Precipitation and excess solid solubility in MgAlZr and MgZnZr processed by spray atomization and deposition

Abstract The microstructures of two magnesium alloys (Mg-5.6wt.%Zn-0.3wt.%Zr and Mg-8.4wt.%Al-0.2wt.%Zr) processed by spray atomization and deposition were examined and subsequently correlated with the processing history. The microstructural characterization was accomplished utilizing semiquantitative energy-dispersive X-ray analysis (EDAX), scanning electron microscopy and X-ray diffraction analysis. The microstructure of the as-spray-deposited MgZnZr alloy consisted of relatively coarse (50–100 μm) equiaxed cells. Lattice parameter measurements using X-ray diffraction revealed a super-saturated solid solutio of zinc in the magnesium matrix. The presence of the Mg7Zn3 phase at the cell boundaries of the MgZnZr alloy was confirmed through semiquantitative EDAX analysis. The microstructure of the MgAlZr alloy in the as-deposited condition consisted of relatively coarse (20–50 μm) cells of magnesium surrounded by an intergranular eutectic microsconstituent which was identified as (Mg) + Mg17Al12; X-ray diffraction results also revealed a supersaturated solid solution of aluminum in the magnesium matrix. The aforementioned microstructural features were related to the solidification conditions of the droplets at the moment of impact with the substrate; these conditions were computed from previously developed numerical models. Furthermore, the effect of the magnitude of Km (growth coefficient in the kinetic relationship between growth rate R and undercooling ΔT ∗ ) on the computed fraction solidified as a function of flight distance is discussed. The present results indicate a good correlation between the experimental findings and the numerical predictions.

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