Precipitation Sequence in Mg-Zn-Sn Based Alloys
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Precipitation Sequence in Mg-Zn-Sn Based Alloys Anton Gorny, Alexander Katsman, Dmitry Shepelev, and Menahem Bamberger Department of Materials Engineering, Technion IIT, Technion City, Haifa, 32000, Israel ABSTRACT Extensive experimental research work has been carried out in order to develop an Mg-Zn-Sn based creep resistant alloy. It was found that the addition of Y improves the structural stability of this alloy. The main goal of this work was to investigate precipitation sequence in Mg-Zn-SnY alloys during aging at different temperatures. In order to elucidate the sequence of phase formation, XRD, TEM, SEM and EDS analyses were applied. It was found that, firstly, an hcpMgZn2 phase nucleates homogeneously, and grows in the form of needles having coherent interphase boundaries with α-Mg-matrix. High-resolution TEM images in combination with Fourier transformed images reveal a topotaxial orientation relationship which is given by {110}MgZn 2 < 100 > MgZn 2 (002) α − Mg < 110 > α − Mg . The cubic fcc Mg2Sn-phase nucleates heterogeneously, mainly at the tips of MgZn2-needles when the needle’s width reaches a certain critical value. The orientation of Mg2Sn-precipitates is usually perpendicular to the one of MgZn2-needles. Two-phase T-like particles are very typical of alloys aged for 1-16 days at 175225°C. The width/length ratio of MgZn2-needles inside T-like particles differs substantially from the one found in single needles. The elastic/surface energy balance of needles and its influence on the morphology and coarsening behaviour have been analyzed. INTRODUCTION Extensive experimental research work was devoted to the Mg-Zn-Sn system that is considered a promising candidate for a creep resistant Mg-alloy due to precipitation hardening [1-6]. Precipitation of MgZn2 and Mg2Sn intermetallic phases distributed within α-Mg grains was reported in [3,4]. It was also found that small yttrium additions improve the structural stability of this alloy [7]. However, sequence and mutual influence of different phases on their nucleation and growth were not investigated in detail. The main goal of this work was in-depth investigation of the precipitation sequence and nucleation peculiarities of MgZn2 and Mg2Sn phases, and their mutual influence during growth and coarsening. EXPERIMENTAL DETAILS Pure Magnesium of 99.98% was melted in a cemented graphite crucible under protective atmosphere of 1 liter/min CO2 and 60cc/min CHF134A gas mixture. 99.8% pure zinc and 99.95% pure Sn and Y were added to the melt. The melt was poured at a temperature of 7200C into a steel disc shaped mold of 60 mm in diameter and 9 mm thick.The composition of casting is Mg-4.01%Sn-4.47%Zn-0.57%Y (Dirats Lab USA). Samples for solution treatment were encapsulated in a quartz tube filled with Ar at a pressure of 400mmHg. The solution-treatment includes the following steps: 96h at 300°C, heating to 440°C at a rate of 1°C/h and holding at this temperature for additional 96h, followed by water quenching. Aging was conducted in a molten salt (sodium
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