Synthesis of Intermetallic (Mg1−x,Alx)2Ca by Combinatorial Sputtering

Abstract

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 (Mg1−x,Alx)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 (Mg1–x,Alx)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.

CALPHAD-Based Modelling of the Temperature–Composition–Structure Relationship during Physical Vapor Deposition of Mg-Ca Thin Films

“…All calculations were performed considering a reference area of 1 m 2 . Thus, for each deposition step, 3.676 × 10 −4 mol Mg and 7.352 × 10 −5 mol Ca (Mg/Ca ratio of 5) are added to the thin-film composition, thus simulating the experimentally reported deposition flux using a composite Mg-Ca target exhibiting a Mg/Ca ratio of 5 [ 12 ]. Similar to the experiments [ 12 ], simulations were performed for temperatures between 20 °C and 420 °C.…”

“…The same effect was also observed for other metals exhibiting high vapor pressure at deposition temperature, such as Al [ 8 ], Mn [ 9 ], Sn [ 10 ], and Ge [ 10 , 11 ]. A pronounced variation in Mg-concentration depending on the synthesis temperature has also been reported for Mg-Ca thin films: minor variations in synthesis temperature yielded pronounced modifications in the thin films’ composition [ 12 ], revealing a complex synthesis–-composition–structure relationship when Mg is involved. Even though PVD is frequently applied to study the properties of elemental Mg [ 13 , 14 ] and Mg-based films [ 7 , 12 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ], the sublimation behavior of Mg has only been considered qualitatively based on ground-state density-function-theory calculations [ 12 ].…”

“…A pronounced variation in Mg-concentration depending on the synthesis temperature has also been reported for Mg-Ca thin films: minor variations in synthesis temperature yielded pronounced modifications in the thin films’ composition [ 12 ], revealing a complex synthesis–-composition–structure relationship when Mg is involved. Even though PVD is frequently applied to study the properties of elemental Mg [ 13 , 14 ] and Mg-based films [ 7 , 12 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ], the sublimation behavior of Mg has only been considered qualitatively based on ground-state density-function-theory calculations [ 12 ]. In this work, thermochemical equilibrium calculations employing the CALPHAD (CALculation of PHAse Diagrams) approach are used to develop a predictive model of the temperature-dependent phase formation during Mg-Ca thin-film growth.…”

Atomistic structures of 〈0001〉 tilt grain boundaries in a textured Mg thin film

“…The chamber was evacuated to a base pressure of 4 × 10 -7 mbar and the substrate was heated to a temperature of ∼100 °C to ensure growth of dense films 33 and avoid Mg desorption during synthesis. 32 Prior to the deposition, a Si {100} substrate was plasma cleaned for 10 min applying an average power of 50 W in pulsed (frequency: 250 kHz) direct-current mode at an Ar partial pressure of 7.5 × 10 -3 mbar. Subsequently, the Mg film was deposited for 30 min at a constant target power of 100 W onto the rotating Si substrate.…”

“…31 It should be noticed that the growth temperature of Mg is limited as thermal desorption has been observed during synthesis of Mg 2 Ca Laves phase thin films at temperatures over 150 °C. 32 Mg-Ca solid solutions with 0.3 at.% Ca were recently synthesized at a substrate temperature of 100 °C, which induced enough adatom mobility to form dense films with low impurity contents (< 0.3 at.% oxygen). 33 In this article, we report the formation of <0001> tilt GBs in a sputtered Mg thin film, which enables a systematic survey on their structure with respect to misorientation angles.…”