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

Faults are no longer a fault !

The demand for light weight structural materials in construction has been on the rise because of the recent trends of high-rise structures. Typically, a material that fits this category should possess certain properties such as high strength (tensile or compressive), be eco-friendly, be readily available, be affordable; but above all, it should be light-weight. To this end, researchers have explored a wide range of materials. So far, magnesium alloys have shown promising properties for use as next generation structural materials. Specifically, ternary Mg alloys containing a few atomic percent of transition metal and rare-earth elements have recently garnered much attention because of their remarkable high-strength, which is attributable to their unique long-period structures termed as the long-period stacking/order (LPSO) phases. The LPSO structures are basically long-period stacking polytypes of an original hexagonal-closed-packed (hcp) Mg structure, for the series of which chemical order along the c-axis occurs so as to synchronize with the relevant stacking order. For further development of the LPSO-Mg alloys, it has become increasing importance to understand their phase formation behaviors and thermodynamic stabilities.

For instance, given the ideal stoichiometric LPSO structure models supported with an energetic stability, thermodynamic analyses have been attempted based on the calculation of phase diagrams (CALPHAD) method and confirmed that the LPSO phases are indeed stable at finite temperatures. Unfortunately, despite the fact that stoichiometric LPSO structures have been well established as the sufficiently stable phase, the thermodynamic origin of the LPSO phase is not fully understood yet. To address this, researchers from the University of Tokyo, National Institute for Materials Science and Tohoku University: Dr. M. Egami, Dr. I. Ohnuma, Dr. M. Enoki, Professor H. Ohtani and led by Professor Eiji Abe employed a thorough thermodynamic analysis that considered the solute partitioning behaviors between the hcp and fcc phases in the Mg-Zn-Y alloys, based on their Gibbs energy comparison using CALPHAD over the Mg-Zn-Y ternary composition ranges. Their work is currently published in the research journal, Materials and Design.

By knowing that the solute enriched stacking-fault (SESF) in the hexagonal-close-packed (hcp) Mg matrix forms a local face-centered-cubic (fcc) environment, the research team conducted a thermodynamic analysis based on the Gibbs energy comparison between hcp and fcc phases over the Mg-Zn-Y ternary composition ranges, using the calculation of phase diagrams method aided by the first principles calculations.

The authors reported that the Zn/Y co-segregations at the SESF provided a remarkable condition whereby the fcc layers become more stable than the hcp-Mg matrix. Furthermore, within the SESF, the following spinodal-like decomposition into the Mg-rich solid-solution and the Zn/Y-rich L1₂-type order phase was reported to cause a significant reduction of the total Gibbs energy of the system.

In summary, the study investigated the thermodynamic origins of the SESFs formed in the dilute Mg-Zn-Y alloys. Based on SEM/STEM observations, the entire composition of the dilute Mg-Zn-Y grain and the volume fraction of the SESFs were experimentally determined. The researchers reported that the spontaneous thermodynamic behaviors observed could explain why the fault layers can be remarkably stabilized in the LPSO-forming ternary Mg alloys. In a statement to Advances in Engineering, Professor Eiji Abe, the lead author highlighted that their approach may further be extended for the other metal matrix for exploring the novel LPSO-related and SESF layer structures; for example, for hcp-Ti, fcc-Al alloys.