HOT SPARKS FLY FROM A NOTCHED SAMPLE OF BULK METALLIC GLASS FRACTURED BY A BLOW FROM A PENDULUM

The alloy of zirconium, beryllium, titanium, copper, and nickel is one of the first metallic glasses that can be made in bulk and formed into strong, hard, useful objects. It was discovered by William L. Johnson and Atakan Peker of the California Institute of Technology.

Working with the discoverers to investigate the mechanical properties of the novel material, which as yet are poorly understood, a team of researchers in Berkeley Lab's Materials Sciences Division led by Robert Ritchie mounted notched specimens of the zirconium-based metallic glass in a pendulum impact device. They were startled to find that, when fractured in air, the alloy shot out showers of bright, hot sparks.

"We were measuring how much energy it took to fracture the material when we stumbled onto the light emission," says Christopher Gilbert, a postdoctoral fellow in Ritchie's group. "As it turned out, scientists at Oak Ridge had seen this last year -- but we've managed to measure the associated temperatures and to explain the mechanism for the first time."

In air, when struck by the pendulum weight, the notched specimens snapped and sent out bright sparks whose color corresponded to a blackbody temperature of 3,175 degrees Kelvin. The same experiment in a nitrogen atmosphere produced no visible sparks, but emission was detected in the infrared at 1,400 K.

"So-called fracto-emission is familiar in brittle insulating solids and somewhat less familiar in ordinary metals," says Gilbert, "but emissions of this intensity are unprecedented in ductile polycrystalline metals. And so far as we know, fracto-emission has never been quantified in amorphous metals."

Digital camera images, plus the experiments in pure nitrogen, showed that the sparks in air were caused by burning particles thrown off from the fracture surface. When the broken specimens were examined under a scanning electron microscope, blobs of melted material were seen on the fracture surface. The heat generated in breaking the metallic glass was enough, apparently, to ignite freshly exposed metal particles.

FRACTURED SURFACE SHOWS EVIDENCE OF LOCAL MELTING

"Zirconium and titanium will burn in air, if you get them hot enough," says Ritchie, "but the real question is the temperature we observe in nitrogen --1,400 K in the absence of intense oxidation and pyrophoric activity."

Gilbert says, "When the metallic glass is broken, the deformation is highly localized in narrow bands, which generates intense heat from plastic work" -- rather the way a wire gets hot if bent back and forth rapidly. Melting observed on the fracture surfaces means local temperatures must have exceeded 935 K, the temperature at which the metallic glass liquefies.

Team member Joel Ager surmises that the temperature rises rapidly as the material is deformed "partly because metallic glass has terrible conductivity for a metal. It can't get rid of the heat. But this can't be the whole story."

Commercial use

Unlike pure metals and most metal alloys, metallic glasses have no regular crystalline structure. This lack of long range order or microstructure is related to such desirable features as strength and low damping -- the ability of some of these alloys to deliver a really big bounce -- which is one reason why the premier use for zirconium-based metallic glass is in the manufacture of expensive golf club heads.

Only recently has it been possible to obtain metallic glass in enough bulk to make a golf club head or to perform extensive mechanical testing. In the past, to prevent segregation and crystallization of the melt required such rapid cooling -- in only about a thousandth of a second, at a rate of about a million degrees Celsius per second -- that only very thin wires and ribbons could be formed.

Zirconium-based metallic glasses can be cast in bulk because they can be cooled much more slowly, at about 10 degrees C per second; they achieve their glassy, disordered state by alloying metals with dramatically different atomic sizes and chemical characteristics. The alloy studied by Ritchie's group, for example, is two-fifths zirconium and one-fifth beryllium.

William Johnson of Caltech and his colleagues at Amorphous Technologies International of Laguna Niguel, California, pioneered the development of these alloys and their commercial uses. Golf clubs made of zirconium-based metallic glass have unusual springiness, a "soft" feel, and an almost ideal density between that of stainless steel and titanium (currently the connoisseur's choice); they also demonstrate, however, that even slow-cooling metallic glass doesn't cool slowly enough for really large castings. For the time being, at least one of the dimensions must be under four inches.

Nevertheless, the properties of bulk metallic glasses -- their high strength-to-weight ratios, high hardness, excellent wear properties, good forming and shaping qualities, and their unusual magnetic properties as well -- hold promise for many important applications. Ritchie's group is pursuing a wide range of measurements, including electrochemical studies by team member and graduate student Valeska Schroeder.

"We're interested in determining which properties of these new materials can be attributed to their individual constituents and which to the amorphous nature of metallic glass," says Ritchie. "Valeska has already shown that the notion that metal corrosion in the zirconium-based glass would be alleviated because of its amorphous nature is simply not true. The crystalline and glassy microstructures show very similar pitting potentials, in a variety of chemical solutions."

As for unexpected light emissions from the zirconium-beryllium alloy, Ritchie says, "These extremely high temperatures aren't Polywater, not a delusion -- they're real. And they demand explanation."

The researchers report their unusual findings in the June 21, 1999, issue of Applied Physics Letters.

The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.