Phase Equilibria And Transformations Of Long-Period Stacking Ordered Phases In The Mg-TM-Gd (TM=Cu, Ni) Systems

Mg-base alloys have been attracted as light weight and high strength structural materials.About Mg-Cu-Gd and Mg-Ni-Gd ternary alloy system, currently focused on alloying’s effect on the properties of alloy. While there were little detail information on phase structure and transformation, especially the phase structure and transformation of a new type of strengthening phase---long-period stacking order structure (LPSO).In this paper, the phase equilibria in the Mg-rich region of the Mg-Cu-Gd and Mg-Ni-Gd ternary alloys have been studied by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) experiments, and further reveals the phase transition rule of LPSO phase. The main results are as follows:(1)Phase equilibra in the Mg-rich region of the Mg-Cu-Gd system at425,445and460℃have been established, and the partial isothermal sections of these systems have been developed.(2) A four-phase quasi-peritectic reaction of L+(Mg)â†'X+Mg2Cu has been determined in the Mg-Cu-Gd ternary system around436℃;While in Mg-Gd side, which are the quasi-peritectic type:L+(Mg)â†'X+Mg5Gd around451℃.(3) Phase equilibra in the Mg-rich region of the Mg-Ni-Gd system at450and490℃have been established, and the partial isothermal sections of these systems have been developed. A ternary eutectic reaction of L℃(Mg)+X+Mg2Ni has been determined in the Mg-Ni-Gd ternary system at482℃.(4) The LPSO phase has been found to be stable in the Mg-Cu-Gd and Mg-Ni-Gd systems. The composition range of the LPSO phase has been determined to be6.9-8.9at.%Cu and6.7-7.5at.%Gd in the Mg-Cu-Gd system.While in the Mg-Ni-Gd system,the composition range of the LPSO phase has been determined to be3.7-5.5at.%Ni and5.5-8.7at.%Gd.