Thermodynamic mechanism of phase formation for W-Mo-Cu composite by electric field sintering

Abstract

Since diffusion alloying and metallurgical bonding interfaces have been confirmed to be existed in W-Mo-Cu composite, the thermodynamic mechanism of phase formation is unclear yet. This paper aims at constructing the Gibbs free-energy change diagram for W-Mo-Cu ternary system and revealing the thermodynamic mechanism of the direct alloying among W, Mo and Cu which is not yet clear. The content combines the analysis of experimental results and calculated results based on Miedema's model. The large current electric field induces field emission and provides sufficient energy to overcome thermodynamic barriers and helps the atom diffusion at a relatively low temperature. The high-resolution electron microstructures show that new crystallite phases of (Mo, W), (Mo, Cu), (W, Cu), and amorphous phase (Mo, W) are formed in large current electric field sintering. Meanwhile, the Gibbs free-energy change diagram for W-Mo-Cu system shows that Mo effectively reduces the value of free-energy change, and when Mo content or sintering temperature is as high as possible, Gibbs free-energy change is as small as possible. Additionally, Gibbs free-energy change diagrams for W-Mo, W-Cu and Mo-Cu binary systems are also constructed during the derivation, and the results show that the crystalline and amorphous phases of (Mo, W) are the most easily to be formed, followed by that of (Mo, W) and finally that of (W, Cu).