Effect of zirconium on thermal stability of nanocrystalline aluminium alloy prepared by mechanical alloying

In the present study, a series of Al-x% Zr (x = 1-10 at.%) compositions were prepared by mechanical alloying (MA) to investigate the solid solubility extension of Zr in Al and its thermal stability. The elemental powder blends were mechanically alloyed under high purity argon atmosphere in stainless steel grinding media using SPEX 8000 M high energy ball mill. The milling was carried out for 8 h at room temperature and the ball to powder ratio was maintained at 10:1. Formation of disordered solid solutions is validated using Miedema's semi-empirical model. X-ray diffraction (XRD) analysis confirms the formation of disordered solid solution up to 1at.% Zr; whereas, Al3Zr and Al9.83Zr0.17 intermetallic phases were found to form as per the XRD pattern of 2-10% Zr alloys. Variation of lattice parameter confirmed the formation of Al-1% Zr solid solution. Crystallite size was estimated to be 41 and 30 nm, respectively, for the as-milled 1 and 10% Zr alloys. The matrix grains were found to be stabilized after annealing at 550 °C, and the XRD crystallite size of Al-10% Zr retained at ∼58 nm. TEM and AFM analysis confirmed that the grain size of the as-milled as well as annealed samples was retained in the nanometre range (<100 nm), which corroborates well with the XRD crystallite size. Vickers microhardness of the as-milled 10% Zr alloy was found to be 2.4 GPa, which decreased to 1.7 GPa after annealing at 550 °C. This demonstrated that the dissolution of Zr in Al has a larger strengthening effect and Zr plausibly played a pivotal role in retaining the matrix grains size in the nanoscale range at high temperature.