Thermodynamic feasibility of solid solubility extension of Nb in Cu and their thermal stability

A series of Cu–xNb (x = 1–15 at.%2) alloys have been investigated to study the metastable solid solubility extension of Nb in Cu by mechanical alloying. Analysis of X-ray diffraction and Gibbs free energy change confirmed that 7.5% of Nb was metastably dissolved in Cu after 8 h of milling at room temperature although Cu–Nb is a system with positive heat of mixing. The solid solubility could be extended up to 10% after enhancing milling duration to 16 h. Detailed thermodynamic analysis revealed that the additional energy stored during mechanical alloying could overcome the required energy barrier as per Miedema's model for the formation of disordered solid solution. The extended solid solubility has been explained along with the other possible mechanisms. Extensive annealing experiments and structural investigation revealed that the supersaturated solid solution is completely stable up to 400 °C. The matrix grains were stabilized and retained their size, ∼25 nm, even after annealing at 600 °C. Microhardness measurement and grain size analysis show that the dissolution of Nb in Cu has a larger strengthening effect than that of free Nb in the compositions.

► Metastable Cu1−xNbx (x = 0.01–0.15) nanostructures developed by mechanical alloying.
► Solid solubility of Nb in Cu detected to be 7.5% after 8 h of milling and extended to 10% after 16 h of MA.
► Gibbs free energy change showed that formation of disordered alloys is thermodynamically feasible as per Miedema's model.
► Matrix grains stabilized their size, ∼25 nm corresponding to a hardness of 4 GPa after annealing at 600 °C.
► Solid solution hardening and grain-boundary segregation are responsible for the high strength.