In-situ formation of NbC in nanocrystalline Cu

We report on an investigation of the in-situ formation of NbC nanoparticles in a Cu nanocrystalline matrix, paying particular attention to the role played by NbC on stabilizing and strengthening the matrix. The material was prepared by milling a Cu-5wt%Nb powder mixture with 1 wt% stearic acid for 36 h, followed by annealing of the milled powder at temperatures ranging from 400 to 1000 °C. Our results show that the NbC nucleates at an approximate temperature of 700 °C and that the associated enthalpy of formation and activation energy of NbC are approximately −0.8 kJ/mol and +312 kJ/mol, respectively. Moreover, the initial size of NbC particles formed at 700 °C was 8 nm, and showed limited coarsening (to 21 nm) even when annealed at 1000 °C for 1 h. With the presence of NbC, dislocation annihilation was impaired and grain boundary mobility was hindered, resulting in a dislocation density as high as 1014 m−2 following annealing at 1000 °C, and a grain size of 96 nm after annealing at 900 °C. The high thermal stability of NbC nanoparticles and the Cu matrix lead to appreciable strengthening of the composite powders as evidenced by a microhardness increase from 2128 MPa for the as-milled powder to 2665 MPa for the powder annealed at 700 °C for 1 h.