Saturday, June 14, 2008

Super-hard nanocrystalline iron that can take the heat

Dr. Carl C. Koch, an NC State professor of materials science engineering

Carl Koch Professor, Associate Department Head

Carl Koch was a research group leader with the Metals and Ceramics Division of Oak Ridge National Laboratory before he joined the NCSU faculty in 1983.

Koch's research in recent years has focused on the synthesis, characterization and properties of metastable materials. Metastable materials with unique structures or microstructures that we have studied include metallic glasses, intermetallic compounds, nanocrystalline materials and polymer alloys. The chief nonequilibrium processing methods used to prepare metastable materials are rapid solidification from the liquid phase (at about 106 oC/s) and mechanical attrition of powders in high-energy ball mills.

Koch was the first researcher to demonstrate that amorphous alloys metallic glasses could be made by ball milling certain elemental powder mixtures by the technique known as mechanical alloying. Recent research has turned to nanocrystalline materials prepared by either mechanical attrition or controlled crystallization of amorphous precursors formed by rapid solidification. His group's interest in these materials is due to their special mechanical and soft magnetic properties.

Researchers at North Carolina State University have created a substance far stronger and harder than conventional iron, and which retains these properties under extremely high temperatures – opening the door to a wide variety of potential applications, such as engine components that are exposed to high stress and high temperatures.

Iron that is made up of nanoscale crystals is far stronger and harder than its traditional counterpart, but the benefits of this “nano-iron” have been limited by the fact that its nanocrystalline structure breaks down at relatively modest temperatures. But the NC State researchers have developed an iron-zirconium alloy that retains its nanocrystalline structures at temperatures above 1,300 degrees Celsius – approaching the melting point of iron.

Kris Darling, a Ph.D. student at NC State who led the project to develop the material, explains that the alloy’s ability to retain its nanocrystalline structure under high temperatures will allow for the material to be developed in bulk, because conventional methods of materials manufacture rely on heat and pressure.

In addition, Darling says the ability to work with the material at high temperatures will make it easier to form the alloy into useful shapes – for use as tools or in structural applications, such as engine parts.

The new alloy is also economically viable, since “it costs virtually the same amount to produce the alloy” as it does to create nano-iron, Darling says.

Dr. Carl C. Koch, an NC State professor of materials science engineering who worked on the project, explains that the alloy essentially consists of 1 percent zirconium and 99 percent iron. The zirconium allows the alloy to retain its nanocrystalline structure under high temperatures. ###

The research will appear in the journal Scripta Materialia. Kris Darling is the lead author on the paper, “Grain-size Stabilization in Nanocrystalline FeZr Alloys,” but co-authors include Koch, fellow NC State materials science professor Dr. Ronald O. Scattergood, NC State doctoral student Jonathan E. Semones, and NC State undergraduates Ryan N. Chan and Patrick Z. Wong.

Note to editors: The paper’s abstract follows.

“Grain-size Stabilization in Nanocrystalline FeZr Alloy”
Authors: Kris A. Darling,* Ryan N. Chan, Patrick Z. Wong, Jonathan E. Semones, Ronald O. Scattergood and Carl C. Koch, North Carolina State University
Accepted for publication: May 7, 2008, in Scripta Materialia

Abstract: Nanocrystalline Fe–Zr alloys with a nominal grain size of 10 nm were synthesized by mechanical alloying. The grain size in pure Fe was >200 nm after annealing for 1 h at T/TM = 0.5. Additions of 1 at .% Zr stabilized the grain size at 50 nm up to T/TM = 0.92. Particle pinning, solute drag and reduction in grain-boundary energy have been proposed as stabilization mechanisms. The stabilization in Fe–Zr alloys is attributed to a reduction in grain-boundary energy due to Zr segregation.
Contact: Matt Shipman 919-515-6386 North Carolina State University

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