Formation of highly saturated Al–Ag precursor by rapid solidification for skeletal silver synthesis

Abstract

The solubility of silver in the Al phase is the most important factor for the synthesis of skeletal silver. Rapid solidification process was used to obtain Al precursor particles with much dissolved silver from various Al100−xAgx (x=15–39at%) alloys and their effect was compared with that obtained by the conventional solidification process. The conventionally solidified specimens showed the primary dendritic α-Al phase and the eutectic was composed of the α and Ag2Al phases in the inter-dendritic region as expected from the equilibrium phase diagram. On the other hand, the X-ray diffraction patterns of the rapidly solidified specimens, except Al61Ag39 alloy, showed almost single α-phase though their micro structure and had a clear network showing different solubilities of silver. The solubility of silver in these alloys has a maximum for Al70Ag30 alloy. The precipitation rate of the Ag2Al phase from the α-Al phase was quite high as derived from a Kissinger plot using DSC measurements. The powder specimens solution treated at 823 K for 1.8×103 s followed by water quenching showed significant precipitation of the Ag2Al phase though most of the silver can be dissolved in the α phase by holding at that temperature. The cooling rate of the powder was not high enough to suppress the precipitation. The rapid solidification had two advantages over the conventional solidification process. The first is that the rapid solidification formed the highly saturated α phase beyond 23.5at%. The second is that particles directly formed from the melt and could circumvent pulverizing of the conventionally solidified ingot.