LPSO STRUCTURE AND AGING PHASES IN Mg-Gd-Zn-Zr ALLOY

At present; long period stacking ordered (LPSO) structures in Mg-RE-Zn (RE = Y, Dy, Ho, Er, Gd, Tb, Tm) alloys have been focused on. According to some reports; Mg-Gd-Zn alloys are classified as type II, i.e., there are no LPSO structures in as-cast alloys; but LPSO structures appear after heat treatment. To further study the formation of LPSO structure in Mg-Gd-Zn(Zr) alloys; a Mg96.32Gd2.50Zn1.00Zr0.18 alloy was prepared by ingot metallurgy (I/M) in this work. Based on OM, SEM and TEM observations; the as-cast microstructure of Mg-Gd-Zn-Zr alloy consists of alpha-Mg solid solution, lamellar 14H-type LPSO structure within alpha-Mg grains and dendritic eutectic structure at grain boundaries; in which the eutectic phase is the beta-phase ((Mg, Zn)(3)Gd)), thus, Mg-Gd-Zn(Zr) alloys should be attributed to type I, i.e.; there are LPSO structures in as-cast alloys. During solid solution treatment at 500 degrees C for 35 h; a solid transformation; dendritic beta-phase with fcc structure -> lamellar X-phase with 14H structure; was observed at grain boundaries. During subsequent peak-aging treatment at 200 degrees C for 128 h, ellipsoidal beta' and rhombus beta(1) phases precipitated within alpha-Mg grains. Tensile tests at room temperature and Vickers hardness tests show that the alloy solution-treated and aging-treated has higher mechanical properties, i.e., sigma(b)=290.7 MPa, sigma(s)=162.5 MPa, delta=10.35% and 108.0 HV. The improvement of mechanical properties are attributed to the composite strengthening-and-toughening effect of the 14H-type LPSO structures, and the beta' and beta(1) aging phases.