Thermal behaviour of mechanically alloyed Ni 50Mo 50 powders and associated kinetics of amorphous phase transformation

Abstract

The thermal behaviour of powders obtained by mechanically alloying elemental 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 exothermic 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 milling and slowly increased as the fraction of amorphous phase increased. The heat of crystallization 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 nucleation and rapid growth of the amorphous phase, and (3) a steady-state reaction governed by the gradual dissolution of molybdenum crystallites into the amorphous matrix. The kinetic parameters for the transformation of the amorphous phase were derived from isothermal DSC. These parameters depend on the milling time. For the later stage of the mechanical alloying process, two-step crystallization was found. The compositionally dependent amorphous phase seems to be the factor most responsible for the changes in transformation kinetics.