Something new under the sun
Laying the foundation for the next generation of materials
A xenon-arc-lamp floating-zone furnace is used to make single crystals and composite materials with well-aligned nanoscale microstructures. These model systems are used to study the fundamentals of materials at extremes. Photo: Jason Richards
Metals have been at the heart of technological advancement for 4,000 years—since people moved from tools of stone, bone and wood to copper, bronze and, eventually, iron and steel. The rise of metallurgy—the ability to extract, refine and mix metals— also coincides with the development of writing and the rise of the first cities. Since then metals have been intertwined with virtually every aspect of civilization, from pots and pans to geopolitics, from manufacturing to macroeconomics.
While this millennia-long fascination with metals has had obvious benefits, it actually presents something of a challenge to scientists trying to modify materials atom by atom to give them radically different physical properties—in this case, ultrastrong steels for nuclear energy systems and other extreme environments.
"People have been working with metals for thousands of years—and very assiduously for the last couple hundred," says materials scientist Malcolm Stocks, director of ORNL's Center for Defect Physics. "So we have a lot to choose from, and they have all been refined to fit very particular purposes. That's why coming up with something that has properties outside of these accepted boundaries—disruptively outside of these boundaries—is a very difficult task."
The goal of the CDP and other research being conducted at ORNL with the support of the US Department of Energy's Office of Basic Sciences is to use advanced synthesis, analytical and computational tools to better understand how materials work at very small scales and under extreme conditions. This knowledge will then be used to make more quantitative connections between the relevant aspects of materials' structure and properties such as strength and radiation resistance.
"We're really looking at two different, but related, definitions of 'extreme,'" Stocks says. "In the case of the CDP, we are trying to develop materials that can resist damage from high levels of external radiation."
"On the other hand, our other work for the Office of Basic Energy Sciences, as well as for applied DOE programs, some of which evolved from our BES work, tends to focus on the intrinsic properties of materials," explains ORNL materials scientist Easo George. Those investigations explore extremes of a somewhat different kind and address questions that deal with how to more closely approach and function effectively near the intrinsic limits of materials, such as their melting points or theoretical strengths.
The knowledge gained by this research will be incorporated into computer models designed to accelerate the development of materials for use in applications such as next-generation nuclear energy systems, where radiation damage is a concern, or turbine blades, where heat-resistant materials play a critical role.
Failure is not an option
The need for accurate descriptions of material behavior is particularly acute in the realm of structural materials, where expectations of performance are particularly high. For example, if you were driving across a bridge and a streetlight went out, you probably wouldn't be too upset; however, if the bridge collapsed, that would be a different story.
"We think of structural materials differently than other materials," says George. "A new consumer electronic device can take over the market in the relative blink of an eye, but it takes a long time for new structural materials to get into service and become useful. Structural materials have to go through a lot of testing—sometimes over long periods of time, depending on their expected length of service—to be sure they are going to work all the time."
Detailed models of material structure validated by experiments like those the center is developing will eventually reduce the time required for testing by identifying the structural characteristics that a material must have in order to achieve desired properties. If you wanted a material with improved radiation resistance, for example, this type of model would identify certain characteristics these materials would need to have and eliminate materials that didn't have them.
The need for advanced modeling capabilities becomes even more acute when you consider that most structural materials have to meet a number of requirements simultaneously.
"There are very few structural applications where only one property is important," George says. "It's almost always a combination of properties. You need strength, you need toughness, you need corrosion resistance, and you need the ability to manufacture and form the material. It's not hard to make a superstrong material that has no other desirable properties. For example, a material can be very strong and very brittle at the same time. However, getting the right combination of properties takes a lot of work. That's a big part of what our research is about."
"Model" materials first
The strength of a material is determined by its defects, so to gauge a structure's strength, researchers need precise knowl-edge of what kinds of defects are present in the material and how those defects are distributed. Generally speaking, the fewer defects there are in a material, the stronger it is. Similarly, if a composite material contains two different materials, the composite's properties generally lie somewhere between the two.
For example, George and his colleagues are studying a composite material that consists largely of a standard nickel-aluminum alloy, but it is reinforced with ultra-strong, defect-free fibers made of either molybdenum or chromium that are five to ten times stronger than their normal dislocation-containing counterparts. Although the fibers represent only a small fraction of the composite's total volume, their contribution to its high-temperature strength is tremendous relative to their size. For example, the composite's ability to resist deformation at high temperatures is a million or more times greater than that of the nickel-aluminum alloy.
Developing the ability to produce defectfree crystalline fibers like these from a range of metals may eventually enable scientists to design materials that combine traits, such as strength and ductility, or strength at extreme temperatures.
"The composites currently being investigated are relatively simple 'model' materials," George explains. "Before we move on to more complex systems, we need to understand how each phenomenon is related to the structure of the material."
Certainty and flexibility
While scientists have known for a long time that understanding defects is the key to controlling other aspects of material behavior, they haven't had a good understanding of precisely how defects work at the small scales Stocks, George and their colleagues are investigating.
"We know that defects that appear at very small scales ultimately control the properties of materials at the macro scale," Stocks says. "The point of our research is to make the link between these scales increasingly quantitative. If we can do that, we think we can overcome many of the conventional limitations of material behavior. If we can incorporate an understanding of the defects at this fundamental scale into our computer models, this will allow engineers greater latitude and enable them to use materials closer to their extremes."
Traditionally, engineers have had to work within the narrow confines of materials whose properties and performance characteristics had been established explicitly through experimentation. ORNL's materials researchers are trying to broaden those parameters by providing the next generation of engineers with the modeling tools needed to identify materials that meet specific requirements or even guide the development of entirely new materials.
"Our goal is to increase the engineer's certainty as well as flexibility," Stocks says. "The ability to predict very precisely how a particular material will behave under a specific set of circumstances is invaluable." —Jim Pearce