MATERIALS RESEARCH: Fracto-emissions from metallic glass hit 3000 kelvins

Zirconium-based bulk metallic glass, a currently favored material for making premium golf clubs, has been observed to emit light at extremely high temperatures when fractured, according to researchers from Lawrence Berkeley National Laboratory and the University of California-Berkeley (both in Berkeley, CA). Research-team leader Robert Ritchie joked that light emissions might make the expensive clubs useful for golfing at night.

In a recent paper, the researchers reported temperatures of 3175 K and 1400 K in air and nitrogen, respectively, at the site of light emission and fracture.1 Ritchie said the finding was "quite a shock. No one has ever seen these massive temperatures before." Increases in temperature during fracture for most metals might get as high as 100 K, he said. So the Berkeley researchers, who are evaluating the metallic glass as a possible structural material, found the observation of light emissions and temperatures an order of magnitude higher than generally accepted upper limits "fascinating."

The light emissions and temperatures were observed in samples of the zirconium, titanium, nickel, copper, and beryllium material formed into 50 x 10 x 8-mm Charpy V-notch impact samples. A pendulum impact device moving at about 3.5 m/s was used to fracture the samples, and the notched impact surface was imaged through a 200-mm f/4 lens onto the entrance slit of a 0.25-m spectrometer. A helium-neon laser directed at the notch provided spectrometer beam alignment. A nitrogen-cooled charge-coupled-device camera collected emission spectra in the visible range, and a mercury cadmium tellurium detector (also nitrogen-cooled) collected near-infrared spectra. Effective blackbody temperatures were determined by applying a weighted nonlinear least-squares regression to spectral data.

In air, the notched specimens snapped on pendulum impact and gave off visible bright sparks with spectra corresponding to a blackbody temperature of 3175 K. When the sample was enclosed in a plastic tent filled with nitrogen, however, the same experiment yielded invisible near-infrared emissions corresponding to a blackbody temperature of 1400 K. The high temperature in air was attributed to oxidation of newly released material.

"Zirconium and titanium will burn in air, if you get them hot enough," Ritchie said. "But the real question is the temperature we observe in nitrogen—1400 K in the absence of oxidation and pyrophoric activity." The lower-temperature anaerobic emission is thought to be caused by a combination of material properties that include high flow stress, lack of work hardening, and low thermal conductivity.

The research is supported by the Basic Energy Sciences Office of the Department of Energy.

REFERENCE

1. C. J. Gilbert et al., Appl. Phys. Lett. 74(25), 3908 (June 21, 1999).