This was written as part of Materials Today’s new section on Thermal Analysis and can be read in its original location here. Many thanks to engineer / metallurgist Lindsay Chapman for speaking to me – couldn’t have written this without her input. She’s a total legend.
“….Laurie Winkless looks at the many challenges surrounding the characterization of nickel-based superalloys.”
Superalloys, also called high-performance alloys, have certainly earned their name – this group of materials have found numerous applications due to their excellent mechanical properties at elevated temperatures. Their resistance to mechanical creep, corrosion and oxidation makes them indispensable to the aeronautical industry, where nickel alloys (Ni-alloys) have been used in turbine engines since World War II. Designing nickel alloys is not without its challenges – in a single alloy, there could be 13 different alloying elements designed in, not to mention any impurities that are present. So they are very difficult to model thermodynamically. And even if their composition is relatively well-defined, often it is difficult to establish what compounds form within the alloy, or where they can be found in the structure. But even with all these challenges, Ni-alloys are everywhere.
The mechanical strength of most metals decreases as the temperature goes up, but nickel based superalloys buck that trend – and it’s all down to their microstructure. Ni-alloys have a base matrix very similar to that of stainless steel – a face centred cubic (fcc) structure, referred to as the gamma (γ) phase. But, what makes them particularly interesting is that they also have a strengthening phase called gamma prime (γ’), consisting of ordered intermetallic particles such as Ni3Al and Ni3Ti, that stop the alloys from softening, even at high temperatures. According to Lindsay Chapman, Senior Scientist at the National Physical Laboratory (NPL), it’s this phase that provides “…the magic of nickel alloys – they can operate at a greater percentage of their melting point than any other alloys – you would struggle to get a steel to be effective so close to its melting point”.
This remarkable property of Ni-alloys also provides a big measurement challenge. Most thermal analysis techniques involve heating and cooling a sample at a defined rate. But in Ni-alloys, there is a strong relationship between heating rate and the material’s microstructure. This is not a surprise – alloy producers use heat treatments to alter the microstructure and ‘tune’ alloys for a particular application (e.g. where creep resistance is key). However, characterisation techniques such as differential scanning calorimetry (DSC) can do the same thing – just in the process of measuring an alloy, it can effectively “heat-treat” it. And as the microstructure can effect both the thermal and mechanical properties of these materials, there’s no guarantee that the sample that comes out is the same one that went in. In short, the process of measuring Ni-alloys can actually alter their properties!
Another measurement challenge is the fact that each technique offers a different thermal environment – systems like laser flash measure thermal diffusivity in a quasi-equilibrium environment, but a standard DSC system ramps up a sample’s temperature by several °C per minute. And neither of those environments reflect that under which Ni-alloys are likely to perform, when used in a power plant turbine, for example. As with all materials research, there is the question of scalability – do the measured properties of a 3 mm sample reflect those of the alloy in a metre long turbine blade? And is it possible to ensure that in a given alloy, each sample machined from a single block all have the same microstructure? It seems that, despite their widespread use, Ni-alloys still offer many challenges to the material producers, kit manufacturers and end users.
NPL sits right at the interface of these stakeholders, working alongside other measurement labs, universities and industrial partners to establish the measurement standards needed to meet these challenges. Chapman is an expert in high temperature thermal analysis and regularly works with customers from the aeronautical, automotive and power plant industries, to help them understand these alloys and their limitations. Historically, very few customers have asked specific questions of the alloy microstructure. But Chapman believes that this is changing. She says, “One of the main problems facing alloy producers is the scarcity of rare earth metals. As they start to look at substituting in other elements, they will need to fully understand the microstructure in order to move forward”.
For many materials-based industries, research from metrology labs such as NPL will help to lead the way and define what is possible at elevated temperatures. Identifying and reducing measurement uncertainties is the cornerstone of this effort, alongside establishing reliable high-temperature thermal analysis techniques for complex materials. For Ni-alloys, measurement is the key.