EARLY AND LATE MECHANICAL ALLOYING STAGES OF THE PD-SI SYSTEM

Four Pd-Si compositions (Pd/Si = 2.5/1; 3/1; 4/1 and 5/1) were mechanically alloyed by a ball-milling technique. X-ray diffraction and fluorescence analysis were used to monitor the mechanical alloying (MA) process which was carried out in planetary and vibratory ball mills. The first step of the milling process consists in a very fine fragmentation of the silicon particles into the palladium matrix. After this early stage of milling, formation of the intermetallic line compound Pd3Si can occur for the 2.5/1, 3/1 and 4/1 composition depending on the milling conditions and/or milling apparatus adopted: i.e. on the conditions of energy transfer experimentally realized. Subsequent milling indicates that amorphization probably occurs starting from the previously formed Pd3Si compound. Long milling times, up to 56 h, promote a demixing process towards the parent elements for the 2.5/1 and 4/1 compositions. Thermal treatments of the long-term milled samples confirm the final products obtained from the near room-temperature solid state reaction induced by MA. For the Pd5Si composition the conditions for Pd3Si formation and subsequent amorphization were never reached.