Bright-field microscopy images of an aluminum single crystal before (top) and after (bottom) mechanical healing, showing how it can remove pre-existing defects. Image: MIT.When designing a new material, whether for an airplane, car, bridge, mobile device or biological implant, engineers strive to make the material strong and defect-free. However, conventional methods for controlling the amount of defects in a material, such as applying heat or mechanical stress, can also have undesirable effects on the material's structure and performance.
An international team of researchers, including Carnegie Mellon University president Subra Suresh, has now developed a new technique called cyclic healing that uses repetitive, gentle stretching to eliminate pre-existing defects in metal crystals. Their results are published in the Proceedings of the National Academy of Sciences.
Most materials, including metals, are crystalline. When materials fail, it is usually due to defects in the crystal or in the arrangement of multiple crystals in a polycrystalline structure. While much research has been done on metal fatigue at larger scales, new technologies are just now allowing researchers to see how atomic-scale defects nucleate, multiply and interact in materials subjected to monotonic or fatigue loading inside a high-resolution microscope.
In this study, the researchers used transmission electron microscopy to look inside sub-micrometer-sized samples of aluminum crystals as they subjected them to stresses like repeated, small-amplitude deformation or fatigue loading. They found that gentle cyclic deformation, a process that repetitively stretches the crystal, helped to unpin rows of atomic defects known as dislocations in the metal samples, allowing them to move. Image forces, which act to minimize the energy of the defects, then attracted these dislocations to free surfaces and forced them out of the crystal. As a result, the crystal ‘heals’, becoming essentially free of pre-existing dislocations, thereby significantly increasing its strength.
This finding came as a surprise to the researchers because cyclic deformation has the opposite effect in larger micro- and macro-sized metal crystals. In these larger crystals, repeated stretching generally leads to the creation, accumulation and interaction of defects, which can lead to cracking and failure.
"This work demonstrates how cyclic deformation, under certain controlled conditions, can lead to the removal of defects from crystals of small volume," says Suresh. "It also points to potential new pathways for engineering the defect structure of metal components in a variety of sub-micro-scale systems."
This story is adapted from material from Carnegie Mellon 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.