Science+Business Media, LLC 2008 Abstract Excellent superplastic elongations (in excess of 1,200%) were achieved in a commercial cast AZ31 alloy processed by low temperature equal-channel angular pressing (ECAP) with a back-pressure to produce a bimodal grain structure. In contrast, AZ31 alloy processed by ECAP at temperatures higher than 200 °C showed a reasonably uniform grain structure and relatively low ductility. It is suggested that a bimodal grain structure is advantageous because the larger grains contribute to strain hardening thus delaying the onset of necking, while grain boundary sliding associated with small grains provides a stabilizing effect due to enhanced strain rate sensitivity. Introduction Magnesium alloys have a great potential as structural materials for aerospace, automotive and electronics appli- cations owing to their low density and high strength-to- weight ratio. However, the fabrication of magnesium parts (including forging and sheet rolling) is limited due to the relatively low ductility of magnesium alloys owing to their hcp crystal structure. The development of thermo- mechanical processing leading to significantly enhanced superplastic behaviour in magnesium alloys would open up new avenues for industrial applications of superplastically formed magnesium parts. In order to enhance ductility and to establish superplastic properties in magnesium alloys, various methods of thermo- mechanical processing, such as hot-rolling and extrusion, have been used [1–6]. In the last decade, significant efforts to use severe plastic deformation methods, such as equal- channel angular pressing (ECAP), to produce materials with extremely fine grain structure have been made [7, 8]. Com- bining ECAP with conventional forming techniques was also considered as a possible processing route [9, 10]. However, even for widely investigated magnesium alloys, such as AZ31, there is no consensus concerning the optimum thermo-mechanical processing route leading to enhanced tensile ductility. At the present time there is no agreement on the type of microstructures most favourable for increasing the elongation-to-failure. A summary of results for AZ31 published to date is given in Table 1. Analysis of the results presented in Table 1 shows that severe hot extrusion or hot extrusion followed by the ECAP leads to extreme grain refinement and significantly enhanced superplastic properties with a maximum strain of 900% obtained at 300 °C[4] and 460% obtained at 150 °C [9]. Most of the published papers report microstructures with uniform grain distributions varying in size depending on the particular processing route from 0.7 to 250 lm. In this study, straight ECAP without any further pro- cessing steps was used for grain refinement of magnesium alloy AZ31. The effect of the ECAP parameters, such as R. Lapovok (&) Á Y. Estrin Á S. Rundell ARC Centre of Excellence for Design in Light Metals, Department of Materials Engineering, Monash University, Clayton, VIC 3800, Australia e-mail: rimma.lapovok@spme.monash.edu.au Y. Estrin CSIRO Division of Materials Science and Engineering, Clayton, VIC 3168, Australia M. V. Popov IWW, TU Clausthal, Agricolastr. 6, 38678 Clausthal-Zellerfeld, Germany T. Williams Monash Centre for Electron Microscopy, Monash University, Clayton, VIC 3800, Australia 123 J Mater Sci (2008) 43:7372–7378 DOI 10.1007/s10853-008-2685-z