Inspection technique reveals secrets of how extremely hot materials degrade over time

A new technique has revealed the secrets of how materials degrade under immense loads and heat, potentially paving the way for the introduction of new materials in extreme conditions.

The process, known as in-situ scanning electron microscopy (SEM) heating and loading, has already led to performance modelling of an alternative stainless steel alloy for nuclear reactors. It was developed by researchers at North Carolina State University and included co-authors from the University of Birmingham, with support from the US Department of Energy and UK Research and Innovation.

“Until now, you could look at a material's structure before exposing it to heat or load, then apply heat and load until it broke, followed by a microstructural observation. That means you'd only know what it looked like before and after loading and heating,” said mechanical and aerospace engineer Afsaneh Rabiei.

The new in-situ process captures SEM images at temperatures up to 1,000ºC and stresses as high as two gigapascal – two billion newtons per metre squared. The technique shows how cracks form and grow, and how microstructures transform as they fail.

The researchers demonstrated the technique on stainless steel alloy 709, which is under consideration for use in nuclear reactors. Samples were heated to 750ºC and put under load cycles ranging from one second to one hour, repeating until they failed. One experiment ran for more than 600 hours.

The technique revealed the role of micro-structural details called twin boundaries in controlling crack growth. Observations showed cracks redirect after reaching twin boundaries, delaying crack growth.

 “Without our in-situ SEM heating and loading technology, such observations could not be possible,” said Rabiei. “Moreover, using this technique, we only need small specimens and can generate data that normally take years to generate. As such we are saving both time and the amount of material used to evaluate the material's properties and analyse its failure process.

“The ability to capture insights like these is a significant advance for research into any number of new, high-performance materials, particularly those that are designed to perform in extreme environments.”

The team used data from the tests to model how the alloy would perform over years of use in a nuclear reactor. The material reportedly outperformed 316 stainless steel, which is currently used in many reactors.

Further tests with the technique could model the performance of many more materials for extreme environments.

The research was published in Materials Science and Engineering.