Increasing the temperature capability of turbine blade materials has been identified as a major requirement to develop efficient and clean aircraft. For high-pressure turbine components, the development of new alloys offering increases in metal surface temperatures of as much as 150 C over the presently used Ni-base, single-crystal super alloys is of strategic importance.
The project aims to provide a sound technological basis for the introduction of innovative materials, namely Mo- and Nb-based Silicide multiphase alloys, which have enhanced high temperature capabilities of up to 1 300o C, compared to the presently used Ni-base single-crystal super alloys, for application in aircraft/ engines and in aero-derivative land-based gas turbines. The objectives of the project are:
- the definition of new alloy compositions with an acceptable balance of mechanical properties and oxidation resistance;
- the development of cost-effective processing technologies;
- the design of coating systems to improve oxidation resistance;
- the creation of a properties database, which will provide data for applications under specific turbine service conditions;
- a preliminary assessment of the implementation conditions of the materials in turbines (machining, joining, etc.).
The expected result of the project is a thorough evaluation of the capability of refractory metals, Nb- and Mo-based Silicide multiphase materials, to withstand future increased temperature turbine service conditions, relying on mechanical, microstructural and environmental investigations in close relation to industrial-scale material processing and component fabrication technologies.
The work plan was constructed as a fast-track programme with simultaneous efforts on all technological aspects leading to representative tests, as well as technical/economic validation:
- alloy composition development with respect to specified property requirements;
- development of cost-effective and reliable processing technologies for both Nb- and Mo-based Silicide alloys. The composition and microstructural control of the multiphase alloys is of prime importance to enhance the current state-of-the-art. Therefore, a range of processes provided by the consortium (including arc-melting, ingot casting, powder-metallurgy processing, thermo-mechanical processing, etc.) together with the expertise in inter-metallic and refractory metal alloy development, will be used extensively for alloy composition screening and improvement up to the manufacturing of a prototype blade;
- development of adequate oxidation protection coatings;
- characterisation of the most relevant alloy properties (mechanical: high temperature yield strength and creep resistance, fracture toughness; physical, thermal, etc.);
- development of fabrication technologies.
To deal with these scientifically and technically harsh problems, the project mobilised specific high temperature facilities for manufacturing, processing and characterisation. Breakdowns of facilities were encountered, which could be overcome by redistributing the work among the partners and by sub-contracting. Significant results have been obtained, even if not sufficiently to develop 'products' (but this was not an objective of the project):
- the project has given a precise view of the promises and limitations of Mo- and Nbsilicide based materials, which is a priceless complement to the work performed here and there by individual institutions and companies, and could be obtained only by the synergetic commitment of all partners at European level;
- alloy compositions have been worked out for both alloy systems, following different development strategies:
- a Mo-silicide based alloy composition is available that provides reasonable mechanical
- properties and oxidation resistance. This alloy, because of its somewhat higher density, could be used in static turbo-engine components;
- Nb-silicide based alloy compositions are available, for which patenting is in progress. These alloys exhibit excellent creep resistance, but oxidation resistance is still insufficient for long duration applications;
- an industrial powder metallurgy route has been developed for Mo-silicide based alloys;
- for Nb-silicide based alloys, investment casting has been developed and validated by the manufacture of blades up to 320 mm in length; it has also been shown that this route can only be used for a certain range of alloys (the main parameter being the melting point).
- The powder metallurgy route could not be investigated as thoroughly as planned, but flawless HIPed materials have been manufactured;
- coating systems have been designed, deposited and tested, focusing on the Nb-silicide based alloys, that exhibit excellent resistance from the intermediate (800°C) to the high temperature (above 1100°C) range;
- a database has been built up, which can readily be used by engine design offices, and also by the partners to further develop the alloy compositions, the protective coatings and the manufacturing processes;
- the machining and joining of both alloy systems has been assessed, and validated with the manufacture of turbine component mock-ups.
In conclusion, the main scientific and technological outcomes of the project are a significant advance in the understanding of synergistic effects of alloying elements on
- phase selection,
- phase stability,
- segregation phenomena,
- phase transformations,
- phase equilibria,
- oxidation behaviour,
- microstructure architecture,
- mechanical properties and
- oxidation resistance of both alloy systems, and the development of processing routes (IM and PM) at industrial scale.
This knowledge would benefit the metallurgical academic community once publication of papers commences following the filing of a patent. This knowledge would also help industry focus its future efforts and resources in selecting alloys for further development.
ULTMAT was a fast-track programme to evaluate the potential of Mo- and Nb-silicide based materials for high temperature applications. In this field, the research was largely exploratory, with little short time applications.