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Ultra high temperature materials for turbines (ULTMAT)

Final Report Summary - ULTMAT (Ultra high temperature materials for turbines)

The ULTMAT project aimed at providing sound technological basis for the introduction of innovative materials, namely Mo- and Nb-silicide based multiphase alloys, with enhanced high temperature capabilities (+100 degrees Celsius to +150 degrees Celsius) compared to presently used Ni-base single-crystal super-alloys, for application in aircraft / rotorcraft engines and in aero-derivative land-based gas turbines.

The scientific and technological objectives of the project were:
- definition of new compositions for Mo- and Nb-silicide based multiphase alloys with an acceptable balance of mechanical properties and oxidation resistance;
- development, throughout the project, of cost-effective processing technologies, adapted from either powder metallurgy (PM) or ingot metallurgy (IM);
- design of coating systems to improve oxidation resistance;
- building of a property database which will provide data for applications under specific turbine service conditions;
- preliminary assessment of the conditions of implementation of the materials in turbines, mainly machining and joining.

The project has been structured in two phases. The main objective of the first phase was to down-select a limited number of alloy compositions that fulfilled a given set of specifications (in terms of density, mechanical properties, etc.) while the development of coating and processing technologies for both Mo- and Nb-based silicide alloys, using ingot metallurgy as well as powder metallurgy, was started.

The second phase aimed at refining the work on the down-selected alloys, thus allowing the optimisation of processing technologies and coating systems, the set-up of a database of physical, thermal and mechanical properties, the manufacture of turbine airfoils and application specific testing.

The basic properties of new blades and vanes materials to be used in future turbo-engines, which could operate at temperatures 150 degrees Celsius higher than those allowed by current Ni-based single crystals super-alloys, have been defined from updated literature data, and three key requirements have been identified:
- good ductility at room and medium temperatures;
- good high temperature strength and creep resistance, up to about 1 300 degrees Celsius;
- good oxidation resistance at intermediate and high temperature.

Work performed prior to project beginning showed the positive influence of oxide particles on the mechanical properties of MoSiB alloys and a limited set of properties as a function of oxide content in the material was available. The objective of alloy development phase was to collect an additional set of data allowing the selection of an optimised composition of a Mo-silicide alloy.

Three candidate oxides were selected: Y2O3, La2O3 and MgO:
- Yttria was selected as a reference.
- Mo-La2O3 alloys are commercially established in the lighting industry for their good workability down to room temperature and their stable microstructure up to 1 800 degrees Celsius. Improvements of both parameters could be beneficial for the definition of components made of Mo-silicides.
- MgO was selected due to experimental results available from Plansee. Metallic Mg additions were shown to improve the oxidation resistance of Mo-3Si-1B whereas Mg was only detected as oxide in the material. There was no evidence of Mg in the molybdenum solid solution, nor in the inter-metallic phases. The primary objective of MgO additions was therefore focused on the improvement of the oxidation resistance.

About 300 kg material were industrially manufactured using a specific powder metallurgy processing route including mechanical alloying, compacting, sintering and extrusion. All processing steps were performed using industrial equipment, hence ensuring that the results of the characterisation could be directly used by engine manufacturers.

The feasibility of producing new materials during primary fabrication at a fairly large scale is essential to the maturation of any new material under development. The investigation of primary fabrication of Nb-silicides alloys was carried out using ingot metallurgy and powder metallurgy at the scale of about 50 kg ingot / billet weight.

The establishment of a property database for the studied Mo- and Nb-silicide based alloys is a key point in the project, allowing on one side the comprehensive assessment of the candidate alloy systems and on the other side the delivery of a data envelope that can be used by engine designers to integrate RM-silicide based components in future engine. The alloys differed by composition (within the two Mo- and Nb-based systems of interest) and also by microstructure, due to the various processing routes investigated. The established database contains the physical, thermal and mechanical properties appropriate to the intended application in gas-turbine hot sections.

Insufficient oxidation resistance at medium (pesting regime) and high temperature is one the weaknesses of the RM-based silicide materials, compared to super-alloys. It is thus of prime importance to characterise the oxidation behaviour and understand the related mechanisms to be able to design and test efficient oxidation resistant coatings. Basically, the work performed can be divided in two major parts: investigation of the oxidation behaviour of uncoated alloys and design, deposition and testing of oxidation resistant coatings, with emphasis on the Nb-silicide based materials.

Two approaches have been followed in parallel during the project.
The first one was based on available coatings developed by some partners mainly for the protection of RM alloys, which have been adapted to RM silicide-based alloys and for which the deposition technique has been optimised.

The second approach was to investigate the possibility to deposit coatings that form alumina scales during oxidation, since it is known that alumina is very efficient in reducing the oxidation rates, at least at high temperature, as it has widely been studied for super-alloys. A literature survey showed that RuAl is a promising candidate coating.

Workable conditions have been defined to perform the main machining operations required on blades and vanes, by performing machining trials on Mo and Nb silicide-based alloys. Machining trials have been carried out on the following operations: EDM, grinding, turning, milling and drilling. Additionally, preliminary joining conditions were investigated for Mo-silicide based alloys. The primary objective was to investigate diffusion bonding, but since brazing is mainly use for joining conventional components, the focus was early set up on brazing technologies. Brazing alloys based on Ti, Zr and precious metals were tested. PdNi based brazing alloys showed good preliminary results without porosity and good gap filling.

Thermal and mechanical calculations have been carried out, in order to have a quantitative evaluation of the benefits of using Mo and Nb silicide-based alloys in future turbo-engines. This has been performed by industrial partners in a first step on existing turbine hot section components (blade, vane, seal segment) with thermo-physical and mechanical properties as measured in the project (or taken from literature). It was proved, for instance, that Nb-silicide based alloys would be useful to enhance operating temperatures of a non-cooled blade or to reduce cooling flow in a cooled one. Nb silicide-based alloys were also assessed for a cooled vane.

A further task was devoted to validate the previously developed manufacturing methods on complex shaped components. Hence, a Mo-silicide based alloy fir tree root specimen was grinded, as well as a fir tree mechanical test sample. Seals were machined from Nb silicide cast parts. Finally, a blade foil was milled.

Significant results have been obtained, even if not sufficiently to develop 'products':
- the project has given a precise view of the promises and limitations of Mo and Nb silicide-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 degrees Celsius) to the high temperature (above 1 100 degrees Celsius) 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 (i) phase selection, (ii) phase stability, (iii) segregation phenomena, (iv) phase transformations, (v) phase equilibria, (vi) oxidation behaviour, (vii) microstructure architecture, (viii) mechanical properties and (ix) 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.