Superstructure in nano-crystalline Al50Cu28Fe22 alloy

This work reports the formation of nano- crystalline Al50Cu28Fe22 by high-energy milling. For obtaining the nano-crystalline material, the Al50Cu28Fe22 alloy synthesized through slow cooling of the molten alloy was subjected to ball milling, which was carried out in attritor mill at 400 rpm for 5 h, 10 h, 20 h, 40 h and 80 h with a ball to powder ratio 40 : 1 in hexane medium. The x-ray diffraction observation of ball-milled samples revealed that the milling duration of $5h$ to $40 hrs$ has led to the formation of nano-phase. The average crystallite size comprising the nano-phase has been found to be $\sim 17 nm$. When the nano-crystalline alloy, Al50Cu28Fe22 was vacuum annealed at a temperature of 500$^0C$ for 5 to 20 hrs, new structural phases representing superstructures of the parent nano-crystalline phase were found . The superstructure have been found to correspond to simple cubic with $a = \sqrt 2a_p$ and face central cubic with a = 2a_p (a_p = lattce parameter of parent nano-crystalline alloy). It has been proposed that the formation of different type of superstructure resulting due to different duration of ball milling followed by annealing is possibly governed by minimization of free energy of the disordered B2 phase.