Research Projects

Equilibrium phase diagram of Cu-Nb binary system indicates very limited solid solubility between Cu and Nb (enthalpy of mixing ~ 9 kJ/mole). In the last decades, attempts have been made to create metastable nanocrystalline Cu-Nb alloys and nanocomposites by various non-equilibrium synthesis methods (mechanical alloying, physical vapour deposition etc.). Interestingly, such metastable Cu-Nb alloys exhibit improved mechanical strength as compared to pure Cu with moderately high electrical conductivity at room temperature. However, Nb composition range over which the alloying occurs strongly depends on the method of alloy synthesis. Whereas by mechanical alloying Cu-Nb solid solution has been possible up to 10at% Nb composition, by physical vapour deposition methods both nanocrystalline and amorphous Cu-Nb alloy thin films have been reported over a wide range of Nb compositions although the thermal stability of such alloy films is much debated. It is to note that such metastable Cu-Nb alloy films possess higher Gibbs free energies as compared to equilibrium which may undergo phase separation during post deposition annealing or even during film deposition. The increased Gibbs free energies mainly originate from increased elastic strain energies due to large films stresses, higher interface energies due to large density of grain boundaries, planar faults etc. in film microstructure and increased surface energies due to average crystallite size less than 100nm. Therefore, study of thermal stability in nanocrystalline Cu-Nb alloy film must take account of temperature dependent microstructure evolution, stress development and crystallographic texture changes in the film. Unfortunately, such detailed study of thermal stability in Cu-Nb alloy films is rare. Thus, the present project proposes detailed investigation of alloy formation, thermal stability and phase separation mechanism in sputter deposited Cu100-x-Nbx (x varies from 3 to 90at%) thin films by in-situ high temperature (temperature range:200oC–600oC) and ex-situ (room temperature) X-ray diffraction (XRD) combined with differential scanning calorimetry (DSC), high resolution transmission electron microscopy (HR-TEM) and atom probe tomography (APT). Microstructure in the as-deposited and heat treated Cu-Nb alloy films will be studied using XRD line broadening analysis and HR-TEM. The measurements of films stresses and crystallographic textures will be performed in as-deposited and heat treated Cu-Nb alloy films using XRD. Additionally, stress generation/relaxation due to structural/microstructural changes and/or phase separation in Cu-Nb alloy films will be monitored (in-situ) by high temperature wafer curvature stress measurements. Finally, the project aims to establish a correlation between the temperature dependent changes of microstructure, residual stress and crystallographic texture (and their thermodynamic consequences) vis-à-vis the phase stability in nanocrystalline Cu-Nb alloy films.