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Dissolution kinetics of Widmanstatten Ag₂Al precipitates Sagoe-Crentsil, Kwesi Kurentsir


A combination of SEM and electron-probe studies have been conducted on the dissolution of Widmanstatten γ(Ag₂Al) plates in Al-15.3 wt.% Ag between the temperatures 458°C and 494°C. Individual rather than averaged kinetic events have been monitored in all cases. The mechanism of dissolution observed at the plate tip was found to be different from that operative at the broadface. Precipitates shorten linearly at volume diffusion controlled rates according to the Horvay-Cahn , Zener-Hillert and Jones-Trivedi analyses, adapted for the case of dissolution. The kinetics of thinning were observed to be two orders of magnitude slower than volume diffusion controlled rates indicating an interface-controlled reaction. This interfacial inhibition has further been confirmed by the consistent fall of the interface concentration value below equilibrium solubility values as determined from the electron probe measurements. The electron probe measurements give similar diffusion distances at the tip and broadface and would require short circuit diffusion from the tip to the broadface to be in agreement with the kinetic results. A mechanism to explain the overall reaction has been proposed.

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