Effect of milling energy on the structural evolution and stability of nanostructured Al-5.7 wt.% Ni mechanically alloyed eutectic alloy

Abstract

In the current research, mechanical alloying (MA) was employed to synthesize nanostructured Al-5.7wt.%Ni eutectic alloy through the variation of the milling parameters. Three-dimensional parametric phase diagrams were constructed for the alloy start and complete formation times over a range of ball-to-powder ratio (BPR), rotational speeds (RPM) and milling time (MT). Total energies of 224kJkg−1 and 1068kJkg−1 were necessary to trigger and complete the Al-5.7wt.%Ni eutectic alloy formation, respectively. X-ray diffraction (XRD) was employed to determine the eutectic alloy powders formation progress as a function of MT and for the investigation of the internal structural evolution in terms of crystallite sizes and lattice strain induced by milling. Prolonged milling up to 40h of MT resulted in changes in the powder morphology and produced nanocrystalline (NC) alloy powders about 20nm in average crystallite size. Isothermal heating (ITH) of the milled loose powders at 500°C for 60min using differential scanning calorimetry (DSC) was employed for the evaluation of the thermal stability produced NC structure. Alloys milled up to the alloy complete formation time at intermediate milling energies of 300RPM and 15:1BPR displayed relatively higher structural stability against grain growth compared to the powders milled at higher milling energies. Prolonged MT strongly influenced the morphology of the eutectic structure, which had a negative impact on the structural stability against growth during heating for both the as milled powders and the hot compact discs.