Thermodynamic analysis on a segregation behavior of alloying elements to stacking faults in Mg-Y-Zn based LPSO structures

A long period stacking ordered (LPSO) structure in Mg-based alloys is characterized by the periodical arrangement of stacking faults introduced to hcp matrix lattice. Alloying elements of rare earth atoms and transition metal atoms segregate to the stacking faults (SFs) and form chemically ordered structures in the layers. The atomic arrangement of matrix phase and SF layer in the LPSO structures corresponds to those in hcp and fcc structures, respectively. Thus in the present work, the free energies for the hcp and fcc phases in the Mg-Y-Zn ternary system obtained from the first-principles calculations combined with the cluster variation method were introduced to the CALPHAD method. Segregation coefficients between hcp matrix and fcc SF were calculated applying the parallel tangent law. The calculated results showed that the Y and Zn atoms segregate to the SFs, mainly because of the difference in chemical potentials for each element between fcc and hcp.