Structures and Properties of Al-based Al-Si-Ni Nanostructures Developed by Mechanical Alloying

Abstract

An attempt was made to synthesize Al-based Al ̶ Si ̶ Ni amorphous and/or nanostructures by mechanical alloying (MA). The Al ̶ Si ̶ Ni alloys containing 14 to 25 % Si and 8 to 25 % Ni was known to generate amorphous phase by rapid solidification processing. Elemental powders of Al, Si and Ni (purity Ni ≥ 99.8%, Al ≥ 99.7%, Si ≥ 98.5%) having an average particle size <70 μm were blended to obtain nominal composition of Al75Si15Ni10, Al70Si20Ni10, Al65Si25Ni10, Al70Si15Ni15, Al65Si20Ni15, Al78Si14Ni8 and Al50Si25Ni25. Mechanical alloying was carried out in a Fritsch high energy planetary ball mill using Cr-steel grinding media at 300 r.p.m. up to 50 h. Toluene was used as the process control agent. The ball to powder weight ratio was maintained at 10:1. The microstructural characterization of the milled powder was followed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The particle size distribution of the 50 h milled samples was carried out using a nano zeta sizer (NZS). Percentage transmittance of the milled sample was evaluated using Fourier transform infrared spectroscopy (FTIR). Partial amorphous structure was obtained in Al75Si15Ni10 and Al70Si20Ni10, whereas, Al-rich solid solution was observed in Al65Si25Ni10, Al70Si15Ni15 and Al65Si20Ni15. Predominantly crystalline structure with intermetallic phases was observed in terminal compositions of Al78Si14Ni8 and Al50Si25Ni25. SEM micrographs showed that the powder morphology was changed from coarse layered structure obtained by very short period of milling to finer as the milling time increased. XRD and energy dispersive X-ray analysis (EDX) showed the formation of a homogeneous phase for all the compositions after milling for 50 h. The crystallite size, lattice microstrain (%) and lattice parameter were analyzed from major Al-peaks of the Al-rich solid solution. The crystallite size decreased very rapidly up to 10 h of milling and then became nearly constant (15-35 nm) with further milling, whereas, lattice microstrain (%) increased gradually up to 10 h very rapidly and then became nearly constant (0.5-0.6 %) with progress of milling. The variation of crystallite size and lattice microstrain (%) of Al-rich solid solutions with milling was found to be similar whereas, variation of lattice parameter of the Al-rich solid solutions was different. The variation of lattice parameter with progress of milling possibly plays an important role in the amorphous phase formation by mechanical alloying.
Keywords: Amorphous; Nanostructures; X-ray diffraction (XRD); Transmission electron microscopy (TEM), nano zeta sizer (NZS), Fourier transform infrared ray (FTIR).