Study on the transformation between W' and W phases in Mg-Zn-Y alloy
LIU Jian-fang;YANG Zhi-qing;YE Heng-qiang;Shenyang National Laboratory for Materials Science,Institute of Metal Research,Chinese Academy of Sciences;
Both face-centered cubic phases W' and W have close relationship with the icosahedral quasicrystal( IQC) phase in Mg-ZnY alloys,but the crystal structure of W' phase remains controversial. Based on electron diffraction analysis,energy dispersive X-ray spectroscopy and aberration-corrected scanning transmission electron microscopy( STEM),it can be concluded that the W' phase has a composition of Zn60. 1Mg23. 9Y16 and which is identified as the Cd45Sm11-type structure. The W' phase is composed of tetrahedral symmetric Friauf polyhedral clusters rather than icosahedral symmetric multiple-shell ones,so the W' phase is not an approximant crystal of IQC. By in situ heating transmission electron microscopy( TEM),a phase transformation from W' to W was observed at 683 K in the as-cast alloy. The W' and W phases have a cube-to-cube orientation relationship,and the W' / W interface is coherent. Close correlation between their basic structural units,cuboctahedral and Friauf clusters,is believed to be responsible for the epitaxial nucleation and growth of W phase on the W' phase.