A place exchange mechanism does not allow the fluxes to be different. A diffusion couple is created by welding together A and B, with intert markers placed at the weld junction. The markers are in the form of wires extending out of the diffusion couple and attached to the laboratory bench. The whole specimen therefore translates along the bench as diffusion proceeds because the flux of A is larger than that of B. Given that there is a net flow of matter there will be an equal and opposite net flow of vacancies which condense to form pores Figure . Light micrographs of a laminate of copper and nickel, before and after heat treatment.
Porosity occurs in the copper because copper diffuses faster in the nickel than vice versa.
Notice that this goes against intuition because copper has a lower melting temperture than nickel. The photograph is reproduced by courtesy of Professor David Matlock of the Colorado School of Mines. The filaments are then heat treated so that the tin reacts with the Nb to produce Nb 3 Sn, which is the superconductor. The production has to be like this because the Nb 3 Sn is too brittle do be drawn into wire form. The heat treatment for the necessary diffusion can lead to Kirkendall porosity.