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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-Sn-Y 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 hcp-MgZn2 phase nucleates homogeneously, and grows in the form of needles having coherent interphase boundaries with Mg-matrix. High-resolution TEM images in combination with Fourier transformed images reveal a topotaxial orientation relationship which is given by {110}MgZn2 <100>MgZn2∥(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 175-225C. 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 has been analyzed.

Ultrasonic waves (frequency 20 kHz and maximum intensity 400 W) were applied in molten Mg alloys during solidification to a cylinder 30 mm in diameter and 100 mm long. Applying this treatment to Mg-4Zn-4Sn-1Si resulted in refinement of the dendritic structure and more uniform dispersion of the Mg 2 Sn and Mg 2 Si precipitates. The same treatment carried out on Mg-1 . 5Ca-3Zn alloy resulted in transition from dendritic solidification to equiaxed structure with a relatively uniform grain size. In addition, the formation of Mg 2 Ca was significantly suppressed.

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