Strengthening particles in superalloys – how the small can have a big influence

Who doesn’t like a good holiday? Going abroad and discovering the world is an established part of today’s jet-setting culture, is for many of us part of modern life. And as jet setters, getting to our holiday destination is not supposed to take up much of our well-earned holiday time.

Air travel is now fast and convenient, but do we ever stop to consider why and how much engineering goes into it? The technology that makes air travel so fast and efficient is high-tech gas turbine engines built by Rolls-Royce plc.

A basic gas turbine engine works as a continuous thermodynamic cycle. This means it operates as a heating and cooling cycle; air is drawn in at the front of the engine, it is compressed and then heated. This is followed by an acceleration of hot air through a turbine, creating a thrust that drives the aircraft forwards.

The law of thermodynamics states that the higher the temperature of the air at the turbine entry point, the more efficient the engine. This is especially true during take-off, when the engine experiences its most demanding conditions, with temperatures reaching more than 1,000 degrees Celsius. When put into context, this is equivalent to the temperature of hot lava coming from an exploding volcano.

This emphasises the importance of materials science as part of the research and development that feeds into turbine engine technology.

The University of Birmingham is part of a strategic partnership in collaboration with Swansea University and the University of Cambridge. It is also one of the Rolls-Royce University Technology Centres in materials research, which focuses on all different types of materials applied in the modern gas turbine engine.

The materials used in the turbine stages of an engine not only have to withstand high temperatures or corrosive environments such as salty air, but they also have to be resilient to high stresses due to parts of the turbine rotating at high speed.

This is why materials used for these applications need to be strong, able to resist reactions with oxygen and salts, and be usable at high temperatures.

Nickel-based superalloys make these applications possible; they are highly complex materials often consisting of more than ten alloying elements. Every element plays a significant role in stabilising a beneficial microstructure, forming a protective oxide layer and shielding the rest of the material, to form strengthening particles creating an incredible strength when exposed to high temperatures.

Nickel alloys are used at temperatures of between 650–1,100 degrees Celsius, reaching up to 80 per cent of their melting temperature. Most materials would show a significant decrease in their mechanical properties when exposed to these kinds of temperatures. However, due to the advantageous microstructure of superalloys their properties are able to remain intact.

These properties are highly stable because the strengthening particles which form within the material act to intercept cracks and stop them growing. These particles are up to 1,000 times smaller than human hair, which is a bigger step change than going from the size of a football field to the size of an ant. The size and quantity of these particles influence the materials properties, providing an optimum size and amount.

Current research being undertaken at the University of Birmingham examines existing theories across physics, chemistry and metallurgy. It shows that these particles are strongly affected by high temperatures and could also be influenced by stresses which occur during the lifetime of the engine, causing the particles to either fully disappear or increase in size. This is important knowledge because the material used in the engine experiences these exact conditions during service.

These conditions are then reproduced in labs using furnaces and mechanical testing machines, and the micro-structure is subsequently analysed using scanning electron microscopes. With these methods, it is possible to look at exactly how strengthening particles are affected and at what rate. This is then linked to how all these changes impact upon the mechanical properties of the material itself, testing various conditions using different methods. Ultimately, this influences the understanding of the performance capabilities of high-tech gas turbine engines.

Fiona Schulz

Doctoral Researcher, Integrated Studies Structural Metallic Systems for Gas Turbine Systems, School of Metallurgy and Materials, University of Birmingham