2014 Volume 49 Issue 3 Pages 181-189
Crystal structures of long-period stacking-ordered (LPSO) phases in the Mg-TM (transition-metal) -RE(rare-earth) systems were investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and transmission electron microscopy (TEM). The 18R-type LPSO phase in the Mg-Al-Gd system is composed by stacking structural blocks with a fully-ordered atomic arrangement. However, the stacking of structural blocks does not exhibit any long-range order along the stacking direction. Because of these characteristics, the LPSO phase in the Mg-Al-Gd system cannot be described as an ‘LPSO’ phase any longer in a strict sense but as an order-disorder (OD) intermetallic phase with a so-called OD structure. Details of the crystal structure analysis of the Mg-Al-Gd LPSO phases are described on the basis of the OD theory.
