J. Mater. Sci. Technol., 2012, 28(5), 407–413. High Temperature Creep and Superplasticity in a Mg–Zn–Zr Alloy S. Spigarelli 1)† , M. El Mehtedi 1) , M. Regev 2) , E. Gariboldi 3) and N. Lecis 3) 1) Dipartimento di Ingegneria Industriale e Scienze Matematiche, Universit´a Politecnica delle Marche, Via Brecce Bianche, Ancona 60131, Italy 2) Mechanical Engineering Dept., ORT Braude College, Karmiel 21982, Israel 3) Dipartimento di Meccanica, Politecnico di Milano, via La Masa 34, 20156 Milano, Italy [Manuscript received June 21, 2011, in revised form November 9, 2011] Creep and superplasticity were investigated by testing a fine-grained extruded Mg–Zn–Zr magnesium alloy under a wide range of applied stress in the temperature range between 100 and 300 ◦ C. Grain boundary sliding became the dominating mechanism at 200 ◦ C, leading to a true superplastic behaviour at 300 ◦ C, where superplasticity was attained even under relatively high strain rates (5×10 −3 s −1 ). By contrast, for lower temperatures, the straining process was controlled by dislocation climb. A comprehensive model, taking into account the simultaneous operation of the different mechanisms, was developed to describe the strain rate dependence on applied stress. KEY WORDS: Creep; Superplasticity; Magnesium alloy 1. Introduction While several studies have analysed the creep re- sponse of Mg–Al alloys (for example, literature [1–3] for a review of some recently published papers), the creep behaviour of Mg–Zn alloys, which were among the earliest materials of the Mg family to be investi- gated and developed, have been investigated by only a few authors [4–6] . Generally speaking, only several recent publications [7–17] indicate that there is a re- newed interest in developing and investigating the high temperature properties of Mg–Zn alloys. Several authors [18–33] have observed a superplastic behaviour in Mg alloys, although in most cases the extremely fine grain size, which is a pre-requisite for superplas- tic forming (SPF), was achieved only by applying se- vere plastic deformation, for example ECAP (equal channel angular pressing). In a few cases, as in the paper published by Watan- abe and Mukai [18] , the ZK60 alloy exhibited a su- † Corresponding author. Prof., Ph.D.; Tel.: +39 071 2204746; Fax: +39 027 2204801; E-mail address: s.spigarelli@univpm.it (S. Spigarelli). perplastic behaviour after a simple extrusion process. This latter conclusion and the lack of creep data on ZK60 alloy let us investigate the high temperature resistance of this material, covering a wide range of experimental conditions. The aim was to identify the different mechanisms governing the high temperature deformation of the extruded ZK60 alloy. 2. Experimental Constant load creep experiments were carried out on samples with 25 mm gauge length and 5 mm di- ameter, machined from a ZK60 extruded rod. Test- ing temperatures were 100, 150, 200 and 300 ◦ C with nominal initial stress ranging from 10 to 150 MPa. Preliminary ageing experiments at 150 ◦ C for up to 216 h were carried out in order to investigate the ther- mal stability of the alloy. The microstructure was studied by means of opti- cal microscopy and using a 20 kV Philips XL30 SEM (scanning electron microscopy) equipped with an Ox- ford energy-dispersive X-ray spectroscopy (EDS) sys- tem and with a 10 kV FEI Quanta 200 scanning elec-