THE SUPPRESSION OF CHEMICALLY-INDUCED LIQUID-FILM MIGRATION IN CO-CU AT HIGH-TEMPERATURE

Abstract

When a Co-20 wt% Cu alloy prepared by liquid phase sintering at 1150-degrees-C is heat-treated at 1300-degrees-C, some intergranular liquid films initially migrate and reverse their directions to return to their initial locations, while others do not migrate at all. Previously, extensive liquid film migration (LFM) has been observed when the sintering and the heat-treatment temperatures have been reversed, When the same alloy sintered at 1150-degrees-C is heat-treated at 1150-degrees-C in contact with a Cu-10Co-10Sn (wt%) powder mixture, extensive LFM occurs, producing Cu-depleted solid behind the migrating liquid films. When the same experiment is carried out at 1300-degrees-C, very little LFM is observed and the grain composition is equilibrated by lattice diffusion. Upon increasing the Sn content in the powder mixture to 15 and 20 wt%, extensive LFM occurs at both 1150 and 1300-degrees-C. These observations are consistent with the prediction of the coherency strain theory that there are high temperature limit and minimum critical driving force for LFM. At high temperatures and low driving forces, the ratio of the solute lattice diffusivity to the migration velocity is so large that the coherency in the frontal diffusion zone is expected to be broken, eliminating the driving force for LFM. The same principle should apply to the grain boundary migration.