Fight with fatigue

A team at Portsmouth University is to study the long-term durability of a superalloy used in the latest generation of Rolls-Royce aero engines.

The EPSRC has granted £200,000 to study the effects of corrosion on RR1000, a nickel base superalloy developed by Rolls-Royce.

Rolls-Royce used powder metallurgy to develop the material that would be able to withstand the increasingly high temperatures and rotational speeds of aircraft engines.

The material is already being used to make the rotating turbine discs in the Trent 1000 engines such as those used in the new Boeing 787 Dreamliner.

In theory, the superalloy is best suited for use as a turbine disc. However, it is not known how resistant it is to the formation of cracks under extreme conditions. Although much research has been done on its fatigue and creep behaviour, the scientists hope to find out how corrosion, or oxidation, brought on by high temperatures, make the cracks grow.

‘There have not been many experiments on the effect of oxidation on alloys, so we hope to make great achievements in the area,’ said Dr Liguo Zhao, the leading researcher on the three-year project, who has also carried out research at Cambridge and Nottingham Universities, and Imperial College.

General progression

‘I have been doing research on alloys for a few years now, working on fracture fatigue and oxidation, so this project is a result of my work — a general progression. Engines are quite a high-temperature environment and the failure of a disc could be catastrophic.’

Through a series of experiments, the researchers will test the material, inducing corrosion to increase the spread of cracks under high heat levels.

This should show how the level of oxygen diffuses at the point of cracking, which in turn increases the cracks in the fine grain nickel base superalloy.

The combined factors of time, temperature, local deformation and the make-up of the material will determine how and by how much the oxygen diffuses.

Specific research will be carried out into how the oxygen diffusion is affected by forces (loading) on the disc, such as cyclic loading, and by the microstructure of the material.

By understanding these processes, the scientists will then be able to assess how and why cracks occur when the superalloy becomes corroded, and develop a model to help predict the safe life of the RR1000 discs.

‘At the moment, Rolls-Royce can predict the specimen life of this very special product, but they have to carry out a lot of expensive tests on specimen models to do so,’ said Zhao. ‘We want to make a model that lets you test the safe life more cheaply.’

Faster tesing

Dr Mark Hardy, materials specialist for nickel discs at Rolls-Royce, said that creating a model would make the development quicker and cheaper.

‘There would be a little less machine testing and it would lower the cost of the development to an extent, though this would be insignificant in the overall development of an engine,’ he said. ‘But it would be quicker to test as you would only have to test just enough specimens to validate the model and understand the trends.’

The researchers will use numerical programming to build a physical model to simulate the mechanical-oxidation behaviour of the RR1000 before they start experimenting early next year.

Part of the project will take place at Cranfield University, which has made its facilities and technical expertise from Prof John Nicholls, on the study of oxidation kinetics, available to the Portsmouth team during a three-month secondment.

Rolls-Royce is providing test pieces, and technical advice from Dr Hardy for the experiments.