Ames Laboratory, U.S. Department of Energy, Ames, Iowa
Ames Laboratory, U.S. Department of Energy, Ames, Iowa
AMES, Iowa -- Together with two other scientists, Rohit Trivedi, a senior metallurgist at the U.S. Department of Energy's (DOE's) Ames Laboratory and distinguished professor of materials science and engineering at Iowa State University (ISU), has won two grants from the NASA's Microgravity Science and Applications Division for research on the processing of materials under conditions that minimize the effects of gravity.
The awards, which extend four years, have a value of about $1.2 million.
One award is for a ground-based research protocol, and the other is for a flight experiment. According to Michael Wargo, program scientist for materials science at NASA headquarters, 53 of the more than 200 proposals that were submitted in response to the NASA announcement were selected for ground-based research and only 10 were selected for flight.
"Many materials scientists introduce their discipline by walking to the blackboard and drawing a big triangle with the words 'structure,' 'processing' and 'properties' at the corners," Wargo comments. Establishing quantitative and predictive relationships between the corners of the triangle is the role of the materials scientist, and Professor Trivedi has been succcssful in describing experiments that provide insight into these relationships."
In both research projects the scientists will study "microstructure selection" in the solidification of molten metals with the goal of improving quantitative models of this process. Quantitative understanding will allow materials with desirable properties to be made by manipulating processing conditions.
"Before, metallurgists would take a microstructure and measure its properties," Trivedi says. "Today we want to be able to alter processing conditions to yield a microstructure with the desired properties. This requires a firm understanding of the physical factors that govern microstructure selection."
Although there has been significant progress in modeling solidification recently, the role of gravity-driven convection is still poorly understood. The problem is that mathematical models cannot account for convective effects, and earth-based experiments cannot avoid them. "This means that sometimes we cannot check the accuracy of the models we've constructed against experimental results," says Trivedi.
"Every researcher--not just the materials scientist--wants to to be able to conduct experiments that have unambiguous answers," Wargo adds, "and in materials science the presence of gravity provides ambiguity. In many cases we can eliminate that ambiguity by conducting experiments in microgravity."
The first research proposal concerns the formation of complex cellular and dendritic structures in metallic systems. Depending on the conditions under which a metal solidifies, the interface between the solid and the liquid can be either flat, cellular (honeycomb-like), or dendritic (treelike). The goal of the research is to obtain unambiguous experimental data in microgravity that will allow Trivedi and his co-investigator, Surendra Tewari of Cleveland State University, to establish the fundamental relations that govern the formation of these complex structures.
The second proposal, which Trivedi wrote with Alain Karma, a physicist at Northeastern University, concerns banded microstructures in what are called peritectic systems. When these systems have compositions within a specific range, they can form microstructures made up of alternating layers of two distinct phases (areas of the solid distinguishable by composition, structure or other properties) perpendicular to the growth direction.
"Theoretically," says Trivedi, "if there were no convection, bands would form all the way through the sample. But when we do the experiment in the earth's gravitational field, we find that the bands become curved, instead of being flat, and quickly disappear. If we suppress convection during the experiment, we can get bands to form, but the spacing of the bands is very different from what theory predicts. This might be because we're missing some physics, but it might also be due to convection." In this case as well, the microgravity experiments will provide benchmark data that will allow scientists to distinguish the physics of solidification from the effects of gravity.
Trivedi points out that microstructure selection is not limited to the solidification of metals. A surface of a bowl of hot soup breaks into cells as the soup begins to cool; cells form in the shock front from an explosion at certain velocities; the flame from a bunsen burner develops dendritic side branches when the gas is turned up; and plants and animals develop by patterned growth and differentiation. For this reason an understanding of the physics of microstructure selection could have wide ramifications.
Ames Laboratory is operated for the DOE by ISU. The Laboratory conducts research in various areas of national concern, including energy resources, high performance computing, environmental cleanup and restoration, and the synthesis and study of new materials.
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