Approaches to the Development of Advanced Alloys Based on Refractory Metals

Approaches to the Development of Advanced Alloys Based on Refractory Metals: Comparison

Please note this is a comparison between Version 2 by Sirius Huang and Version 1 by Igor Razumovskii.

The most promising directions of the development of heat-resistant alloys (HRAs) based on refractory metals are analyzed. The microstructures characteristic of HRAs, which it is advisable to form in promising alloys, are considered. The stability factors of the microstructure with respect to the diffusion coarsening of the hardening phases are discussed. Two groups of alloys are considered as the most promising HRAs based on refractory metals. First, the principles for design of HRAs based on (Pt, Ir)-Sc with heterophase γ-γ’ microstructure, where γ-matrix is a (Pt, Ir) solid solution with a FCC lattice, and γ’ is a strengthening phase with the structure L1₂ by analogy with Ni-base superalloys, are developed. The resistance of γ-γ’ microstructure in Ni, Pt and Ir alloys against the process of diffusion-limited coarsening is analyzed. It is shown that the diffusion permeability of Pt is several times less than that of Ni, so one should expect that Pt-based HRAs will not be inferior to Ni-based HRAs in terms of structural stability. The second group includes HRAs based on many not noble refractory metals. It is shown that solid solutions of the system (Ti, Zr, Hf, Ta, Nb) with a BCC lattice can be considered as a matrix of advanced refractory HRAs. The results of experimental studies of alloys based on (Ti, Zr, Hf, Ta, Nb) additionally alloyed with elements contributing to the formation of strengthening intermetallic and silicide phases are discussed. The issues of segregation of alloying elements at the grain boundaries of refractory alloys and the effect of segregation on the cohesive strength of the boundaries are considered.

high-temperature alloys
microstructure
diffusion
structural stability
refractory metals
Pt-Ir-Sc alloys
alloys based on many refractory metals
cohesive strength

Critical parts of gas turbine engines of aircraft are made mainly of Ni-based heat-resistant alloys (HRAs), the operating temperatures of which are fundamentally limited from above by the melting point T_m located near 1450 °C [1,2,3,4]^[1][2][3][4]. The development of aerospace technology requires an increase in engine power, which is possible only by increasing the gas temperature at the turbine inlet. For operation in such conditions, materials with higher temperature parameters typical for refractory metals and compounds are required. When choosing the composition of new alloys, it should be assumed that their operating temperatures usually do not exceed ~0.75 T_m. Thus, to ensure the operability of gas turbine engines at 1500 °C, refractory HRAs with T_m ≥ 2000 °C are necessary. The search of promising HRAs with high melting points for practical use compared to Ni-based HRAs is an urgent task of physical materials science.

Traditional HRAs based on one of the refractory metals, among which alloys based on W, Ta, Mo, and Nb ^[5] have the greatest practical application, have high values of T_m, and satisfy the criterion of T_m ≥ 2000 °C; however, they have a number of significant disadvantages characteristic of metals with a BCC lattice. These include a low ductility at room temperature, especially characteristic of Mo and W, and low oxidation resistance.

Currently, the possibility of creating metallic HRAs with high melting temperatures based on many refractory metals (so-called “high-entropy alloys”) is being actively investigated ^[6]. The matrix of such alloys is a solid solution with a BCC lattice, which is characterized by a tendency to brittle fracture. Therefore, it is necessary to develop approaches to the choice of alloying system for such alloys with an acceptable deformation ability.

Among HRAs based on a single refractory metal, a special place is occupied by noble metal alloys based on Pt (T_m ≈ 1769 °C) and Ir (T_m ≈ 2447 °C) with a FCC lattice, free from the disadvantages of BCC structures. In the field of structural materials, the main application of Pt and Ir is their use in coatings that protect the blades of gas turbine engines from the effects of aggressive environment at elevated temperatures under loads ^[7]. As industrial structural materials, Pt, Ir and their alloys are used for the manufacture of heat-resistant vessels for various purposes: for melting glass, which is used in the production of fiberglass composite materials, growing special-purpose crystals, etc. [8,9]^[8][9].

