Quantitative Thermal Analysis of Transient Liquid-Phase-Sintered Cu-Ni Powders

Abstract

Differential scanning calorimetry (DSC) was used to study the transient liquid phase sintering (TLPS) of elemental Ni and Cu powder mixtures. The initial melting behavior, kinetics of isothermal solidification, and remelt behavior of these powder mixtures were quantified, using DSC and metallographic techniques. The effects of the initial liquid distribution and processing temperature on these events were investigated. Quantitative DSC analysis indicates that suppressed liquid fractions form during TLPS at peak temperatures of 1090 °C and 1140 °C, due to solid-state interdiffusion prior to melting. Larger liquid fractions form at 1140 °C compared to 1090 °C, due to the recovery of some liquid previously lost to interdiffusion by dissolution at the higher peak temperature. This resulted in higher sintered densities at 1140 °C. Complete isothermal solidification of the Cu-rich transient liquid requires more time at 1140 °C due to higher liquid fractions initially formed at this temperature compared to those formed at 1090 °C. When the initial liquid is distributed nonuniformly, isothermal solidification times are also longer. The DSC and metallographic data indicate that the transient liquid phase (TLP) isothermally solidifies by limited long-range Cu diffusion into the Ni particles and by the epitaxial growth of a surrounding Cu-rich “layer” via the gradual outward progression of the solid/liquid interface at compositions given by the liquidus and solidus.