- This work investigates the effects of the metal substrate on the thermochemistry of active brazing alloys during ceramic/metal joining applications. The thermodynamics and kinetics of the interactions between the metal substrate and the braze alloy are examined. Numerical simulations are used to rationalize experimental observations reported elsewhere. It is shown that using Ni metal substrates at temperatures much higher than the liquidus of the brazing alloys can dramatically decrease the chemical activity of Ti in Cu-based brazing alloys. This effect is much less intense when Fe-metal substrates are in contact with the same active brazing alloy systems. It is additionally proven that the time scales necessary to observe such detrimental decrease in chemical activity are on the order of minutes. 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.