Synthesis of Intermetallic (Mg_1-x,Al_x)₂Ca by Combinatorial Sputtering

The synthesis-composition-structure relationship in the Mg-Ca-Al system is studied using combinatorial magnetron sputtering. With increasing deposition temperature, a drastic decrease in Mg concentration is obtained. This behavior can be understood based on density functional theory calculations yielding a desorption energy of 1.9 eV/atom for Mg from a hexagonal Mg nanocluster which is far below the desorption energy of Mg from a Mg₂Ca nanocluster (3.4 eV/atom) implying desorption of excess Mg during thin film growth at elevated temperatures. Correlative structural and chemical analysis of binary Mg-Ca thin films suggests the formation of hexagonal Mg₂Ca (C14 Laves phase) in a wide Mg/Ca range from 1.7 to 2.2, expanding the to date reported stoichiometry range. Pronounced thermally-induced desorption of Mg is utilized to synthesize stoichiometric (Mg_1-x,Al_x)₂Ca thin films by additional co-sputtering of elemental Al, exhibiting a higher desorption energy (6.7 eV/atom) compared to Mg (3.4 eV/atom) from Mg₂Ca, which governs its preferred incorporation during synthesis. X-ray diffraction investigations along the chemical gradient suggest the formation of intermetallic C14 (Mg_1-x,Al_x)₂Ca with a critical aluminum concentration of up to 23 at.%. The introduced synthesis strategy, based on the thermally-induced desorption of weakly bonded species, and the preferential incorporation of strongly bonded species, may also be useful for solubility studies of other phases within this ternary system as well as for other intermetallics with weakly bonded alloying constituents.