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–4.0wt%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)-MgZn₂phase nucleates and grows in the form of needles having coherent interphase boundaries with α-Mg matrix. Then the face-centered cubic (fcc)-Mg₂Sn-phase nucleates heterogeneously, mainly at the tips of MgZn₂needles. A very certain mutual orientation of crystal lattices of MgZn₂, Mg₂Sn, and α-Mg matrix was revealed. The orientation of Mg₂Sn precipitates is perpendicular to that of MgZn₂needles. They grow in the form of plates parallel to the basal planes of α-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 MgZn₂needles 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.