Principle of dynamic decompression and cooling for materials processing
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Principle of dynamic decompression and cooling for materials processing Robert E. Apfel and Ning Qiu Department of Mechanical Engineering, Yale University, New Haven, Connecticut 06520-8286 (Received 16 October 1995; accepted 1 June 1996)
When a melt seeded with droplets of a volatile liquid is rapidly decompressed, the droplets explosively vaporize, taking their latent heat of vaporization from the melt and, therefore, homogeneously cooling and expanding it. The possibility of using this dynamic decompression and cooling (DDC) technique for producing an amorphous metallic foam is theoretically investigated. To test the premises of this theoretical model, preliminary experiments with an organic melt ( p-terphenyl) seeded with water drops were performed. Results of these experiments demonstrate the potential of this novel approach to materials processing. I. INTRODUCTION
Since the pioneering work on metallic glasses at P. Duwez’s lab in the 1960s,1 it has been generally agreed that amorphous metallic glass can be produced if the cooling rate is high enough (e.g., ,106 KyS) to suppress crystal nucleation and growth from the liquid state.2–4 Recently, the methods of amorphous metallic glass processing have been extended from liquid quenching to vacuum evaporation,5 cathode sputtering process,6,7 ion implantation,8 and solid state reaction.9–11 Among these available processing methods, the rapid solidification technique is still the most commonly used. While every method offers some special advantages, a common limitation is that the processed materials are either in the form of thin ribbons or fine particles. This is because a large surface area has to be produced in order to achieve a high quenching rate. In the case of metallic glass produced by solid state reaction, fine particles have to be produced for effective diffusion in the crystalamorphous phase transformation. However, a bulk form of material is often required in many applications and, therefore, has to be produced by powder consolidations which suffer many shortcomings. Thus, processing that can directly produce as-cast bulk amorphous metallic glass is desirable. The basic idea of bulk foam metallic glass production is the application of sudden decompression of a melt that is seeded with a volatile liquid.12 This dispersed “foaming” liquid will vaporize upon decompression, taking its latent heat of vaporization from the melt, thereby adiabatically and homogeneously cooling it. If the cooling rate is sufficiently great (e.g., ,106 Kys), a foam metallic glass is expected for some alloys. The resulting “foam” metallic glass will take the form of an open, solid, bulk structure that may possess glass properties and low density. One of the advantages of amorphous materials is the absence of structural defects such as dislocations and grain boundaries, providing the 2916
J. Mater. Res., Vol. 11, No. 11, Nov 1996
possibility of improved mechanical properties, such as high strength per unit mass. Since these foam me
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