Thermodynamic mechanism for direct alloying of immiscible tungsten and copper at a critical temperature range

It has been proved that for an immiscible W-Cu system with positive heat of reaction, the immiscibility can be overcome and direct alloying can be realized between the constituent elements at a critical temperature range closes to the melting point of copper (T-mCu) in our earlier researches. However, the thermodynamic mechanism of direct alloying between W and Cu is not yet clear. In this paper, a thermodynamic model has been established for direct alloying between W and Cu based on Miedema's theory. Combining the differential scanning calorimetry (DSC) tests for the storage energy in W and Cu, the analysis results based on the model show that the initial energies of W-Cu system including the surface energy, the storage energy and energy caused by external pressure are able to overcome the positive reaction heat and serve as the driving forces for the direct alloying. In other words, the direct alloying between immiscible W and Cu is thermodynamically feasible. Additionally, the thermodynamic analysis results also show that the final alloying microstructure of W-Cu system depends on whether the initial energy is larger than the Gibbs free energy change of W/Cu amorphous phase formation and the cooling rates during the alloying. These thermodynamic calculation results agree well with the experimental results. The amorphous phase is more likely formed during powder metallurgical sintering than during diffusion bonding of W and Cu rods. Lastly, the analysis results explain well why the key to realize direct alloying between W and Cu is to control the alloying temperature at a critical temperature range closes to T-mCu. (C) 2018 Elsevier B.V. All rights reserved.