Revisited precipitation process in dilute Mg-Ca-Zn alloys

To obtain thorough understandings of precipitation process in heat-treatable Mg-Ca-Zn alloy, we revisited the precipitation process of a Mg-0.3Ca-0.6 Zn (at.%) dilute alloy during isothermal aging at 200 °C using an aberration-corrected scanning transmission electron microscope, atom probe tomography, and first-principles calculations. The monolayer G.P. zones form on the (0002) _α plane in the peak-aged condition and transform into tri-atomic layer η`` and η` plates with a thickness of a single unit-cell height. The η` plates, then, form in pairs and stacks with energetically favorable 4–5 atomic layers of pure magnesium between the plates. While such a transformation path is similar to that seen in Mg-RE-Zn alloys (RE: rare-earth elements), the unique structure of coarse η<sub>1</sub> plates that precipitate after the η` plates leads to a different precipitate microstructure evolution from the Mg-RE-Zn system. The η<sub>1</sub> phase (Mg <sub>7</sub>Ca <sub>2</sub>Zn <sub>3</sub>) is unevenly distributed in the matrix after 100 h of aging and finally evolves to the equilibrium η phase (Mg <sub>10</sub>Ca <sub>3</sub>Zn <sub>6</sub> ) phase with a hexagonal structure. First-principles calculations of energetics were performed to further identify the crystal structure and stability of the precipitates, supporting the following new precipitation sequence: S.S.S.S. → G.P. zones → η`` → η` → η` pairs and stacks / η₁ → η