Phase equilibria and transformations in ternary Mg-rich Mg–Y–Zn alloys J. Gro ¨ bner a , A. Kozlov a , X.Y. Fang b,c , J. Geng b , J.F. Nie b , R. Schmid-Fetzer a,⇑ a Institute of Metallurgy, Clausthal University of Technology, Robert-Koch-Str. 42, D-38678 Clausthal-Zellerfeld, Germany b Department of Materials Engineering, Monash University, Victoria 3800, Australia c Monash Centre for Electron Microscopy, Monash University, Victoria 3800, Australia Received 1 February 2012; received in revised form 21 May 2012; accepted 29 May 2012 Available online 1 September 2012 Abstract The long-period stacking ordered structures 18R and 14H formed in Mg–Y–X (X = Zn, Cu, Ni) systems have received considerable interest over the past decade, but their thermal stability and relationships with other intermetallic phases in the Mg–Y–X systems remain to be unambiguously established. In this study, the occurrence and transformations of long-period stacking ordered structures 18R and 14H are clarified in as-cast and heat-treated Mg–Y–Zn alloys. The 18R structure is a stable equilibrium phase that forms directly from the melt whereas the 14H cannot form directly from the melt but forms in a solid-state transformation. That explains the absence of 14H in the as-cast microstructures of the alloys. These findings are embedded in the complete description of Mg–Y–Zn phase equilibria, gen- erated by Calphad-type thermodynamic calculations and verified for a range of Mg-rich alloys by electron microscopy and thermal anal- ysis. It is found that the 18R is a stable equilibrium phase that exists in the high temperature range from 753 to 483 °C, and that the 14H is an equilibrium phase below 537 °C. In the small temperature range between 537 and 483 °C, the 18R and 14H can co-exist in equi- librium in some special alloy compositions. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Mg alloys; Long-range ordering; CALPHAD; Phase diagram; Transmission electron microscopy 1. Introduction Magnesium alloys based on the Mg–Y–X (X = Zn, Cu, Ni) systems have received considerable interest over the past decade for producing high strength products, bulk metallic glasses, and hydrogen storage materials [1–3]. Among this group of Mg–Y–X alloys, the Mg–Y–Zn ter- nary alloys can exhibit an impressive yield strength of over 600 MPa, together with an elongation to fracture of 5%, when they are produced by rapid solidification processing and hot extrusion [1]. These unique tensile properties are reportedly due to the formation of nanoscale intermetallics of a long-period stacking ordered (LPSO) structure and their ability to prevent the growth of deformation twins [1,4]. The LPSO structure was initially designated 6H [5] which was subsequently proved to be incorrect. An 18R structure was proposed for the intermetallics [6]. The 18R structure is observed not only in the rapidly solidified sam- ples, but also in samples produced by the conventional casting methods such as permanent mould casting. The 18R structure was replaced by another LPSO structure des- ignated 14H after isothermal heat treatment at tempera- tures of 350–500 °C [4,6–10]. This 14H structure was rarely observed in the as-cast microstructures. While the accumulated experimental evidence indicates that the 14H is very likely an equilibrium phase in the Mg–Y–Zn ternary system, it remains to be established (i) whether the 18R structure is a metastable phase or an equilibrium phase that exists only at a narrow high temperature range in the ter- nary Mg–Y–Zn system, (ii) whether the 14H structure can form directly from the melt during conventional casting, and (iii) whether 18R and 14H can co-exist at some particular temperatures. The unambiguous answers to such 1359-6454/$36.00 Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2012.05.035 ⇑ Corresponding author. Tel.: +49 5323 722150; fax: +49 5323 72 3120. E-mail address: schmid-fetzer@tu-clausthal.de (R. Schmid-Fetzer). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 60 (2012) 5948–5962