IMPRESSive research - the IMPRESS project
The Intermetallic Materials Processing in Relation to Earth and Space Solidification Integrated project (IMPRESS) aims to unravel the important relationships between the processing, structure and properties of intermetallics. In contrast to normal alloys, intermetallics are ordered chemical compounds of two or more metals. Two specific families of intermetallics are being examined by IMPRESS, titanium aluminides (TiAl) and nickel aluminides (NiAl).
The multidisciplinary consortium comprises 150 scientists from 42 European universities and companies, with a total budget of approximately 41 million euros. Titanium aluminides have remarkable mechanical and physical properties at temperatures of up to 800ºC. The combination of high melting point, high-temperature strength and creep resistance, and low density, makes TiAl ideal for high-performance gas turbine blades. The objective is to produce, for the first time, low-pressure turbine blades, which are up to 40cm in length, via cost-effective casting.
Nickel aluminides, on the other hand, have good catalytic properties making them particularly useful for hydrogenation reactions in the chemical industry, as well as electro-catalysts in alkaline fuel cells. The goal is to develop low-cost catalysts with double the activity of traditional sponge nickel, as well as improved selectivity and stability.
The starting point for IMPRESS was the decision that turbine blades must be produced by investment casting, as opposed to using more costly forging. Due to the reactivity of liquid TiAl at 1,700°C, numerous problems in casting were highlighted. In addition ductility of cast TiAl components was shown to be intrinsically poor, thus highlighting the need for further alloy development. The work within IMPRESS targets two linked but separate areas – firstly, the development of casting technology, and secondly, the development of blades to meet end-user specifications, including a minimum room temperature ductility of one per cent.
Experimental work within these areas has been supplemented by multiscale modelling. Accurate thermodynamic phase diagrams are in place. In addition, thermophysical property databases for liquid TiAl intermetallics have been created with the help of benchmark experiments carried out in space. These data are essential for understanding and optimising melting, mould-filling, solidification and subsequent heat treatment. The success of the experimental programme is assured by comprehensive measurements of the mechanical and oxidation properties of the test-pieces.
Casting of low-pressure turbine blades has been carried out successfully using a centrifugal investment casting technique. An example of blades produced during IMPRESS is shown in the image left. They have been used to assess the tensile, creep, fatigue and fracture toughness properties, as well as oxidation resistance, of the cast and heat-treated samples. Larger samples are being produced using tilt-casting that is still being evaluated. From a detailed assessment it appears that the manufacturing technology developed within IMPRESS has led to a significant improvement in the yield of sound net-shape castings.
The intermetallic selected after an extensive alloy development programme is Ti 46Al 8Ta (at %) Adding tantalum permits the required heat treatment. Samples treated so far are up to 20mm in diameter. The treatment applied consists simply of air cooling, which leads to the intermetallic being transformed, after which it is hot isostatically pressed (HIPped). This produces a complex ‘convoluted’ microstructure, which is illustrated together with the coarse as-cast microstructure. It is this refined microstructure that confers the balance of mechanical properties to the turbine blade.
The mechanical properties obtained at room temperature are 0.2% proof stress of over 620MPa, tensile ductility of over one per cent, fracture toughness of about 16MPam1/2 and lifetimes of more than 100,000 cycles for stresses up to 660MPa. Creep life at a stress of 350MPa at 750°C is over 450hrs and oxidation resistance is also satisfactory.
If successfully implemented, this material could result in a 50% weight reduction in low-pressure turbine stages. Such a reduction could lead to improved thrust-to-weight ratios of aero-engines, higher efficiencies, reduced fuel consumption and lower exhaust emissions. The next stage beyond IMPRESS will be industrial scale-up, testing and supply chain management.
Sponge nickel catalysts are based on NiAl precursors and have been in industrial use for over 80 years, but there are limitations to the traditional manufacturing process. It involves casting and crushing ingots of Ni-50wt%Al intermetallic, followed by caustic leaching in sodium hydroxide to remove the aluminium atoms and create a large and catalytically-active surface area, typically 50m2/g.
In the case of cast-and-crushed material, the microstructure of the precursor material is an equilibrium one, which does not lend itself to further modification. However, by adopting gas atomisation, one can produce rapidly-solidified NiAl powder with tailored non-equilibrium micro- and nanostructures for improved catalytic performance.
Hundreds of catalytic trials in the IMPRESS programme have shown that the features of the gas atomised powder can be tuned – particle size, volume fraction of different phases, fineness of the dendritic network and the location of dopant elements. These properties are related to the superheat and melt flow rate during gas atomisation.
‘For the first time in this field, a selection map is emerging that links process, structure and final catalytic properties,’ says Dr Nick Adkins, Technology Manager Powder Group, CERAM, UK.
The IMPRESS team has identified a number of atomised NiAl precursors, with double the activity of standard Ni-50wt%Al, for hydrogenation reactions of commercial interest. Some useful improvements in selectivity are also observed.
In addition to hydrogenation testing, trials in alkaline fuel cells and standard half-cells are also being undertaken. Promising results have been found as sponge nickel provides good catalytic performance as part of the anodic electrode. The powder’s performance is good enough to replace platinum catalysts, albeit at higher loading. The implications of this are self-evident, when one considers that platinum prices are considerably higher than nickel. Commercially, this offers the opportunity to bring alkaline fuel cells to the market at a more affordable level.