Afsaneh Rabiei with the device she developed that can capture SEM images in real time at temperatures up to 1000°C while applying stresses as high as 2 gigapascal. Photo: NC State University.A new microscopy technique allows researchers to track microstructural changes in materials in real time, even when the material is exposed to extreme heat and stress. Using the technique, researchers were able to confirm that a stainless steel alloy known as alloy 709 shows potential for use at elevated temperatures, such as in nuclear reactor structures.
"Alloy 709 is exceptionally strong and resistant to damage when exposed to high temperatures for long periods of time," says Afsaneh Rabiei, corresponding author of a paper in Materials Science and Engineering: A on the new findings and a professor of mechanical and aerospace engineering at North Carolina State University. "This makes it a promising material for use in next-generation nuclear power plants.
"However, alloy 709 is so new that its performance under high heat and load is yet to be fully understood. And the US Department of Energy needed to better understand its thermomechanical and structural characteristics in order to determine its viability for use in nuclear reactors."
To address the Department of Energy's concerns, Rabiei came up with a novel solution. Working with three companies – Hitachi, Oxford Instruments and Kammrath & Weiss – Rabiei developed a new technique that allows her lab to perform scanning electron microscopy (SEM) in real time while applying extremely high heat and high loads to a material.
"This means we can see the crack growth, damage nucleation and microstructural changes in the material during thermomechanical testing, which are relevant to any host material – not only alloy 709," Rabiei says. "It can help us understand where and why materials fail under a wide variety of conditions: from room temperature up to 1000°C and with stresses ranging from zero to 2 gigapascal."
Rabiei's team collaborated with the University of Birmingham in the UK to assess the mechanical and microstructural properties of alloy 709 when exposed to high heat and load. The researchers exposed a 1mm-thick sample of the alloy to temperatures as high as 950°C until the material ‘failed’, or broke.
"Alloy 709 outperformed 316 stainless steel, which is what's currently used in nuclear reactors," Rabiei says. "The study shows that alloy 709's strength was higher than that of 316 stainless steel at all temperatures, meaning it could bear more stress before failing. For example, alloy 709 could handle as much stress at 950°C as 316 stainless steel could handle at 538°C.
"And our microscopy technique allowed us to monitor void nucleation and crack growth along with all changes in the microstructure of the material throughout the entire process," Rabiei says. "This is a promising finding, but we still have more work to do. Our next step is to assess how alloy 709 will perform at high temperatures when exposed to cyclical loading, or repeated stress."
This story is adapted from material from North Carolina State University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.