To build a better jet engine, Kevin Hemker believes you have to start small. Very small.
Hemker, an associate professor of mechanical engineering in the Whiting School of Engineering, is starting with a powerful new microscope that allows him to see how rows of atoms are arranged in metal alloys. Knowing how these atoms arrange themselves, he says, can help predict how well these materials will be able to withstand the high temperatures, centrifugal forces and corrosive gases that exist inside a jet engine. By looking at defects in the geometric patterns formed by atoms, Hemker and his students are collecting information that may someday help scientists use a computer to devise durable new aerospace materials.
"The U.S. Air Force and others in the aviation industry want to be able to predict in a computer how well new metal alloys will behave without having to physically cast these alloys and test them," says Hemker. "That's a time-consuming and expensive process. What we're doing is providing the bench marks that will help them get to the point where they can evaluate these new materials by using computer models."
To advance this line of research, Hemker is using a $1.3 million high-resolution transmission electron microscope recently installed at the Homewood campus. The state-of-the-art instrument, one of a handful in use at universities throughout the United States, uses a field emission gun to send a powerful beam of electrons through a very thin foil. This foil has been ground and polished to a height of less than 100 atoms. The electron beam travels through it, producing pictures of the atomic structure that can be viewed on a phosphorescent screen, captured on film or videotape, or preserved as digital information.
"Not only can we take pictures of what the microstructure looks like, we can do more complicated chemical analyses," Hemker says. "You can take the electrons that come through the specimen and pass them through an imaging filter that analyzes how much energy the electrons lost as they passed through the specimen. Electrons lose different amounts of energy as they run into different types of atoms, so this is one way you can tell what kind of atoms are present in the specimen and where they are located on a near-atomic scale."
In his own research, funded by the Air Force, Hemker is using the high-tech tool to study imperfections in the atomic structure of pure metals and intermetallic alloys. "It's the imperfections and defects in the crystal structure that control the metal's mechanical properties, such as strength and toughness," he says.
Hopkins researchers are collaborating with Northwestern University scientists, who are developing computer models to predict how the properties of new materials might change as different metals are mixed into the recipe. By directly observing the arrangement of these atoms, Hemker will help determine whether these computer models are valid. Eventually, such models may be used to design new aerospace materials.
Hemker is one of at least two dozen Johns Hopkins faculty members from many science and engineering departments who are eager to conduct experiments with the new high-resolution electron microscope. He and David Veblen, a professor in the Department of Earth and Planetary Sciences in the Krieger School of Arts and Sciences, obtained grants to purchase the microscope and supervised its installation. Primary funding came from the National Science Foundation and the W. M. Keck Foundation.
In addition to Hemker and Veblen, Hopkins researchers in chemistry, physics, environmental engineering, biomedical engineering, chemical engineering and materials science will use the new instrument to study diverse specimens, ranging from water and soil pollutants to mineral crystals, nanostructured materials and amorphous and crystalline alloys.
"This will be an invaluable tool for a wide range of research projects throughout the university," Hemker says. "We'll be able to collect structural information and chemical characterizations at the atomic scale. If we want to stay at the cutting edge of science and engineering, we had to have this microscope."