Precipitation hardening of Zr-modified Mg–Ca–Zn alloy

The microstructure and mechanical properties of Mg–Ca–Zn alloys with 1 wt.% Zr were investigated in as-cast and heat-treated conditions. A substantial decrease in grain size (from 65 µm for the Mg–Ca–Zn base alloy to 22 µm) was observed. The alloy was solution treated at 410 °C for up to 96 h followed by aging at 175 °C for up to 24 h. Conventional techniques, X-ray diffraction, EM + EDS, and TEM were used to characterize the microstructure of the alloy. The microstructure obtained after heat treatment had equiaxed grains with evenly distributed binary phase Zn₂Zr. The binary Mg₂Ca and ternary Mg₂Ca₆Zn₃ phases were identified in the matrix and at grain boundaries surrounded by precipitate-depleted zones (PDZs). The thermal stability of the Zr-modified alloys was examined by microhardness measurements conducted after prolonged exposures of the alloys to elevated temperatures. It was found that Zr is a structure-stabilizing factor. Its influence was associated with the formation of Zn₂Zr phase that does not undergo coarsening at the elevated temperatures used (due to the low diffusivity of Zr). The nanoscale mechanical properties of grain boundary PDZs were analyzed using combined nanoindentation and atomic force microscopy. These mechanical properties were then correlated to the composition and precipitate distribution in PDZs. An increase in the solution treatment duration from 10 to 96 h at 410 °C resulted in expansion of PDZs from ~0.75 to ~3 µm, while the following aging at 175 °C for up to 24 h did not lead to a detectable change in PDZs. The analysis indicates that the lowest hardness was found in the region where Zn₂Zr precipitates density was low, regardless of the solute concentration.