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We investigate thermodynamic behaviors of dilute Mg-Zn-Y ternary alloys toform a unique solute-enriched stacking-fault (SESF), which is an intrinsic-IItype stacking-fault (I2-SF) enriched by the Zn and Y atoms and represents thestructural-unit of the long-period stacking/order (LPSO) phase. SESF in thehexagonal-close-packed (hcp) Mg matrix forms a local face-centered-cubic (fcc)environment, and hence our thermodynamic analysis is based on the Gibbs energycomparison between hcp and fcc phases over the Mg-Zn-Y ternary compositionranges, using the calculation of phase diagrams (CALPHAD) method aided by thefirst principles calculations. Segregation behaviors of solute Zn/Y atoms intothe SESF are firstly estimated according to the Hillert's parallel tangent law,followed by the possible disorder-order phase transformation within the SESFusing the multiple-sublattice model. We find that the Zn/Y co-segregations atthe SESF provide a remarkable condition that the fcc layers become more stablethan the hcp-Mg matrix. Besides, within the SESF, the following spinodal-likedecomposition into the Mg-rich solid-solution and the Zn/Y-rich L12-type orderphase causes a significant reduction of the total Gibbs energy of the system.These thermodynamic behaviors explain fairly well a phenomenological origin ofthe Zn-Y clustering with the L12-type short-range order, which is known tooccur for the LPSO phases and also confirmed for the present SESF by electronmicroscopy experiments. Therefore, strong Zn-Y interactions even in diluteconditions play a key role to stabilize firmly the SESF in the Mg-Zn-Y alloys.

Thermodynamic origin of solute-enriched stacking-fault in dilute Mg-Zn-Y alloys