Phase Structures And Transformations Of Long-Period Stacking Ordered Phase In The Mg-TM-Y(Tm=Cu, Ni, Zn) Systems

As a new type of effective strengthening phase in magnesium-based alloys, Long-period ordered (LPSO) structure attracts many attentions at present. The LPSO phase has been reported in the Mg-Cu-Y, Mg-Ni-Y and Mg-Zn-Y systems, but the phase structure and transformation of the LPSO is not clear. In this paper, the phase equilibria in the Mg-rich region of the Mg-Cu-Y, Mg-Ni-Y and Mg-Zn-Y ternary alloys have been studied by SEM, EPMA and XRD experiments.The phase transformations of the alloys have been studied by DSC. The LPSO structures of as-cast alloys and heat-treated alloys have been studied by TEM. The experimental results are as follows:(1) Phase equilibra in the Mg-rich region of the Mg-Cu-Y system at300,400and450℃, of the Mg-Ni-Y system at300,400and500℃, of the Mg-Zn-Y system at400and500℃have been established, and the partial isothermal sections of these systems haveen developted.(2) The LPSO phase has been found to be stable in the Mg-Cu-Y, Mg-Ni-Y and Mg-Zn-Y systems. Structure analysis showed that14H structure is an equilibrium state in the above ternary alloys at low temperature. It has limited composition ranges of3.8-4.7at.%Cu and3.8-5.8at.%Y in the Mg-Cu-Y system,3.1-4.4at.%Ni and3.4-6.5at.%Y in the Mg-Ni-Y system and3.6-4.3at.%Zn and5.0-6.0at.%Y in the Mg-Zn-Y system.(3) Except for14H structure, a few of18R structure occurs in the as-cast Mg-Cu-Y, Mg-Ni-Y and Mg-Zn-Y alloys. The18R structure, after long-time diffusion at lower temperatures, shrinks and finally disappears, which means that18R is metastable at low temperatures. The18R structure usually has higher alloy contents than the14H structure.(4) A four-phase reaction of L+(Mg)→14H+Mg2Cu has been determined in the Mg-Cu-Y ternary system around442℃; While in the Mg-Ni-Y system, two invariant reactions have been detected, which are the eutectic type:L→(Mg)+14H+Mg2Ni happening around492℃, and the quasi-peritectic type:L+(Mg)→14H+Mg24Ys around545℃; In the Mg-Zn-Y system, three invariant reactions have been detected around527,537and542℃.