Effect of Nb, Y and Zr on thermal stability of nanocrystalline Al-4.5 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%Cu−Y, Al-4.5%Cu−Nb and Al-4.5%Cu−Zr 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%Cu−Y, Al-4.5%Cu−Nb 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− Cu−Nb, Al₃Y and Al₃Y₅ in Al−Cu−Y and Cu₁₀Zr₇, Al₃Zr and Cu₅Zr in Al−Cu−Zr alloys. - Highlights: • Metastable Al-4.5 wt% Cu-1at.% X (X=Nb, Y & Zr) alloy powders was developed by MA for industrial applications. • ΔG required for formation of metastable alloy powders confirmed by Miedema's and Toop's model. • Stabilization effect & high hardness ascribed to segregation of Nb, Y, Zr & precipitation of Al₃Nb, Al₃Y, Cu₁₀Zr₇, Al₃Zr etc.