Crystal Chemistry of a Biodegradable Mg65Ca28Au7 Alloy: Thermal Evolution and Refinement of CaAuMg2

Phase constitution affects localized dissolution and mechanical retention in biodegradable magnesium alloys. We examined melt-spun nominal Mg65Ca28Au7 by X-ray diffraction before and after annealing at differential scanning calorimetry (DSC). Area EDX estimated 64.4 at.% Mg, 26.9 at.% Ca and 8.7 at.% Au. The as-spun pattern is described by Mg2Ca and CaAuMg2 with normalized crystalline fractions of 56.9 and 43.1 wt.%. After-DSC heating in argon to 400 °C, Mg3Au is clearly resolved alongside both phases, with fractions of 51.8 wt.% Mg2Ca, 37.8 wt.% CaAuMg2 and 10.5 wt.% Mg3Au. Mg3Au is not securely distinguishable before heating; its absence is not established. Rietveld refinements yield Rwp = 2.06% and 3.54%. The after-DSC pattern permits refinement of four CaAuMg2 positional parameters in Cmcm, with a = 4.3121(1), b = 10.7642(2) and c = 8.2596(2) Å. Au-centered Mg6 prisms retain the known MgCuAl2 topology; detailed distortions remain texture-sensitive. No significant localized DSC transition is resolved. Phase fractions remain semiquantitative. The results provide a structural basis for subsequent degradation studies; improved corrosion resistance or biological performance is not demonstrated.