BALL-MILLING-INDUCED AMORPHIZATION IN NIXZRY COMPOUNDS - A PARAMETRIC STUDY

A vibrating frame grinder has been instrumented for evaluating the amplitude and frequency of ball oscillation, as a function of the amplitude of vibration of the frame and of the ball mass. Based on such measurements, it is found that Ni(x)Zr(y) compounds become fully amorphized provided the specific milling intensity is greater than a temperature- and composition-dependent threshold value. The specific milling intensity is the momentum transferred by the ball to the unit mass of powder per unit time. Below this threshold, a two-phase structure (crystalline+amorphous) is stabilized; the steady proportion of the amorphous phase increases with the milling intensity and decreases on increasing the milling temperature. The intensity threshold increases with temperature and varies in the opposite direction as the hardness of the starting crystalline compound. The amorphous NiZr phase can be obtained by two distinct routes, either starting from the crystalline compound NiZr, or from a mixture of Ni10Zr7 and NiZr2 amorphous phases. These routes correspond, respectively, to an increase and a decrease in internal energy.