Precipitation- and stress-influenced coarsening in Mg-based Mg-Zn-Sn-Y and Mg-Zn-Sn-Sb alloys

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.Owt%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)-MgZn2 phase nucleates and Grows in the form of needles having coherent interphase boundaries with alpha-Mg matrix. Then the face-centered cubic (fcc)-Mg2Sn-phase nucleates heterogeneously, mainly at the tips of MgZn2 needles. A very certain mutual orientation of crystal lattices of MgZn2, Mg2Sn, and alpha-Mg matrix was revealed. The orientation Of Mg2Sn precipitates is perpendicular to that of MgZn2 needles. They grow in the form of plates parallel to the basal planes of alpha-Mg matrix. Two-phase T-like particles are very typical of alloys aged for I to 16 days at 175 to 225 degrees C. The width/length ratio of MgZn2 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.