Engineering Strength: Oxide Dispersion for Bulk Alloys
Metals like iron and aluminum make up much of the modern world around us – from cars to wind-energy turbines – because they are strong, lightweight and cost-efficient materials. But as the heat is turned up on these metals, their mechanical properties and strength can be compromised. The need, therefore, continues to grow for advanced structural materials that can withstand high temperatures for applications in expanding industries like power generation and chemical processing.
Carnegie Mellon University (CMU) researchers are answering the need for heat-tolerant materials by developing a new way to strengthen metals using oxide particles. Bryan Webler, assistant professor of materials science and engineering, and his research group have created a two-step method to disperse oxide particles into solid bulk alloys.
First, the bulk alloy is introduced to specific atmospheric conditions to quickly cause internal oxidation. Second, a process called severe plastic deformation is performed to evenly distribute and refine the internally formed oxide particles.
Fig. 2. Scanning electron micrograph (5,000x magnification) of oxidized Fe-Y alloy after severe plastic deformation by equal-channel angular pressing. The light areas are Y2O3, and the gray matrix is Fe. The project goal is to create the most uniform possible dispersion of the Y2O3 in the Fe matrix.
To explain, Webler gives the example of an iron-aluminum alloy. “If you take a solid bulk material with aluminum atoms dissolved in an iron matrix and you expose it to the right set of atmospheric conditions, you’ll form little aluminum-oxide particles inside the metal matrix,” Webler said. “By evenly dispersing these little oxide particles in your metal matrix, you have a composite material that exhibits better properties than just the metal on its own. The oxides are very stable and don’t change at high temperatures.”
Webler’s research at Carnegie Mellon focuses around metals reacting with their environment. So, the inspiration for this work was drawn from a new twist on a simple chemical reaction.
“Most often, when researchers think about how metals react in their environments, we are thinking about preventing a reaction that could be potentially harmful to the structure of a material. But it’s interesting when we can use chemical reactions in metal processing as a tool to improve the properties of a material,” Webler said.
The method is a unique potential alternative to current commercial methods for strengthening metals with oxide particles. Currently, oxide-dispersion strengthened (ODS) alloys are all made using powder-metallurgy processes where a metal powder and an oxide powder are mixed together before exposure to pressure and heat to consolidate the powder particles.
Webler’s bulk processing method for ODS can be applied to multiple alloy systems, but the group is first using iron-yttrium as a model system to study fundamentals (Fig. 2). Iron-yttrium’s particularly fast internal oxidation makes it an interesting system from which to gather data on oxidation behavior. Currently, the research group is working toward a few main research goals.
The group is studying the rate of oxidation, as well as working to better quantify the dispersion of oxide particles. In addition, Webler is working with Carnegie Mellon colleague Yoosuf Picard, associate research professor of materials science and engineering, on characterization of the materials. Picard is leading a research project to gain a more detailed understanding of how oxide particles change the way the metal behaves.
Although still in the research stage, this bulk processing method is less expensive than a powder-metallurgy process and also has promise to be more precise.
Lab Reveals the Secret Life of Materials
In 1915, Sir William Henry Bragg and his son, William, received the first Nobel Prize ever given for materials characterization. Using X-rays, the Braggs revealed the crystal structure of rock salt, the first crystal structure ever characterized. Their success launched a century of exploration into the field of materials characterization, or the practice of peering into a material’s internal structure to learn its secrets – namely, its microstructure and chemistry.
Transmission electron microscopes (TEM) and scanning electron microscopes (SEM) are now the tools of choice for materials research. These are multimillion-dollar pieces of complex equipment that require significant training to use. At Carnegie Mellon University’s Materials Characterization Facility (MCF), students and faculty have access to the full gamut of today’s most advanced materials characterization technology. The lab also welcomes users from other universities and local industry.
“The microstructure is what gives a material its properties and performance,” said Marc De Graef, professor of materials science and engineering at CMU and co-director of the MCF. “If we can understand the microstructure, we can begin to think about how we might change it to affect the behavior of the material – make it stronger, more conductive, more resistant to corrosion or anything you’d like.”
The MCF houses an array of microscopes, spectrometers and diffractometers for the structural and chemical characterization of materials. It allows users to take materials characterization in novel directions.
Take Isha Kashyap, for example. As a Ph.D. candidate in materials science and engineering, Kashyap studies the structure of ferromagnetic shape-memory alloys. Using Lorentz transmission electron microscopy (LTEM), one of only a handful of university-based LTEMs in the U.S., Kashyap investigates defects and qualities of these alloys, which are used in magnetically induced mechanical actuators, a kind of motor that transforms energy into motion.
“The performance of the actuators depends on the properties of the alloys, which, in turn, depend on atomic-level interactions. These interactions can only be observed using powerful instruments with nanometer-level resolution,” Kashyap explained.
A computational research example is Saransh Singh, who spent three years developing software to simulate the images and diffraction patterns observed in scanning electron microscopes. Singh’s custom software, which he has made available for free online, can predict the outcome of materials characterization experiments without having to run the materials through high-powered, high-cost microscopes.
The Materials Characterization Facility at CMU houses an array of microscopes, spectrometers and diffractometers for the structural and chemical characterization of materials. It allows users to take materials characterization in novel directions.
One of the main applications of Singh’s software so far is in industry. Manufacturers will often develop a material they want to use, but they need to ensure that it does what it is designed to do. Not only that, but they need assurance it will continue to behave predictably throughout its entire life span. The instruments in the MCF, used in tandem with Singh’s software, can gather the information necessary to predict how these materials will respond in the future.
“The instruments in the MCF are really essential for my work,” Singh said. “I can immediately go check my theory to see if it is correct. You can come up with any theory you want, but unless you have some experimental validation of what you’re doing, it’s of no use to anyone.”
In just a period of four years, the Materials Characterization Facility enabled the publication of more than 320 research papers in peer-reviewed journals.
For more information: Contact Hannah Diorio-Toth, communications manager, Carnegie Mellon University, College of Engineering, Pittsburgh, Pa.; tel: 412-268-1208; e-mail: email@example.com; web: www.engineering.cmu.edu. Adam Dove is communications manager for Carnegie Mellon University’s College of Engineering.