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 degrees 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)& alpha; plane in the peak-aged condition and transform into tri-atomic layer & eta;'' and & eta;' plates with a thickness of a single unit-cell height. The & eta;' 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 & eta;1 plates that precipitate after the & eta;' plates leads to a different precipitate microstructure evolution from the Mg-RE-Zn system. The & eta;1 phase (Mg7Ca2Zn3) is unevenly distributed in the matrix after 100 h of aging and finally evolves to the equilibrium & eta; phase (Mg10Ca3Zn6) 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. & RARR; G.P. zones & RARR; & eta;'' & RARR; & eta;' & RARR; & eta;' pairs and stacks / & eta;1 & RARR; & eta;