Effect of Nb, Y and Zr on thermal stability of nanocrystalline Al-4.5Â wt.% Cu alloy prepared by mechanical alloying

In the present study, effect of Nb, Y and Zr (having low equilibrium solid solubility in Al) in the formation of nanocrystalline Al-4.5 wt.% Cu-1at.% X (X = Nb, Y & Zr) solid solutions by mechanical alloying and their thermal stability have been investigated. The mechanically alloyed samples were annealed in batches for 1 h at different temperatures ranging from 150 to 550 °C. The thermal stability has been studied through X-ray diffraction (XRD) phase analysis and variation of microhardness as a function of temperature. Transmission electron microscopy (TEM) and SAED analysis showed that 1 at.% Y and 1 at.% Nb could be dissolved into Al-4.5% Cu solid solutions after mechanical alloying up to 8 h of milling. The spontaneity of formation of Al-4.5%CuY, Al-4.5%CuNb and Al-4.5%CuZr solid solutions has been explained from the change of Gibbs free energy as per Miedema's and Toop's models. It demonstrated that the summative energy due to the reduction in crystallite size and accumulation of dislocation density exceeds the theoretical energy barrier required for the formation of the Al-4.5%CuY, Al-4.5%CuNb disordered solid solutions. The grain sizes were found to retain <100 nm even after annealing at 550 °C. This is attributed to the stabilization of metastable grains by segregation of large size solute atoms (Nb, Y and Zr) along grain boundaries and/or Zener pinning by intermetallic precipitates like Al3Nb in Al CuNb, Al3Y and Al3Y5 in AlCuY and Cu10Zr7, Al3Zr and Cu5Zr in AlCuZr alloys.