A Study On The Nanocrystalline Fe-Al Intermetallics And Its Composites Made By Mechanical Alloying

Mechanical alloying (MA) is a ball milling process. In terms of the technique, pure element powders or their mixtures are placed in the ball mill and subjected to high energy collision during the ball milling, which results in the solid reactions occured in the milling powders. The technology has been extensively used to prepare dispersive strengthening alloys, supersaturated solid solution, intermetallic compounds, amorphous alloys, nanocrystalline materials, and functional materials such as magnetism and superconductor.In this paper, nanocrystalline Fe₃Al and nanocrystalline Fe₃Al/TiB₂ composite powders were prepared by mechanical alloying with subsequent heat treatment. The microstructures of MA powders were studied using X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The studies are mainly focused on the change of the structure, morphology and size of powders during the milling process, and, the effect of heat treatment on the evolution of the structure, morphology and size of the ball milled powders. Meanwhile, during the heat treatment, the grain growth kinetics of the Fe₃Al phase of the milled powder was also discussed.The results show that the supersaturated Fe(Al) solid solution was formed during the milling the Fe₇2-Al₂8 powder. At early stage of the MA process, the composite particle with the structure of Fe circle covered with Al was detected. When increasing the MA time, the Al content of the supersaturated Fe(Al) solid solution increases to about 27.4wt%. The average grain size is about 6.9nm when the Fe₇2Al₂8 powder ball milled for 25h. When heat treating from 800℃ to 1100℃, Fe(Al) solid solution milled for 25h was transformed into the DO₃ Fe₃Al intermetallics, along with the growth of Fe₃Al grain size. The Fe₃Al grain growth kinetic equation is K=1.58 × 10^-9exp (-540.48 ×10³/RT) m^-2s^-1.During ball milling the (Fe₇2-Al₂8)-10wt.%(Ti₃3-B₆7) powder, the Fe(Al,Ti,B) solid solution was first formed. When prolonging the MA time, the mixtures of Fe(Al,Ti,B) solid solution and an amorphous alloy were formed. The grain size of the Fe(Al,Ti,B) solid solution is 9.6nm when the powder ball milled for 40h. When heat treating from 800℃ to 1100℃, the nanocrystalline Fe₃Al/TiB₂ composite powders were obtained. The Fe₃Al grain growth kinetic equation is K= 1.5 × 10^-5exp (-578.7 × 10³/RT) m²s^-1.