The alloys were prepared in Mg-rich corner of Mg-Zn-Ce system. Partial phase equilibrium relationships of these alloys at 350 ℃ were identified by using scanning electron microscopy(SEM), electron probe microanalysis(EPMA), X-ray diffraction(XRD) analysis and selected area electron diffraction(SAED) pattern analysis of transmission electron microscopy(TEM). Partial isothermal section of Mg-Zn-Ce system in Mg-rich corner was identified. The results show that there is one ternary compound (T-phase) in Mg-Zn-Ce system. The T-phase is a linear ternary compound in which the content of Ce is about 7.7% (molar fraction); while the content of Zn is changed from 19.3% to 43.6% (molar fraction). The crystal structure of T-phase is C-centered orthorhombic. In addition, one two-phase region of Mg+T-phase and one three-phase region of Mg+T-phase+MgZn(Ce) exist in the Mg-rich corner of Mg-Zn-Ce system at 350 ℃.
Alloys with different compositions in Mg-rich corner of Mg-Zn-La system at 400 ℃ were prepared,the phase equilibrium in the Mg-rich corner of Mg-Zn-La system was determined by scanning electron microscopy(SEM),electron probe microanalysis based on energy dispersive X-ray spectroscopy(EPMA-EDS) and X-ray diffraction(XRD),and the isothermal section at 400 ℃ was established. The results show that there exists a ternary compound(T-phase) with a constant La content and changable Mg/Zn contents in the Mg-Zn-La system. The mole fraction of La in T-phase is about 8%,and that of Zn is from 16% to 43%. There exist a two-phase equilibrium which consists of T-phase plus hcp-Mg and a three-phase equilibrium which is composed of T-phase,hcp-Mg and a liquid phase(L-phase) at 400 ℃ . The L-phase consists of 70% Mg,30% Zn and is free of La.
Alloys of Mg-Zn-La system in Mg rich corner were prepared, and the phase relationship was investigated at different temperatures such as 200, 300 and 350 ℃ by scanning electron microscopy(SEM), X-ray diffraction(XRD) and electron probe microanalysis with energy dispersive X-ray spectroscopy (EPMA). Two types of phase equilibrium were identified at the different temperatures. One is two-phase equilibrium contained the Mg solid solution and T-phase. The other is three-phase equilibrium which contained the Mg solid solution, MgZn phase and T-phase. T-phase was stable as the temperature changed. The La content in T-phase is constant which is about 8±0.3% (atom fraction); but the Mg content and Zn content of that is changed, and the Zn content in T-phase was from 16.1% to 44%. The solubility of La in MgZn phase was increased from 1.2% to 1.6% as the temperature increasing from 200 to 350 ℃.
The phase equilibria in the Mg-rich corner of the Mg-Zn-La system at 350°C have been investigated by scanning electron microscopy, X-ray diffraction, and electron probe microanalysis. It has been shown that the linear compound (Mg,Zn)17La2 existed in the Mg-Zn-La system at 350°C. The linear compound (so-called T phase) was with the C-centred orthorhombic crystal structure induced by the solution of significant quantities of the third element. The three-phase region α(Mg) + MgZn(La) + T and the two-phase region composed of the α(Mg) and the linear-compound T phase existed in the Mg-rich corner of the Mg-Zn-La system at 350°C.
LI Hongxiao REN Yuping HUANG Mingli CHEN Qin HAO Shiming
