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

A series of Cu- xNb (x= 1-15. at.%. ²2All the compositions are expressed in at.% after this unless otherwise stated.) 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.