Quantifying metallurgical interactions in solid/liquid diffusion couples using differential scanning calorimetry

A novel method using differential scanning calorimetry (DSC) has been developed to quantify the interface kinetics in a solid/liquid diffusion couple. The Ag–Cu binary eutectic system is investigated by heating an assembly of Ag base metal and Ag–Cu eutectic foil to 800 °C and holding. The fraction of liquid remaining after various isothermal hold periods is measured by comparing the melting endotherms with the solidification exotherms. Detailed analysis of the results show the per cent liquid remaining is inversely proportional to the square root of isothermal hold time; however, effects of the base metal cause an apparent loss of 25% of the liquid immediately after heating. The fundamental understanding of the effects of bi-phase diffusion couple geometry is advanced to manifest the mechanisms resulting in the error. Further carefully devised experiments reveal that primary solidification during cooling is not included in the enthalpy of solidification measured by the DSC. Furthermore, baseline shift across the melting endotherm increases the measured melting enthalpy. These effects combine to systematically underestimate the fraction of liquid remaining. Development of a modified temperature program and application of an appropriate correction can remedy these effects. The experimental results compare well with a prediction generated by an analytical model. Successful quantification of these phenomena has broadened the knowledge of DSC operational characteristics in the solid/liquid diffusion couple treatment, which can now be applied to other material systems.