The structure of traditional HRAs based on Pt and Ir is a solid solution based on these metals. Such alloys have a single-phase FCC structure, and they are strengthened by a solid-solution hardening mechanism ^[8]. It should be noted that the early nickel based HRA Nimonic has a similar structure, in which the Ni-Cr solid solution was alloyed with refractory metals at the maximum purification from low-melting components [2,4]^[2][4].

However, it was later shown that higher heat resistance of metal alloys at elevated temperatures is provided by a polyphase structure ^[10] in which the solid solution is a metal matrix that is strengthened with intermetallic, carbide or silicide phases. It is possible to distinguish two main groups of polyphase structures characteristic of HRAs obtained using traditional metallurgical technologies.

The first group includes a structure consisting of a solid solution (matrix) reinforced with isolated particles of the second phase, which are formed during the decomposition of a supersaturated solid solution. Such a structure is formed in traditional dispersion-strengthened Ni-based HRAs. The second group includes eutectics, which are formed during the crystallization of the melt and are a “mechanical” mixture of two phases ^[11]. One of the phases in eutectic is usually a metal solid solution, and the second phase, which strengthens the solid solution, is various chemical compounds.

In this paper, the most promising approaches to the development of the newest HRAs based on refractory metals are analyzed. The features of the microstructure of HRAs, which provide high long-term strength of alloys, and the mechanisms of diffusion coarsening of different types of structures are considered. The design principles of refractory HRAs based on precious metals and high-entropy alloys are proposed.

References

Sims, C.T.; Stoloff, N.S.; Hagel, W.C. Superalloys II: High-Temperature Materials for Aerospace and Industrial Power; John Wiley & Sons, Ltd.: New York, NY, USA, 1987.
Kablov, E. Cast Blades of Gas Turbine Engines: Alloys, Technologies, Coatings; MISiS: Moscow, Russia, 2001. (In Russian)
Reed, R.C. The Superalloys: Fundamentals and Applications; Cambridge University Press: New York, NY, USA, 2006.
Logunov, A. Nickel Superalloys for Gas Turbine Blades and Disks; Izd. Dom Gazoturb. Tekhnol.: Rybinsk, Russia, 2017. (In Russian)
Arzamasov, B.N.; Solov’eva, T.V. (Eds.) Handbook of Structural Materials; Publishing House of Bauman Moscow State Technical University: Moscow, Russia, 2005. (In Russian)
Senkov, O.N.; Tsakiropoulos, P.; Couzinié, J.-P. Special Issue “Advanced Refractory Alloys”: Metals, MDPI. Metals 2022, 12, 333.
Fisher, G.; Datta, P.K.; Burnell-Gray, J.S. An assessment of the oxidation resistance of an iridium and an iridium/platinum low-activity aluminide/MarM002 system at 1100 °C. Surf. Coat. Technol. 1999, 113, 259–267.
Rytvin, E.I. Heat Resistance of Platinum Alloys; Izd. Metallurgiya: Moscow, Russia, 1987. (In Russian)
Timofeev, N.I.; Ermakov, A.V.; Dmitriev, V.A.; Panfilov, P.E. Fundamentals of Metallurgy and Production Technology of Iridium Products; Ural Branch of the Russian Academy of Sciences: Yekaterinburg, Russia, 1996. (In Russian)
Kablov, E.N. Scientific and Biographical Sketch: S. T. Kishkin. In S. T. Kishkin. Creation, Research and Application of Heat-Resistant Alloys. Selected Works; Nauka Publishing House: Moscow, Russia, 2006. (In Russian)
Chandra Sekhar Tiwary, Prafull Pandey, Suman Sarkar, Rakesh Das, Sumanta Samal, Krishanu Biswas, Kamanio Chattopadhyay. Five decades of research on the development of eutectic as engineering materials. Prog. Mater. Sci. 2022, 123, 100793.