ORDER-DISORDER TRANSFORMATION IN FE-AL UNDER BALL-MILLING

Ball-milling experiments of the ordered intermetallic compound Fe50Al50 are carried out in a calibrated vertical vibrating frame grinder at various temperatures T and milling intensities I defined as the momentum transferred by the ball to the unit mass of powder per unit time. The long-range order (LRO) parameter, as quantified by x-ray diffraction, decreases as a function of milling time and reaches a steady-state value which is a function of T and I. The time evolution of the LRO parameter can be fitted to a simple model of order-disorder transition under external forcing, in which the atomic exchanges occur because of two mechanisms: the forced atomic jumps, the frequency of which is found to be proportional to I and the thermally activated jumps, the frequency of which is found to be a linear function of I. The forcing parameter, defined as the ratio of the latter two jump frequencies, is found to be the control parameter of the steady degree of LRO achieved under ball milling. Moreover, the kinetic path of the LRO parameter depends on the milling intensity and milling temperature and is in semiquantitative agreement with the model.