A new study demonstrated the capabilities of the first "defect microscope" that can monitor how line defects move at the subspace of macroscopic materials - promising wide applications in the fields of physics, materials science, and engineering.

Led by a scientist from the Lawrence Livermore National Laboratory (LLNL), together with collaborators from France, Denmark, and Austria, the new project shows a classical example of the end of a line defect, or its dislocation boundary, before showing how it starts moving at the threshold of melting temperatures. They present the design and demonstration of the defect microscope in the report "In situ visualization of long-range defect interactions at the edge of melting," appearing in the Science Advances journal, July 14.

TEM Micrograph Showing Dislocations and Precipitates in Stainless Steel
(Photo: Wikityke at English Wikipedia via Wikimedia Commons)

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The Disconnect Between Macroscopic Properties and Microscopic Line Defects

"This work presents a large step forward for materials science, physics and related fields, as it offers a unique new way to view the 'intermediate scales' that connect microscopic defects to the bulk properties they cause," said Leora Dresselhaus-Marais, corresponding author and an assistant professor of Materials Science and Engineering at Stanford, in a news release from LLNL.

One of the oldest and most persisting challenges in materials science as a field is in relating the bulk materials' macroscopic properties with its microscopic defects. While long-range interactions between supposedly separate dislocation populations have been known to play critical roles in the deformation or melting of materials, definitely connecting these phenomena to a material's macroscopic properties remains unavailable until now. Another study, published in the Materials journal last February, has examined the interactions between dislocations and deformation twins against boundaries such as grain boundaries, twin boundaries, and phase interfaces during deformation at ambient temperatures.

Now, with the defect microscope, materials scientists can better understand how subsurface defects in macroscopic materials move. Defects might be related to a material's mechanical, thermal, or even electronic properties. For example, researchers noted line defects occurring in the atomic lattice that allows crystalline materials to deform when the load is applied. Meanwhile, ductile materials exhibit a wider range of hardness and malleability thanks to the interactions and behavior of their line defects or dislocations.

Real-Time Monitoring of Line Defects in the Macroscopic Level

To achieve the real-time monitoring of the defect microscope, researchers utilized a time-resolved dark field X-ray microscope (DFXM). This process directly visualizes the movement and interaction of dislocations over hundreds of micrometers on the subsurface of bulk aluminum. Creating a "real-time movie," researchers demonstrated that the movement and interaction of these dislocations are activated by temperature change and create a "boundary." They also showed how the weakening of the material's binding forces begins to destabilize the structure as it approaches 99 percent of the melting temperature.

Researchers also included images in the study mapping the dislocation boundary migration along a low-angle boundary as the single-crystal aluminum sample is heated from 97 percent to 99 percent of the material's melting point at 1220 degrees Fahrenheit (660 degrees Celsius).

"By visualizing and quantifying thermally activated dynamics that were previously limited to theory, we demonstrate a new class of bulk measurements that is now accessible with time-resolved DFXM, offering key opportunities across materials science," Dresselhaus-Marais added.


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