The effect of different zirconium contents on the grain size of Mg-9Gd-4Y alloys and the grain refinement mechanism of zirconium have been studied. The results reveal that zirconium can refine the grains of the alloys to a large extent, and the grains become finer with an increase of zirconium content. Microstructural analysis shows that there is at least one zirconium rich core in almost each grain in alloys with a high zirconium content, whereas the characteristic zirconium rich cores are not found in the alloy with a low zirconium content. It is suggested that the grain refinement mechanism of zirconium in the low zirconium alloy is different from that in the high zirconium alloys: the zirconium works mainly by restricting grain growth in the low zirconium alloy, and by generating nucleants in the high zirconium alloys.

Sn and Al þ Sn containing coatings were prepared on the surface of AZ91D magnesium alloy by laser surface treatment. The microstructure and composition of specimens were analyzed by metallurgical microscope, scanning electron microscope (SEM), field emission scanning electron microscope (FESEM) equipped with energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The corrosion resistance of the as-received and laser treated magnesium alloys were evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests. The effect of Sn and Al þ Sn addition on the microstructure, composition and corrosion behavior of magnesium alloys was investigated. Results showed that the surface presents different phase composition. The corrosion resistance of magnesium alloys sharply decreased after laser surface cladding (LSC) of Al þ Sn, for the galvanic corrosion between Al and Sn. Conversely, the corrosion resistance was improved by laser surface alloying (LSA) of Sn due to the refinement effect and the protection of Mg 2 Sn and SnO 2 in the grain boundary to the a-Mg matrix.