In a study that advances lightweight magnesium alloys for more fuel-efficient, affordable vehicles, University of Michigan researchers used powerful X-rays to capture the first 3D views of the formation of microscopic structures that can help the material absorb stress without breaking.
The results, funded by the U.S. Department of Energy and published in Science, will improve understanding of the alloy's complex reaction to mechanical stress. Magnesium alloys weigh 30% less than aluminum. Today, some car manufacturers have started using them for nonload-bearing parts, but they could have much wider adoption if their behavior under stress can be optimized.
Each metal's crystalline structure—the highly ordered, repeating arrangement of atoms—influences how it responds to stress. Steel and aluminum, the most commonly used metals in cars, can stretch when pulled in any direction. They do this using what's referred to as slip systems, in which atoms stay in their row but shift in the stack to misalign the columns. Slipping is like a deck of cards on a table where the bottom card stays in place, but a push on one side makes the whole stack slant as each card moves a little further than the one below it.
Magnesium's crystalline structure only allows atoms to slip easily in a handful of directions. When pulled in directions it can't slip, magnesium alloys create "deformation twins"—mirror-image sections of the crystal structure—by shifting the orientation of atoms in a certain area. This can be thought of as an accordion fold in a sheet of paper, with the plane coming off the fold forming the mirror image at an angle.
Deformation twinning allows the material to stretch in more directions without breaking, creating ductility, but at a certain point too much twinning can create a concentration of defects that cause cracks to form.
"We were surprised to find all three twins formed in triple junctions, where three crystals touch, and defects always formed where the twin touches another crystal. This consistency can help us understand twin microstructures to optimize the material lifetime," said Ashley Bucsek , U-M assistant professor of mechanical engineering and materials science and engineering and corresponding author of the study.
Before the experiment, the researchers used a small-scale CT scanner to map how crystal grains were oriented within a magnesium alloy sample. From here, they selected a specific grain of interest with a good orientation for following the twinning process.
They leveraged the European Synchrotron Radiation Facility in France, one of a few facilities in the world able to generate powerful enough X-rays to image the grain of interest at an ultra-high resolution without destroying the sample.
The technique, called dark-field X-ray microscopy, or DXFM for short, works by accelerating charged particles around the synchrotron and directing the bright X-rays they emit into the alloy. X-rays diffract through the crystal grains at different angles depending on their orientation. The "dark field" technique selectively captures X-rays diffracted at a specific angle—in this case, the angle associated with the previously selected grain of interest—and magnifies the diffracted X-ray image using a series of lenses.
They applied three typical car part loads that would stretch the alloy—0.6 megapascals (MPa), a unit of tensile strength, 30 MPa and 45 MPa—imaging the sample between each.
"Real-space X-ray images gave us a front-row seat to observe twinning as stress was applied. We literally watched the twin appear and evolve with our own eyes for the first time," said Sangwon Lee , U-M doctoral student of mechanical engineering and lead author of the study.
The high-resolution images are the first step toward optimizing the material's ductility without compromising stability. As a next step, the research team plans to capture changes in real time.
The research is funded by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering (Award DE- SC0008637) as a part of the Center for Predictive Integrated Structural Materials Science (PRISMS).
Researchers from the European Synchrotron Radiation Facility, Los Alamos National Laboratory and National Institute of Standards and Technology contributed to the study.
Study: Three-dimensional nucleation and growth of deformation twins in magnesium (DOI: 10.1126/science.adv3460)
