210 Gasmetal eutectic solidification

Numerous metal alloy-hydrogen binary phase diagrams exhibit a eutectic; these include Al-, Be-, Cr-, Cu-, Fe-, Mg-, Mn- and Ni-based alloys. The alloys are melted, saturated with hydrogen under pressure, and then direction-ally solidified, progressively reducing the pressure. During solidification, solid metal and hydrogen simultaneously form by a gas eutectic reaction, resulting in a porous material containing hydrogen-filled pores. These materials are referred to as GASARs (or GASERITE).

A schematic diagram of the basic approach is shown in Figure 2.10. A furnace placed within a pressure vessel is used to melt an alloy under an appropriate pressure of hydrogen (typically 5-10 atmospheres of hydrogen). This melt is then poured into a mold where directional eutectic solidification is allowed to occur. This results in an object containing a reasonably large (up to 30%) volume fraction of pores. The pore volume fraction and pore orientation are a sensitive function of alloy chemistry, melt over-pressure, melt superheat (which affects the hydrogen solubility of the liquid metal), the temperature field in the liquid during solidification, and the rate of solidification. With so many process variables, control and optimization of the pore structure are difficult. The method poses certain safety issues, and in its present form is a batch process. As a result, materials manufactured by this route are costly. Though GASAR materials were among the first highly porous materials to attract significant interest, they remain confined to the laboratory and are not yet commercially available.

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