THE DEVELOPMENT OF HIGH SURFACE AREA MICROSTRUCTURES WITH DIMENSIONS CONTROLLED BY MASS TRANSFER PROCESSES

摘要

Exposure of alloys to environments which can selectively dissolve an alloy constituent leads to dealloying and the formation of a porous residue. Two examples are described : leaching of aluminium from Cu-AI alloy into aqueous alkali solution, and formation of gaseous SiO during high temperature exposure of Ni-Si alloys to low oxygen activity gases. In each case the reaction product consists of a layer of fibres oriented normal to the surface, hundred of microns in length but about 50nm in diameter. Fibre elongation is controlled by diffusion in the fluid phase and parabolic kinetics result. Fibre width and spacing are thought to be controlled by diffusion in the parent alloy or at the alloy-reaction product interface. Product morphologies are discussed in terms of cellular phase transformation reactions.