Formation and Stability of Solute Enriched Stacking Fault in the Mg-Zn-Y, Mg-Co-Y and Mg-Zn-Ca Ternary Systems

Thermodynamic behaviors of the Mg-Zn-Y, Mg-Co-Y and Mg-Zn-Ca ternary systems to form a unique solute-enriched stacking-fault (SESF), which is regarded as the structural-unit of the long-period stacking/order (LPSO) phase, have been investigated. The SESF in the hexagonal-close-packed (hcp) Mg matrix forms a local face-centered-cubic (fcc) environment, and hence our thermodynamic analysis is focused on the Gibbs energy comparison between hcp and fcc phases for arbitrary chemical compositions at finite temperatures in these ternary systems, using the calculation of phase diagrams (CALPHAD) method aided by the first principles calculations. It has been reported that the Zn/Y co-segregations at the SESF provide a remarkable condition that the fcc layers become more stable than the hcp-Mg matrix in the Mg-Zn-Y. Within the SESF, furthermore, the following spinodal-like decomposition into the Mg-rich fcc solid-solution and the Zn/Y-rich L1(2)-type order phase causes a significant reduction of the total Gibbs energy of the system. It was suggested that the L1(2)-type ordering of the Co-rich phase would not take place even though the spinodal-like decomposition is predicted to occur, which makes the LPSO phase metastable due to insufficient gain of the Gibbs energy in the Mg-Co-Y system. In the Mg-Zn-Ca system, the formation of SESF layers is also expected. However, neither the spinodal-like decomposition nor the chemical ordering would occur, which suggests that no Gibbs energy gain is provided for the formation of the LPSO phase. These spontaneous thermodynamic behaviors explain well why the SF layers can be remarkably stabilized in the LPSO-forming ternary Mg alloys, and also clarify phenomenological criteria of the LPSO formation in the Mg-based ternary systems.