THERMAL-BEHAVIOR OF MECHANICALLY ALLOYED NI50MO50 POWDERS AND ASSOCIATED KINETICS OF AMORPHOUS PHASE-TRANSFORMATION

The thermal behaviour of powders obtained by mechanically alloying ele mental crystalline nickel and molybdenum powders for various times was investigated using differential scanning calorimetry (DSC) and linked to structural data obtained by X-ray diffraction. The two observed ex othermic heat effects were correlated with the milling time. The heat of relaxation reached a value of about 8.5 kJ mol-1 after 100 h of mil ling and slowly increased as the fraction of amorphous phase increased . The heat of crystalization reached a maximum value close to 5 kJ mol -1 after 300 h of milling and then slowly decreased. The formation of the amorphous phase by mechanical alloying occurred in three distinct stages: (1) the formation of very fine composite powders, (2) the nucl eation and rapid growth of the amorphous phase, and (3) a steady-state reaction governed by the gradual dissolution of molybdenum crystallit es into the amorphous matrix. The kinetic parameters for the transform ation of the amorphous phase were derived from isothermal DSC. These p arameters depend on the milling time. For the later stage of the mecha nical alloying process, two-step crystallization was found. The compos itionally dependent amorphous phase seems to be the factor most respon sible for the change in transformation kinetics.