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Glassy Steel

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02/12/2006

ORNL researchers have developed a new bulk amorphous steel that is non-magnetic at room temperature and significantly harder than conventional steel.

ORNL researchers have discovered a new steel that is significantly harder and potentially stronger than ordinary steel yet has a jumbled atomic structure like glass. The glassy steel, which has been described in Physical Review Letters and Nature, also may have better magnetic properties and corrosion resistance than industrial steel, which has an orderly crystalline structure.

Possible applications for this bulk amorphous steel, or iron-based bulk metallic glass, could include tougher medical implants, lighter aircraft, die tools, tennis rackets, and golf clubs.

Golf clubs made of bulk metallic glass

The properties of bulk metallic glasses—alloys whose atoms are almost randomly arranged—are usually superior to those of counterpart materials whose atoms are arranged in repeating patterns typical of crystals.

Recent advances at ORNL in projects initially supported in 1996-97 by the internally funded Laboratory Directed Research and Development (LDRD) Program and later by the Department of Energy, as well as work in other laboratories throughout the world, have stimulated efforts to fabricate bulk amorphous steels. The LDRD project led by ORNL Corporate Fellow C. T. Liu produced an unusually thick zirconium-based bulk metallic glass.

Liu and Zhao Ping Lu, both of ORNL's Metals and Ceramics (M&C) Division, sought to make a bulk amorphous steel using ordinary industrial processes and alloying materials. Such a material would be much cheaper than zirconium-based bulk metallic glasses and would theoretically possess greater strength, hardness, and corrosion resistance than conventional steel.

The discovery of the unique steel was reported in Physical Review Letters by Lu and Liu, who heads the Alloying Behavior and Design Group in the M&C Division; Wallace Porter of M&C's Diffraction and Thermophysical Properties Group, and James R. Thompson, who holds a joint position in ORNL's Condensed Matter Sciences Division and the University of Tennessee's Physics Department.

Nanoindentation tests by Hongbin Bei, a UT postdoctoral researcher, revealed that the new steel is 3 to 4 times harder than ordinary steel, suggesting it is stronger. "The ORNL steel, unlike conventional steel, is not magnetic at room temperature," Thompson says. "The alloy has high-tech applications where lack of ferromagnetism is an asset, such as in accelerators, medical imaging devices, and submarines."

Based on his experimental work, Lu discovered that, by adding yttrium to a molten material that is 44 atomic percent iron and contains smaller percentages of boron, carbon, chromium, cobalt, molybdenum, and manganese, the material could be cooled so as to freeze into a noncrystalline instead of a crystalline structure. The stumbling block to producing a practical bulk amorphous steel has been the great difficulty in achieving the low critical cooling rate, or large glass-forming ability, of any iron-based alloys.

The addition of yttrium enabled the alloy to remain in a "liquid-like" structure at very low temperatures and thus stay amorphous as it solidified. Also, yttrium retarded the growth of iron carbide crystals, making the steel less likely to become crystalline.

Gravity enabled the molten iron liquid to drop cast into a copper mold, producing a bulk amorphous steel rod with a diameter of 12 millimeters (mm), 3 times the thickness of previously fabricated bulk amorphous steel rods containing no yttrium additions. The ORNL rod's glasslike structure was verified using X-ray diffraction and optical microscopy.

The research was sponsored by a private company, DOE's Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, and DOE's Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technology Program as part of the HTML User Program.

A group at the University of Virginia also produced a 12-mm rod made of a bulk amorphous steel of a different composition. Liu says that the challenge is to learn how to easily make bulk amorphous steel as thick as 20 to 30 mm.

A reviewer of the ORNL paper says that fabrication of a thicker bulk metallic glass of structural steel is "an extremely important discovery that should have a large impact on society."

Source: http://www.ornl.gov/info/ornlreview/v38_1_05/article17.shtml

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