Analysis of the Structure and Orientation of the Plate-shaped Precipitate in a Mg-Gd-Zn Alloy

Precipitates in a Mg-2Gd-1Zn (at%) alloy, aged at 573 K for 100 h after forging, have been characterized using transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). TEM observation reveals that the precipitates have thin plate morphology. The energy dispersive X-ray spectrometry (EDX) and selected area electron diffraction (SAED) pattern have indicated that the plate-shaped precipitate has an fcc structure and a lattice parameter of approximately 2.23 nm, which is identical to β-Mg5Gd. It is suggested that the most probable dominant orientation relationship between the β-Mg5Gd precipitate and α-Mg matrix is (1120)α // (001)β and [0001]α // [110]β based on the crystal orientation analysis using EBSD. It is speculated that the strengthening in the Mg-Gd-Zn alloy is attributable to the plate-shaped β-Mg5Gd precipitates. The precipitates which are formed on prismatic planes of α-Mg matrix during the heat treatment can provide the most effective obstacles to basal dislocation slip.