The precipitation of the Mg-8Gd-2Er （wt.%） alloy at 200 ℃ was investigated by transmission electron microscopy （TEM） and X-ray diffraction （XRD）. The precipitation sequence was described as supersaturated solid solution （S.S.S.S., cph）→β（D019）→β（bco）→ β-phase（fcc）, which was different from previously reported four stages sequence： （S.S.S.S., cph）→β（DO19）→β（bco）→β（fcc）→β-phase（fcc）. The morphologies and crystal structures of both β＇ and,β precipitates in the alloy were studied in the present investigation. The orientation relationship between the precipitate phases and the α-matrix was observed. The ultimate tensile strength （UTS） and tensile yield strength （TYS） of the as-cast alloy were 189 and 110 MPa, respectively, and the UTS and the YTS were improved greatly after ageing treatment. The values of UTS and YTS after peak ageing process were 308 and 246 MPa, respectively. The improved mechanical properties were mainly ascribed to the formation of β phase. The age hardening response decreased with increasing ageing time because the β phase got coarse as ageing time increased.
Tensile test of the as-cast Mg-6Zn-2Er alloy was conducted at room temperature. The results indicate that the alloy is inclined to failure when the strain reaches 5.6%. The coarse secondary phases are responsible for the failure, especially for the Mg3Zn3Er2 phase （W-phase）. It is indicated that the existence of the W-phase activates the stress concentrations due to the incapacity of W-phase for the load transfer, which results in the void at the inner of the W-phase. In comparison, the interface between the matrix and the secondary phase is stable. In conclusion, the characters of the secondary phases with respect to size, distribution, morphology and type, play an important role in the plastic deformation behavior of the alloy.
Microstructure evolution of the cast Mg-9Gd-2Er-0.4Zr alloy during solid solution treatment at temperature of 460-520 ℃ for 3-12 h was investigated by using optical microscope（OM）,scanning electron microscope（SEM） and transmission electron microscope（TEM）.The results indicated that the grain size and the shape of second phase were obviously changed with time and/or temperature going on.At 460 ℃ for 3 h,the morphology of the Mg5（GdEr） phase was changed into fragmentized island morphology and the volume faction of the phase decreased.After solution solid treatment at 460 ℃ for 6 h,the Mg5（GdEr） phase was already completely dissolved,but some cuboid-shaped RE-rich phase precipitated.As the temperature increased,the morphology of the Mg5（GdEr） phase was transformed into the same morphology as that at 460 ℃ for 6 h.It was suggested that the microstructure evolution of the alloy during the solid solution treatment was concluded as follows：Mg5（GdEr） eutectic phase→Gd/Er atom diffusing into matrix→spheroidic Mg5（GdEr） phase→cuboid-shaped RE-rich phase→grain boundary immigration.
The microstructure and mechanical properties of the Mg-6Zn-xEr alloys in the as-cast, as-extruded and extruded-T5 states were investigated. It is found that the addition of Er has an obvious effect on improving mechanical properties of Mg-6Zn based alloys. The results suggest that the Mg-6Zn-0.5Er alloy in the peak aged state shows the best tensile strength. The ultimate tensile strength and the yield tensile strength of the peak-aged Mg-6Zn-0.5Er alloy are about 329 MPa and 183 MPa, respectively, companying with a good elongation of 12%. The addition of 0.5% Er into the Mg-6Zn based alloys has a great effect on improving ageing hardening response. The highest tensile strength of the peak-aged Mg-6Zn-0.5Er alloy is mainly due to the refinement of microstructure and the precipitation of β1 phase.
