Materials Science and Engineering A 387–389 (2004) 41–45 High-temperature behaviour of as die-cast and heat treated Mg–Al–Si AS21X magnesium alloy E. Evangelista a , E. Gariboldi b,∗ , O. Lohne c , S. Spigarelli a a INFM-Mechanical Department, University of Ancona, Via Brecce Bianche, 60131 Ancona, Italy b Mechanical Department, Polithecnic of Milan, Via La Masa 34, 20158 Milano, Italy c Sintef Materials Technology, N-7465, Trondheim, Norway Received 26 August 2003; received in revised form 13 February 2004 Abstract The mechanical behaviour of the magnesium alloy AS21X (Mg–2% Al–1% Si alloy AS21 modified by addition of rare-earth elements) was experimentally investigated in the as die-cast and in the solution treated and artificially aged conditions in the temperature range from 70 to 210 ◦ C. The as die-cast microstructure was characterized by coring effects and by the presence of Mg 2 Si and Mg 17 Al 12 particles. Holding at temperatures below 180 ◦ C caused massive formation of fine-scale precipitates at the grain boundaries. The homogeneous distribution of Al in the Mg-rich -phase of heat treated material caused at these temperatures the formation of further amounts of Mg 17 Al 12 particles, also within the grains. At 210 ◦ C, Mg 17 Al 12 particles dissolved in both material conditions. At low temperatures, longer times to creep rupture and better creep resistance were displayed by the as cast (AC) material. The properties in the heat treated (HT) condition approached those of the as cast material at the highest test temperature. The suggested deformation mechanism of the alloy up to 180 ◦ C is climb-controlled dislocation creep, for which core diffusion could play a significant role at 70 ◦ C in the as cast material. © 2004 Elsevier B.V. All rights reserved. Keywords: AS21X magnesium alloy; Heat treatment; Creep; Hot tensile behaviour; Microstructure evolution 1. Introduction The high temperature structural stability and mechanical behaviour of Mg die-cast alloys are of great importance for structural parts where thermal exposure up to 150–180 ◦ C can be experienced during service. Nevertheless, even mod- erate temperatures, below 100 ◦ C, were reported to cause marked changes of the non-equilibrium structure and me- chanical properties of die-cast alloys [1,2], and led to creep phenomena of importance for industrial components [3–5]. Among Mg die-cast alloys, the Mg–2% Al–1% Si alloy (AS21) was specifically developed as a creep-resistant alloy for use in car engines [6]. The presence of Mg 2 Si particles at grain boundary was reported to enhance the long-term creep properties of this alloy so that in the low strain rate and high temperature regime, the material exhibits better creep resis- tance with respect to the widely diffused Mg–9% Al–1% Zn ∗ Corresponding author. Tel.: +39-02-3994823; fax: +39-02-70638202. E-mail address: elisabetta.gariboldi@polimi.it (E. Gariboldi). (AZ91) and to the other alloys containing only Mg 17 Al 12 particles [7]. The AS21 alloy has recently been modified by minor additions of rare earth in order to improve its corro- sion resistance [8]. The high-temperature behaviour of such modified alloy, AS21X, has been studied in the present pa- per in as die-cast and solution treated and aged conditions. 2. Material and experimental procedures The Mg–Al–Si alloy AS21X, of chemical composition (mass%): Al = 2.1, Mn = 0.09, Si = 0.94, Zn = 0.16, RE = 0.1, Mg = balance, was investigated in the present study. Test bars having the shape of cylindrical specimens of 6-mm gauge diameter were die-cast at Hydro Light Metals Re- search Centre in Norway by a cold-chamber machine. Ex- periments were carried out both on specimens in the as cast (AC) condition and in the heat treated (HT) condition. The heat treatment consisted in a solution quenching at 415 ◦ C for 2 h and water quenching followed by aging at 150 ◦ C for 4 h. Such heat treatment corresponds to that reported as optimal for the mechanical properties of AZ91D, usually 0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.02.077