SOLID-STATE REACTIONS AND MICROSTRUCTURAL EVOLUTION OF AL-NI POWDERS DURING HIGH-ENERGY BALL-MILLING

The solid state reactions and the microstructural evolution during hig h-energy ball milling of Al-Ni powder mixtures in the composition rang e 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 amorp hous Al-rich phase containing a few at.% oxygen and with a Ni content not exceeding approximately 50 at.% has been detected in the equiatomi c 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 t o by-pass the nucleation of AI,Ni in these samples. The self-sustainin g high-temperature synthesis of the AlNi B2 phase has been found to oc cur over the 40-60 at.% range of Al concentration after about 3 h of m illing. The same reaction has been observed in the Differential Scanni ng Calorimeter if equiatomic samples premilled for about 3h (unreacted ) are heated at a sufficiently high heating rate.