Abstract
The phase formation of PtIrCuAuX (X = Ag, Pd) compositionally complex thin films is investigated to critically appraise the criteria employed to predict the formation of high entropy alloys. The formation of a single-phase high entropy alloy is predicted if the following requirements are fulfilled: 12 J∙K−1 mol−1 ≤ configurational entropy ≤ 17.5 J∙K−1 mol−1, −10 kJ∙mol−1 ≤ enthalpy of mixing ≤ 5 kJ∙mol−1 and atomic size difference ≤ 5%. Equiatomic PtIrCuAuX (X = Ag, Pd) fulfill all of these requirements. Based on X-ray diffraction and energy-dispersive X-ray spectroscopy data, near-equiatomic Pt22Ir23Cu18Au18Pd19 thin films form a single-phase solid solution while near-equiatomic Pt22Ir23Cu20Au17Ag18 thin films exhibit the formation of two phases. The latter observation is clearly in conflict with the design rules for high entropy alloys. However, the observed phase formation can be rationalized by considering bond strengths and differences in activation energy barriers for surface diffusion. Integrated crystal orbital Hamilton population values per bond imply a decrease in bond strength for all the interactions when Pd is substituted by Ag in PtIrCuAuX which lowers the surface diffusion activation energy barrier by 35% on average for each constituent. This enables the surface diffusion-mediated formation of two phases, one rich in Au and Ag and a second phase enriched in Pt and Cu. Hence, phase formation in these systems appears to be governed by the complex interplay between energetics and kinetic limitations rather than by configurational entropy.
Highlights
Platinum and its alloys are known for their spectacular performance in most corroding environments at elevated temperatures [1,2]
Recalling that the integrated COHP (ICOHP) value is an indirect but qualitatively accurate descriptor for the bond strength, we summarize that the average strength of bonds formed between Ag and all other constituting elements is significantly weaker than that of
Pd) thin films was studied various length scales. Both compositions fulfill the criteria for the formation for single-phase high entropy alloys, namely
Summary
Platinum and its alloys are known for their spectacular performance in most corroding environments at elevated temperatures [1,2]. These materials are extensively used in applications demanding corrosion resistance [1,3] as well as high strength such as organic chemical synthesis [4,5], electrode materials in fuel cells [6,7], implantable medical devices [8,9] or precision glass molding [10,11,12,13].
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