Investigation On Phase Equilibria Of Mg-Ni-RE(Re=Y, Gd) Alloys In The Mg-rich Regions

Recently more attentions have been paid to "LPO"(long period ordered) magne-sium alloys for their excellent performances. In2001, Kawamura et al. prepared an LPO-reinforced Mg97Zn1Y2alloy by rapidly solidified powder metallurgy, which has extremely high yield strength up to610MPa with an elongation of5%at room tem-perature. Further studies have shown that such high strength mainly comes from the refined grain and the LPO phase formed in the alloy. It has been reported that there is also LPO phase in Mg-Ni-RE(RE=Y,Gd) magnesium alloys, but the investigation of the content is scarce, and especially, the composition and thermodynamic stability of the LPO phase has not been clear. Accurate and reliable information on the Mg-Ni-RE(RE=Y,Gd) phase diagrams is important for the development of LPO magnesium alloys.In this work, the phase equilibria of the Mg-Ni-Y and Mg-Ni-Gd ternary systems in the Mg-rich region from300to500℃have been studied systematically by ex-perimental methods, including differential thermal analysis, scanning electron mi-croscopy, electronic probe analysis and X-ray analysis. The results are as follows.(1) There is one three intermetallic compound of X phase, that is the LPO phase, in the Mg-Ni-Y ternary system getting equilibrium with (Mg). The X phase has a high thermodynamic stability which can be stable up to500℃. The composition range of the X phase has been determined to be3.42-5.85at.%Ni and3.82-11.50at.%Y. The two-phase region of (Mg)+X has a narrow composition region, and a higher Ni con-tent will bring about the precipitation of a Laves Mg2Ni phase, while a higher Y con-tent induces Mg24Y5phase.(2) Ternary isothermal sections of the Mg-Ni-Y system at300,400and500℃have been established. In the Mg-rich region at lower temperatures there exist (Mg)+X+Mg2Ni and (Mg)+X+M24Y5ternary-phase equilibria, while at higher tem-peratures above500℃, there exist (Mg)+X+M2Ni and (Mg)+X+L instead.(3) With the results of SEM and DSC analysis, and based on the phase relations determined at400and500℃, a ternary eutectic reaction of Lâ†'(Mg)+X+M2Ni has been determined in the Mg-Ni-Y ternary system at492℃.(4) Similar to the Mg-Ni-Y system, the ternary X phase is also stable in the Mg-Ni-Gd system, which gets equilibrium with (Mg). The composition range of the X phase has been determined to be3.68-5.34at.%Ni and5.67-8.68at.%Gd. The (Mg)+X two-phase region has been determined, and at a higher Ni content, Mg2Ni precipitates, while at a higher Gd content, Mg5Gd phase precipitates.(5) The ternary isothermal sections of the Mg-Ni-Gd system at450and500℃have been established. Ternary-phase equilibria of (Mg)+X+Mg2Ni and (Mg)+X+have been detected at450℃; while at500℃, the Mg2Ni phase disappears from the Mg-rich region.t...