Solid state reactions and microstructural evolution of Al-Ni powders during high-energy ball milling

The solid state reactions and the microstructural evolution during high-energy ball milling of Al-Ni powder mixtures in the composition range 25-75 at.% Al have been investigated. Experimental observations have shown that the microscopic mechanism underlying the alloying process in this system is the diffusion of Ni atoms in the Al-rich layers and that an important role is played by the oxygen contamination. An amorphous Al-rich phase containing a few at.% oxygen and with a Ni content not exceeding approximately 50 at.% has been detected in the equiatomic and Ni-rich samples milled for a few hours. This phase upon further milling transforms to a Ni-rich fee solid solution thus allowing one to by-pass the nucleation of AI,Ni in these samples. The self-sustaining high-temperature synthesis of the AlNi B2 phase has been found to occur over the 40-60 at.% range of Al concentration after about 3 h of milling. The same reaction has been observed in the Differential Scanning Calorimeter if equiatomic samples premilled for about 3h (unreacted) are heated at a sufficiently high heating rate.