Page 133 - Read Online

Page 2 of 18 Chong et al. J Mater Inf 2023;3:21 https://dx.doi.org/10.20517/jmi.2023.17

INTRODUCTION
Aiming to design ultra-high-temperature materials for use in harsh environments has led to the
development of Pt-based superalloys. These superalloys are superior to Ni-based superalloys since Pt has a
higher melting point (2,042 K for Pt vs. 1,728 K for Ni ) and is more chemically inert for outstanding
[1]
[2]
oxidation resistance, c.f., the predicted Ellingham diagrams in the present work. Currently, a higher
application temperature is urgently needed in gas turbine sections to improve engine efficiency and reduce
fuel consumption and carbon dioxide emissions for a greater thrust. For example, the current application
[3]
temperature of Pt-based superalloys can be extended to 1,350 °C (1,623 K) . Although Pt-based alloys are
expensive, they are the most reliable materials under ultrahigh temperatures and are widely used as bonding
coating alloys in the thermal barrier coating (TBC) system and in the aerospace engine nozzle. The excellent
oxidation resistance of Pt-based superalloys is their great advantage. The oxidation behavior of pure Pt has
been fully investigated ; however, the oxidation of Pt-Al-based alloys is far from fully understood.
[4]
The Pt-Al system is one of the widely used superalloys, fueling up considerable studies such as the strength
[5,6]
and creep properties . The mechanism of oxidation resistance for pure Pt and Pt-Al alloys is quite
different. It shows that the Pt-Al alloy can be further stabilized at high temperatures in terms of the L1 -
2
based γ’ phase by adding alloying elements such as Cr, Hf, and Ta. Especially, Cr can form the dense and
continue Cr O coating to provide a protective scale on Cr-containing alloys; Hf is a reactive element to
2
3
reduce the growth of a thermally grown Al O scale in alloys such as the NiAlCr and NiAlPt ; additionally,
[7]
2
3
Ta can hinder ion diffusion and improve oxidation resistance in, e.g., the Ni-10Al alloy . One of the
[8]
purposes of the present work is to develop a series of Pt-Al-based alloys with strong oxidation resistance
used as bonding coating in the TBC system. Notably, oxidation resistance for an alloy of interest is an
important indicator to determine whether it can be applied to high temperatures or not [9,10] , with the best
oxide being α-Al O (represented by Al O in the present work if no further explanation), which will be
3
2
2
3
formed on the surface of Al-containing alloys at high temperatures (> 1,100 °C) [11-13] . Here, Al O usually acts
2
3
as the oxidation resistance barrier of the alloys at high temperatures, thus further preventing oxidation
inside the alloys [14,15] .
In the present work, four alloying elements, Al, Cr, Hf, and Ta, are selected to investigate oxidation
resistance of the Pt-based alloys. The forming mechanism of Al O and other oxides is understood by the
3
2
predicted Ellingham diagrams with experimental verifications. Here, the free energy (or partial pressure of
[2]
gas species) versus temperature diagram, i.e., the Ellingham diagram, is a predictive tool to tailor
[16]
thermodynamic stability of oxides that form in the present Pt-based alloys . In addition, the oxidation
kinetic curves are also measured to examine the formation rates of oxides. The present computations of
Ellingham diagrams with experimental verifications provide fundamentals to develop Pt-based superalloys
with superior oxidation resistance that can be used in ultra-high temperatures.
MATERIALS AND METHODS
Ellingham diagram
o
[2]
An Ellingham diagram is a plot regarding the change of the standard Gibbs energy (ΔG ) as a function of
temperature for a given reaction, evaluating the equilibrium phases obtained by chemical reactions. In the
present work, the calculated Ellingham diagrams can judge whether the Pt-based alloys (i.e., metals M = Al,
Cr, Hf, Pt, and/or Ta) can be oxidized or not in terms of the following two reactions (scenarios),