  The composition, crystal structure and precipitation crystallography of a
newly found precipitate are characterized by a Cs-corrected scanning
transmission electron microscopy (STEM). The composition of the plate-like
precipitate could be expressed as MgxAl2-xGd (x = 0.38), and its crystal
structure is the same as the fcc type Laves phases Mg2Gd and Al2Gd with the
lattice parameter of 7.92 Angstrom (space group No. 227, Fd-3m). The
orientation relationship between the matrix and precipitate is found to be
(0001)m // (111)p and [10-10]m // [1-10]p, and the habit plane is parallel to
(0001)m // (111)p plane. In addition, this preferred crystallography of phase
transformation is well explained based on the atomic matching in the interface.
