http://journals.cambridge.org Downloaded: 10 Feb 2015 IP address: 132.68.209.156 Precipitation- and stress-influenced coarsening in Mg-based Mg–Zn–Sn–Y and Mg–Zn–Sn–Sb alloys Anton Gorny and Alexander Katsman a) Department of Materials Engineering, Technion—Israel Institute of Technology, Haifa 32000, Israel (Received 4 July 2007; accepted 18 October 2007) Extensive experimental research work has been carried out to investigate precipitation peculiarities in Mg–Zn–Sn-based alloys during aging at different temperatures. This in-depth research was conducted on Mg–4.4wt%Zn–4.0wt%Sn–0.6wt%Y and Mg–4.4wt%Zn–4.4wt%Sn–1.1wt%Sb using x-ray diffraction (XRD), transmission electron microscopy (TEM) including high-resolution TEM, and scanning electron microscopy (SEM) equipped with an energy-dispersive x-ray spectrometer (EDS). It was found that, first, a hexagonal close-packed (hcp)-MgZn 2 phase nucleates and grows in the form of needles having coherent interphase boundaries with -Mg matrix. Then the face-centered cubic (fcc)-Mg 2 Sn-phase nucleates heterogeneously, mainly at the tips of MgZn 2 needles. A very certain mutual orientation of crystal lattices of MgZn 2 , Mg 2 Sn, and -Mg matrix was revealed. The orientation of Mg 2 Sn precipitates is perpendicular to that of MgZn 2 needles. They grow in the form of plates parallel to the basal planes of -Mg matrix. Two-phase T-like particles are very typical of alloys aged for 1 to 16 days at 175 to 225 °C. The width/length ratio of MgZn 2 needles inside T-like particles differs substantially from that found in single needles. The elastic/surface energy balance of needles and its influence on the morphology and coarsening behavior has been analyzed. I. INTRODUCTION Extensive experimental research work was devoted to the Mg–Zn–Sn system, which is considered a promising candidate for a creep-resistant Mg alloy due to precipi- tation hardening. 1–6 Precipitation of hexagonal MgZn 2 and cubic Mg 2 Sn intermetallic phases distributed within -Mg grains was reported in Refs. 3 and 4. Needlelike MgZn 2 precipitates and platelike Mg 2 Sn precipitates were found to be responsible for the strengthening of the -Mg matrix. The coarsening process leading to overag- ing substantially diminishes the strengthening effect. 1,3 It was also found that small yttrium and antimonium addi- tions improve the structural stability of this alloy by in- troducing additional subgrain microstructure. 7 High- temperature phases, MgSnY or Mg 3 Sb 2 , formed during solidification are concentrated at subgrain and grain boundaries preventing a subgrain growth during heat treatment. They remain unchanged during solution treat- ment and aging at 175 to 225 °C and do not directly influence precipitation of MgZn 2 and Mg 2 Sn phases. Their presence manifests itself in retaining an additional subgrain microstructure (in comparison with the base Mg–Zn–Sn alloy) that leads, after aging, to decreasing the effective grain size. 8 At the same time, some important features of the mi- crostructure evolution remained unclear, for example, the occurrence of thin single MgZn 2 needles elongated dur- ing long aging times simultaneously with the formation and coarsening of T-like particles consisting of MgZn 2 and Mg 2 Sn phases. The mutual influence of different phases on their nucleation, growth, and coarsening was not investigated in detail up until now. The main goal of this work was in-depth investigation of the precipitation sequence and nucleation peculiarities of MgZn 2 and Mg 2 Sn phases in Mg–Zn–Sn–Y(Sb) alloys and their mu- tual influence during growth and coarsening. II. EXPERIMENTAL DETAILS Pure magnesium of 99.98% was melted in a cemented graphite crucible under protective atmosphere. Pure zinc (99.8%) and 99.95% pure Sn were added to the melt. Yttrium was added to the melt as a Mg–Y prealloy con- taining about 30 at.% Y (alloy 1). Pure Sb (99.95%) was added to the melt (alloy 2). The melt was poured at 720 °C into a steel disc-shaped mold 60 mm in diameter and 9 mm thick. The mold was heated up to 300 °C. The a) Address all correspondence to this author. e-mail: akatsman@tx.technion.ac.il DOI: 10.1557/JMR.2008.0166 J. Mater. Res., Vol. 23, No. 5, May 2008 © 2008 Materials Research Society 1228