Abstract
Thin films of the CrMnFeCoNi high entropy alloy were deposited by magnetron sputtering from a sintered equimolar target. The substrate temperature and bias were varied during deposition, and the structure, morphology and elemental distribution were studied in detail. All films formed phase mixtures of multiple crystal structures. This contrasts with studies on the bulk alloy, where it typically forms a single phase with a simple cubic closed packed (ccp) structure, with other phases precipitating only after long annealing times. For higher substrate temperatures, we observed a mixture of phases with ccp and bcc (body centered cubic) structures, and the intermetallic phases σ-phase and L10, the first three being the predicted equilibrium phases at the deposition temperature. For room temperature depositions, we found evidence of very limited diffusion of metal atoms during the deposition. These films formed a mixture of a ccp and the intermetallic χ-phase. Two mechanisms can be distinguished that govern the phase formation at lower and higher temperatures. From the present results and comparisons with the literature, we also discuss why the small grain size, the low process temperature, and the fast surface diffusion during synthesis causes magnetron sputtering to yield different results compared to bulk synthesis from the melt. These principles explain why it is easier to form the equilibrium phases by sputtering, and why a single ccp phase should not be expected as a rule for this deposition method. Following the thermodynamic principles of high entropy alloys, this may also be the case in other high entropy alloy systems.
Highlights
High entropy alloys (HEAs), called multicomponent alloys, were introduced as a new concept for materials design in 2004 by Cantor et al [1] and Yeh et al [2]
The deposited films can be separated into two groups; the temperature series with a floating bias and the bias series deposited at room temperature (RT)
The films consisted of ccp and a phase identified as χ phase
Summary
High entropy alloys (HEAs), called multicomponent alloys, were introduced as a new concept for materials design in 2004 by Cantor et al [1] and Yeh et al [2]. Yeh defined a HEA as a solid solution of at least five metallic elements at close to equal atomic concentrations and found that these alloys often crystallize into a single phase with simple crystal structures, such as a cubic close packed (ccp) or body centered cubic (bcc) structure. This was unexpected for materials with such a high number of elements, which typically segregate into multiple phases and form intermetallics. Another example is avoiding the formation of intermetallics to preserve ductility
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