New technique enables high-heat, high-load real-time microscopy

Researchers have developed a technique to track microscopic changes in metals or other materials in real time, even when exposed to extreme heat and loads over a long period of time, a phenomenon known as name of "creep". This technique will accelerate the development and characterization of materials intended for use in extreme environments, such as nuclear reactors.

"Until now, you could examine the structure of a material before exposing it to heat or a load, then apply heat and a charge until it breaks, followed by an observation of the microstructure Afsaneh Rabiei, corresponding author of an article on the book and professor of mechanical and aerospace engineering at North Carolina State University.

"Our technique, called" in-situ scanning electron microscopy (SEM) heating and loading, "allows us to see the microscopic changes that occur throughout the process – you can see how cracks are formed and how they develop, or how the microstructure is transformed during failure This method is extremely useful for understanding the characteristics of a material and its behavior under different loading and heating conditions. "

Rabiei developed the in situ SEM technique for high temperatures and load (voltage) as a means of performing high throughput evaluations of the behavior of advanced materials. The goal was to be able to predict how a material reacts under various heating and loading conditions. The project was supported by the Ministry of Energy. The instrument can capture SEM images at temperatures up to 1000 degrees Celsius (C) and stresses up to two gigapascals, which equates to 290,075 pounds per square inch.

For their recent demonstration of the potential of the technique, the researchers performed "creep fatigue" tests on a stainless alloy alloy called 709 alloy, the use of which is being considered in nuclear reactors.

"Creep fatigue testing involves exposing materials to high temperatures and repeated, extended loads, which helps us understand the behavior of structures when they are placed under loads in extreme environments," explains rabiei. "This is clearly important for applications such as nuclear reactors, designed to operate for decades."

To this end, Rabiei and his associates tested 709 alloy samples at temperatures of 750 ° C, which underwent repeated charging cycles, ranging from charging for one second to charging for one hour, up to one hour. At break. In one iteration, where the sample was repeatedly exposed to a load for one hour, with seven-second intervals between charges, the experiment lasted for more than 600 hours. And SEM in situ captured everything.

"SEM in situ allowed us to track the microscopic development of cracks in the material and the evolution of the microstructure during creep fatigue tests," Rabiei explains. "We were then able to use this data to model the behavior of the 709 alloy over the years of use in a nuclear reactor.This alloy 709 outperformed the 316 stainless steel, which is currently used in many reactors.

"This is good news, but what's more exciting here is the methodology we used. For example, our SEM in situ technique allowed us to see the role played by the micro-structural details called twin boundaries in controlling the growth of cracks in alloy 709. Our observations showed that when a crack It reaches 709 in such a range, it deviates and makes a detour, which delays the growth of the crack and improves the strength of the material. Without our in situ heating and SEM charging technology, such observations would be impossible. In addition, using this technique, we only need small samples and can generate data that normally takes years, saving us both time and the amount of material used to evaluate properties. material and analyze its failure process.

"The ability to capture such information is a significant step forward for research into a large number of new high performance materials, especially those designed to operate in extreme environments," said Rabiei.

The document "Performance of alloy 709 under fatigue creep at different times" is published in the newspaper Materials Science and Engineering: A.