Study On The Phase Equilibria Of Long Period Ordered (LPO) Phase In The Mg-Cu-RE(RE=Y, Gd) Alloys

Magnesium alloy have wide applications due to their high specific strength, specific stiffness and excellent damping characteristic.However, the low yield strength, ductility and poor elevated temperature creep limit the development of magnesium alloys. Recently it has been found that an LPO phase increases the strength effectively without destroying the ductility. Since then, more attentions have been paid to the preparation and performances of the LPO magnesium alloy, however, the works on the phase equilibria of the LPO magnesium alloys are scarce. In this work, the phase relationships in the Mg-rich region of the Mg-Cu-Y and Mg-Cu-Gd alloys have been studied by experiments. The results will provide reliable date for alloy design of the LPO magnesium alloys.Different experimental techniques including optical optical microscope, scanning electronic microscope, energy-dispersive X-ray analysis and X-ray diffraction analysis have been done to the Mg-rich region of the Mg-Cu-Y alloys at300、400and450℃,and to the Mg-Cu-Gd alloys at440℃. The experimental results are as follows:(1) A ternary intermetallic compound X phase, which is the LPO phase, has been found to be stable in the Mg-Cu-Y system. The X phase gets equilibrium with (Mg) phase, with a composition range of4.33～5.36at.%Cu and5.41-7.15at.%Y. The two-phase region of (Mg)+X is narrow, and at a higher Y content, Mg24Y5phase precipitates, while at a higher Cu content, the brittle Mg2Cu phase precipitates at300and400℃, and at a higher temperatures above450℃, the liquid phase occurs.(2) Similar to the phase relationship in the Mg-Cu-Y system, the X phase is also stable in the Mg-Cu-Gd system, which gets equilibrium with (Mg) phase. The X phase has a composition of6.90～7.88at.%Cu and6.86～7.36at.%.Gd. Three-phase equilibria of (Mg)+X+Mg5Gd and (Mg)+X+Mg2Cu have been detected at lower temperatures.(3) With the analysis of scanning electricity microscope and differential scanning calorimetry, and based on the phase relationships established at400and450℃, a four-phase reaction of L+(Mg)�'+Mg2Cu has been determined in the Mg-Cu-Y ternary system happening around442℃.