Gas turbines are prominent pieces of engineering with high material standards. As explained in the previous article “How new materials can improve gas turbines”, these engines require of some of the highest performance materials known, to achieve higher efficiency rates. Increasing the operational temperature is usually seen as the most important factor to improve efficiency, and for that, new materials are required. In this article, we will go deeper into the different high-temperature materials that are currently being explored to be used in the future of gas turbines.
Although a variety of materials are used for different parts of the engine (Fig. 1), when thinking about new materials for gas turbines, many times we think about the blades because they are the part that requires the best material properties. They rotate at high speed; therefore, they need to sustain high mechanical loads, and they need to work at temperatures over 1000°C. The challenge is to make materials that preserve the high mechanical resistance over a very large range of temperatures. Furthermore, the complicated shape of the turbine blades requires to use materials that can be easily manufactured.
Usually, metallic materials possess the adequate mechanical properties, but at high temperatures, they easily get deformed by creep and oxidation becomes a problem. On the other hand, materials with excellent oxidation resistance at high temperatures, such as ceramics, are usually too brittle for the requirements of a turbine.
To solve this issue, the scientific community proposed the use of composite materials formed by two or more constituents with different properties. The result is a material that can have resistance to high temperatures and good mechanical properties at the same time. Metal Matrix Composites (MMCs) are one such class of materials, they have a ductile metal matrix with high-temperature resistant particles or fibers distributed on it. Ceramic Matrix Composites (CMCs) are also an alternative, especially due to their light weight.
Metal Matrix Composites (MMCs)
Within the family of MMCs, metal alloys with intermetal phases are the most promising solution for high-temperature applications in gas turbines. A variety of alloys exist that can be applied in different types and different parts of turbines.
Molybdenum-based alloys with silicide particles have been getting a lot of attention in recent years and are now in their latest phase of development. Molybdenum caught the attention due to its high melting point (2617°C) and the Mo-Si-B alloy, pioneered by Douglas M. Berczik  in the 90s, is expected to run in engines at temperatures significantly higher than current materials . This alloy has extraordinarily good properties at high temperatures; paradoxically, it is at low temperatures where the problems arise as it becomes more brittle. This brittleness also brings additional challenges for processing the material into the right shape.
Many other MMCs are being explored for applications in different parts and kinds of gas turbines. Niobium or rhenium are metals with similarities to molybdenum that can also be alloyed with silicon or cobalt to create MMCs and could possibly be used in specific situations [4,5].
Iron aluminide (Fe-Al) based alloys are a cheaper and lighter option for parts working at lower temperatures and could also be used in steam turbines. They could be a replacement for stainless steels for structural applications working at up to 1000°C . Vanadium silicide alloys also provide a combination of high strength at temperatures up to 1000°C and even lower density . Titanium alloys present an excellent combination of mechanical properties and are widely used in the aerospace, petrochemical and biomedical industries.
Additionally, they have been processed by additive manufacturing thanks to their favourable response to a wide range of processing parameters . Intermetals such as titanium carbides and titanium silicides are seriously being considered for applications requiring low density, such as for the aerospace industry, and titanium aluminides are already being used in some parts of commercial aircraft engines.
Ceramic Matrix Composites (CMCs)
CMCs have been under research for decades and are now set to appear in hot components of gas turbine engines. They possess resistance to high-temperatures and good mechanical properties. However, they are especially interesting due to their low weight, approximately one third the weight of current nickel-based super alloys. Additionally, ceramics are implemented as thermal barrier coatings (TBC), adding resistance to environmental barrier coatings (EBC).
Silicon Carbide (SiC) based CMCs can already be used in commercial aircraft engines, such as the LEAP jet engine, and the GE9X engine. At the moment, they are used for the nozzle or shrouds which are parts that do not demand as high mechanical properties, but they may soon be also introduced in blades [9,10]. One of the challenges of SiC-based CMCs is their vulnerability to combustion environments; however, alternatives based on eutectic-oxide CMCs are already being developed (Fig. 2).
The materials science community is developing a wide range of materials for gas turbine applications. This variety will enable engineers to choose materials with tailored properties for different kinds of gas turbines or for every part of the engine, making it more flexible and efficient. Additionally, new high-temperature resistant materials will enable a higher overall performance, subsequently reducing environmental impact and operational costs.
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