AMORPHOUS METAL DISCOVERY AND BREAKTHROUGH

(From http://www.metglas.com/)

Metal alloys typically possess crystalline atomic structures in which individual atoms are arranged in ordered, repeating patterns. Amorphous-metal alloys differ from their crystalline counterparts in that they consist of atoms arranged in near random configurations devoid of long-range order.


Although such non-crystalline structures are common in nature, they have normally been associated only in non-metallic systems. For example, non-crystalline solids can be formed from silicates by continuous cooling from liquid state. The disordered liquid structure is preserved when the cooling rate is sufficiently great to prevent atoms or molecules from aligning into ordered, crystalline configurations.

In silicates, which consist of three-dimensional atomic clusters, the liquid state is viscous. Individual molecules have limited mobility and crystallization proceeds slowly. Only modest cooling rates are required to suppress crystallization entirely.

By contrast, liquid metal alloys are characterized by low viscosity and high diffusivity, partly because they consist of loosely bonded atoms rather that bulky clusters or molecules. The individual atoms in a liquid metal alloy can move about freely. On cooling, atomic rearrangement and crystallization occur rapidly, suggesting that extraordinary cooling rates would be necessary to bypass crystallization.

The first observation of metallic alloys with non-crystalline atomic structures was made in 1950 at the National Bureau of Standards. A. Brenner reported that amorphous Ni-P alloy films could be produced by electro deposition. Although amorphous Ni-P is widely used as a hard-surfacing material. Brenner’s starling observation of the amorphous structure has gone relatively unnoticed.

The discovery of amorphous metals is generally credited to P. Duwez, who in 1960 produced amorphous samples by rapid quenching an AU75Si25 alloy from the liquid state. Duwez used a pressurized gas gun to propel small droplets of the molten alloy onto a polished copper plate. On impact, each droplet deformed into a thin film. Intimate contact with the highly conductive copper plate allowed the molten film to cool rapidly and solidify into flake or ”splat” form. Ironically, this discover came as a surprise. Duwez adopted the rapid solidification, “splat quenching” process to study solid solubility and phase separation in crystalline metal-alloy systems.

H. Cohen and D. Turnbull suggested that the formation of the amorphous Au75Si25 structure was related to the presence of a deep eutectic in the Au-Si alloy system near 25 at.% Si coupled with the rapid solidification rate achieved in Duwez’s experiment. The incorporationof25% Si into molten Au reduces the melting point of Au from1336K to about 970K.

Thus the moltenAU75Si75 alloy could cool to a relatively low temperature without solidifying. At the reduced temperature, atomic diffusion would proceed slowly, permitting the alloy to solidify without crystallization. The association of amorphous-metal formation with both rapid solidification rates and eutectic compositions has formed the basis for virtually all subsequent studies of this material form.

In the early 1970s, H.S. Chen and D.E. Polk at Allied Signal conducted the most exhaustive study of amorphous-metal formation. This work defined alloy compositions that on rapid solidification formed stable, amorphous structures. The alloys were described by the general formula M70-90 Y10-30Z 0.1-15, where M is one or more transition metals, Y is a non-metallic element (such as Si, Al, or Ge). Virtually all amorphous-metal products manufactured follow this basic recipe.

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