Precipitation in lean Mg-Zn-Ca alloys

While lean Mg-Zn-Ca alloys are promising materials for temporary implants, questions remain on the impact of Zn and Ca on the microstructure. In this context, the precipitation of Zn and Ca in Mg-1.5Zn-0.25Ca (in wt.%), initially extruded at 330 & DEG;C, towards Mg-Ca binary precipitates or Ca-Mg-Zn ternary precipitates was probed in a multiscale correlative approach using atom probe tomography (APT) and an-alytical transmission electron microscopy (TEM). Particular focus was set on the ternary precipitate phase whose structure is debated. In the as-extruded material, the binary precipitates are made of hexagonal C14 Mg2Ca containing up to about 3 at.% of Zn. The ternary ones are based on the hexagonal Ca2Mg5Zn5 prototype structure with a composition close to Ca3Mg11Zn4, as deduced from atomically resolved EDS mapping and scanning TEM imaging, supported by simulations. The precipitation sequence was scru-tinized upon linear heating from room temperature to 375 ?, starting from the solutionized material. Three exothermic differential scanning calorimetry (DSC) peaks were observed, at respectively 125, 250 and 320 & DEG;C. Samples were taken after the peak decays, at respectively 205, 260 and 375? for structural analysis. At 205 & DEG;C, APT analysis revealed Ca-rich, Zn-rich and Zn-Ca-rich clusters of about 3 nm in size and with a number density of 5.7 x 10 23 m -3. At 260 ?, APT and TEM showed mono-layered Zn-Ca-rich Guinier-Preston (GP) zones of about 8 nm in size and with a number density of 1.3 x 10 23 m -3. At 375 ?, larger and highly coherent elongated precipitates were found, with a size of about 50 nm. They occur as binary Mg-Ca precipitates or ternary Ca2Mg6Zn3 precipitates, as deduced from scanning TEM-based energy dispersive X-ray spectroscopy (EDS) and nanodiffraction in TEM. Here, the binary precipitates outnumber the ternary ones, while in the as-extruded material the ternary precipitates outnumber the binary ones, which corresponds well to the calculated phase diagram. We correlated the microstruc-ture to hardness probed by Vickers testing. The largest hardening relates to the end of the 125 ? DSC peak and thus to GP zones, which outperform the hardening induced by the nanometer-sized clusters and the larger intermetallic particles. The complexity of the precipitation sequence in lean Mg-Zn-Ca alloys is discussed.(c) 2022 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )

Automated summary

The impact of Zn and Ca on the microstructure of lean Mg-Zn-Ca alloys was investigated. The precipitation of binary and ternary precipitates was observed at different temperatures. Hardness testing revealed that GP zones were the most effective factor in hardening.