High Temperature Materials Development beyond Ni-Base Superalloys

Dear Colleagues,

Gas turbines have benefited from decades of development of nickel-base superalloys. For the future, however, a new material class is required to obtain further efficiency gains and environmental friendliness. The last couple of decades saw a worldwide development effort for new materials in gas turbines.

Presently, Ni-base superalloys are the dominant material class in the hot section of gas turbines, where operational temperature has already reached 80% of the Ni-superalloy melting temperature. The gas entry temperature in present turbines is even higher, and considerably exceeds the melting temperature. Innovative cooling strategies and thermal barrier coatings (TBC) make it possible to continue to use Ni-base superalloys. Clearly, this development cannot be sustained indefinitely and a solution beyond the temperature capabilities of Ni-based superalloys is essential.

The choice of alternative materials is, however, limited. In addition to high-temperature metallic alloys, only a limited number of material classes (e.g., ceramics, intermetallic and refractory metal alloys) can possibly meet the severe demands of gas turbines. Some of these developments show promise, but there is still a long way to go before substituting Ni-base superalloys in gas turbines.

Examples of some known development efforts can be mentioned. Co-based superalloys, which made their debut at the same time as Ni-base superalloys in the 1940s, and are also used in gas turbines, are challenging Ni-superalloys with their new developments. γ/γ’ type alloys of the Co-Al-W system, as well as the Co-Re-based alloys with vastly improved melting temperatures, show promise. The refractory metal alloys of Mo and Nb also have high melting points and satisfy many of the requirements for engine applications, but their oxidation resistance poses a challenge. Silicon-containing alloys (silicides) of these refractory metals (along with B or Cr addition) have also been explored and they show good oxidation resistance.

Although some prospects are on the horizon, many hurdles are still to be overcome. Therefore, the search for the new material, particularly metallic alloys with their many advantages, continues. For the case of high-temperature material development, the role of in situ investigation techniques simulating operation conditions (temperature, mechanical load) should be stressed. Here, neutron and synchrotron diffraction methods have established themselves as powerful tools to observe structure and microstructure changes, both during heat treatment and during operation. These techniques more and more frequently complement the standard established methods, like electron microscopy, for the characterization of high-temperature materials.

We invite all researchers in the challenging area of high-temperature materials and their development to contribute to this Special Issue “High Temperature Materials Development beyond Ni-Base Superalloys” in the journal Metals. In this summary, only several known development directions and material characterization routes are mentioned; the Special Issue will, nevertheless, cover all new materials in this area investigated using any suitable method.

Dr. Debashis Mukherji
Dr. Pavel Strunz
Guest Editors

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• High-temperature materials
• Gas turbine materials
• Superalloys
• Refractory metal alloys
• Ceramics
• Intermetallic alloys
• Mo-Si-B alloys
• Neutron scattering
• Synchrotron
• In-situ measurements