Abstract
Abstract To solve the uncertainty of the platinum (Pt)–palladium (Pd) phase diagram, especially the existence of a suggested miscibility gap, atom probe tomography (APT) was used to determine the time evolution of the composition after heat treatment. Due to the extraordinarily slow diffusion in the temperature range of the controversial phase separation, the investigated volume was limited to nano-sized multiple layers deposited by ion beam sputtering (IBS). The evaporated volume was reconstructed from the obtained datasets and the respective diffusion coefficients were determined using the Fourier series solution of the diffusion equation. Beginning with pure Pt and Pd layers annealed at 673, 773, 873, and 973 K, the mixing appears to be purely diffusion controlled in the chosen annealing times, but the state of complete mixing was still not observed. Therefore, extended isothermal annealing sequences at 673 and 773 K with pre-alloyed layers have been carried out. They clearly suggest complete mixing even at the lowest investigated temperatures.
Highlights
Understanding the thermodynamics of an alloy system is fundamental and a prerequisite for interpreting the properties of the materials
Due to the extraordinarily slow diffusion in the temperature range of the controversial phase separation, the investigated volume was limited to nano-sized multiple layers deposited by ion beam sputtering (IBS)
It is safe to say that the critical temperature of the miscibility gap, if any exists, is certainly below 973 K, lower than Raub’s (1959) expectation of 1,043 K
Summary
Understanding the thermodynamics of an alloy system is fundamental and a prerequisite for interpreting the properties of the materials. In some cases, required data are missing, since the very slow diffusion kinetics demands extremely long annealing times to reach the equilibrium state. One of these systems is the Pt–Pd binary system, an important alloy system essential for catalysis. Beeskow (Raub, 1959) and Bharadwaj et al (1991) tried to prove the phase diagram, especially the phase separation in the miscibility gap, even though the alloying time takes more than a year, but it still remains as an unclear dashed line in the published phase diagrams (Villars & Okamoto, 2012; Raykhtsaum, 2013)
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