Revisited precipitation process in dilute Mg-Ca-Zn alloys

• Published in: Acta materialia Vol. 257; p. 119072
• Main Authors: Li, Z.H., Cheng, D., Wang, K., Hoglund, E.R., Howe, J.M., Zhou, B.C., Sasaki, T.T., Ohkubo, T., Hono, K.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15.09.2023
• Subjects: 3DAP; First-principles calculation; HAADF-STEM; Magnesium alloys; Precipitation; 3DAP; Precipitation; First-principles calculation; HAADF-STEM; Magnesium alloys
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2023.119072

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 η1 plates that precipitate after the η' plates leads to a different precipitate microstructure evolution from the Mg-RE-Zn system. The η1 phase (Mg7Ca2Zn3) is unevenly distributed in the matrix after 100 h of aging and finally evolves to the equilibrium η 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. → G.P. zones → η'' → η' → η' pairs and stacks / η1 → η [Display omitted]