Turbine technology

Materials World magazine
,
3 May 2015

Dr Justin Burrows, Project Manager of University Research at Rolls-Royce, stresses the importance of structural materials research to the aerospace industry.

Gas turbines in aerospace provide one of the ultimate challenges for materials science and metallurgy. Performance demands materials that deliver minimum system weight while resisting increasing loads and temperatures. In the core of the engine, for example, turbine blades under full power conditions reach a load of 18 tonnes, equivalent to a small lorry. They also operate at 200°C above their melting point, which is enabled by complex film-cooling technology and coating systems. In other words, if you placed an ice cube in a domestic oven at 200°C, this level of thermal protection would prevent it from melting. 

The modern gas turbine is made possible by the use of cutting edge materials technology throughout the engine, ranging from carbon-fibre composites in the fan system to high-temperature nickel superalloys and the introduction of ceramic matrix composites (CMC) in the turbine core. 

Developing a new nickel superalloy for a high-integrity component, such as a turbine disc, requires a detailed understanding of composition, microstructure and property relationships to enable the design of an optimised alloy chemistry and processing route. The resultant component must withstand high tensile and fatigue loading in the bore, while at the rim demonstrate an extreme resistance to creep in a corrosive, high-temperature environment. Through careful control of powder processing and hot forging, the necessary control of the microstructure and elemental constituents can be achieved. 

A range of advanced analytical techniques are employed to study these materials, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD) beam line experiments, 3D atom probe and nano secondary ion mass spectrometry (SIMS), where we can look at micro-mechanical behaviour in situ. Small-scale alloy prototyping techniques are used to explore novel phase diagrams, which allow consideration of material systems that are beyond conventional nickel-based superalloys. In parallel, to minimise the requirement for physical experiments and to understand alloy behaviour, advanced modelling and computing systems are used to complete work virtually. 

Temperature, temperature

Several technologies have to be integrated into the design of a gas turbine. Up to a limit, the efficiency of the turbine is heavily dependent on the temperature that the engine core can tolerate. Materials R&D has aimed at delivering materials (such as those used for the high-pressure turbine blades) with increasingly high temperature capabilities. Creep strength is a crucial factor, due to the continuous high axial loading from the rotation of the turbine. Other important attributes are fatigue strength, and corrosion and oxidation resistance. The development of alloys has enabled  a continuous increase in turbine entry temperature (TET) from wrought to cast alloys, then to directionally-solidified blades. 

Creep resistance was maximised by developing single crystal casting technology and removing grain boundaries as a source of weakness. Further developments enabled control of the major and minor axis orientation through seeding. Alongside these developments, the single-crystal casting technology assists the design and fabrication of the complex internal cooling passages, which allow cooler air from the compressor to be fed into the turbine blades during operation to achieve the high levels of cooling required. 

To further increase TET, coating technology has been developed that allows the turbine to withstand even greater temperatures. A thermal barrier coating (TBC) can not only withstand high temperatures, it also has low thermal conductivity and oxidation resistance, augmenting the capability of the existing system. 

Steel yourself

However, it is not just emerging technology areas such as CMCs and other novel high-performance materials where advances are being made. Equally important
is research into the behaviour of more established materials systems, to enable their safe and cost-effective operation in increasingly arduous conditions. Critical to future engines and challenging to materials scientists are the high-strength steels and advanced bearing materials needed to transfer power via shafts throughout the engine. 

Steels are an established technology, but the physical and mechanical requirements of high-bypass engines (with larger fan blades) and smaller cores with increased TET mean that the shafts need to be strong, while retaining their mechanical properties under increased temperature. For example, the shaft connecting the turbine to the fan will experience a dramatic change in temperature along its length, while reacting to extreme torque loads. One solution can be to join dissimilar alloys together using advanced inertia welding techniques to meet different requirements, although the development of a material with properties to meet both environments would reduce manufacturing costs and save weight. 

A large Rolls-Royce gas turbine has three concentric shafts connecting low-, medium- and high-pressure systems across the engine. This is unique in the industry but means that bearings have to react to cyclic mechanical loads between shafts. In addition, architecture that looks at minimising the turbine core size also requires smaller diameter shafts, enhancing the need to increase mechanical and physical properties.

One recent significant breakthrough has been achieved by implementing titanium aluminides – low-density materials that, when used in low-pressure turbines, reduce engine weight, fuelburn and emissions. This material has low density and good high-temperature strength, but low ductility. As such, thorough research into mechanical behaviour was required to enable the design, manufacture and assembly process. A novel testing method was developed during an EPSRC project at Swansea University, which produced data validating the mechanical behaviour during assembly and under service conditions. Even before the engine operates for the first time, assembling a blade set onto a rotor requires physical handling, and it was essential to reliably understand this behaviour ahead of manufacture and testing. 

The universities of Birmingham, Cambridge and Swansea have been at the forefront of this work within the EPSRC and Rolls-Royce Strategic Partnership. Also within this programme is the development of next-generation nickel alloy disc materials and associated lifing technology. 

Over the past 50 years, gas turbine materials technology has made huge advances, revolutionising almost every aspect of the engine and enabling great strides in performance. This has only been possible through close interaction of industry and academia to deliver high-quality research on new materials systems, and their performance and service integrity. But the journey is far from over. The gas turbine engines being designed today for introduction to service in the next 10 years will require more advanced materials systems if they are to deliver the levels of efficiency demanded by the industry. This provides further exciting challenges to the community across the full range of materials technologies, from composites to high-performance alloys, coatings and surface engineering, to fluid systems.  

The Strategic Partnership in Structural Metallic Systems for Advanced Gas Turbine Applications

This 10-year partnership between Rolls-Royce and EPSRC integrates research and training programmes, aiming to recruit and train 20 PhD and EngD students a year. This is a programme led by industry (with industrial and academic partners) that provides a stable, long-term funded plan to allow the development of fundamental underpinning research for next-generation materials. 

For more information on  advances in structural metallic systems for gas turbines, visit www.maneyonline.com/toc/mst/30/15