Acceleration dependence of formation of quasicrystals and amorphous alloys by a high-energy planetary ball-mill

The mechanical alloying process has been recognized as a novel technique in synthesizing stable and metastable alloys through solid-state reaction without involving melting. However, many investigators are well aware that the milling time necessary to complete the formation of metastable phases is strongly dependent on the milling conditions such as the type of mill, the size and number of balls and the motion of the balls. For instance, mechanical aUoying experiments have been extensively carried out on the Ni-Zr alloy system. The milling time for the completion of the amorphization for Ni:Zr--1:1 powders is distributed over 20-400 h [1-5]. This clearly indicates that the milling conditions affect very sensitively the milling time to produce a final product. Ivanov et al. [6] recently reported that an icosahedral quasicrystal can be formed within 1 h by mechanical alloying for mixtures of Mg, Zr and A1 elemental powders with the composition Mg:AI:Zn = 3:3:2 and for the Frank-Kasper intermetallic compound Mg32(Zn,A1)49. This milling time is surprisingly short, compared with the data for the Ni-Zr system mentioned above. It was claimed that their "high-energy" planetary ball-mill is operated with an angular velocity of 900 r.p.m, or with a ball acceleration of 600 m s-2, about 60 g on the scale of the gravity of the Earth. Their maximum centrifugal acceleration is much higher than that of about 14 g at the intensity setting of 9 in the Fritsch P-5 planetary mill. Their results suggest that the milling time depends strongly on the centrifugal acceleration exerted on the balls and powders. In this work we studied the acceleration dependence of the milling time needed to complete the formation of the quasi-crystalline phase in the Mg-AI-Zn alloy system and that of the amorphous phase in the Ni-Zr alloy system in otherwise fixed milling conditions. The planetary mill employed in this experiment consisted of a turntable with an angular speed of revolution 0) 1 and a pair of mill pots rotating around its central axis with an angular speed of 0) 2 relative to the revolution speed. According to Zhao et al. [7]