✍ Scribed by L. Lü; M.O. Lai; S. Zhang
No coin nor oath required. For personal study only.
Ni3A1 intermetallic powders were produced in solid state with controlled microstructure using mechanical alloying (MA) technique by milling Ni and A1 powders in a planetary ball mill. 33 grams of Ni and AI powders with 3:1 atomic percentage were milled each time in argon atmosphere. The evolution and formation of Ni3AI intermetallic compound were examined at different milling stages using scanning electron microscope, optical microscope and X-ray diffraction. It was observed that Ni content rapidly decreased from the initial 75 at.% to about 22 at.% after 30 rain of MA. Since the soft Ni powders can easily be welded to the inner surface of the milling container and the balls, Ni content decreased at the very beginning of MA. With prolonged milling time, part of the Ni powders stuck on the surthces of the milling tools (container and balls) were impacted off so that Ni content was observed to increase with milling time after 30 min of MA. After 300 min, the Ni content had reached 79 at.% indicating that more A1 powder was stuck on the surface of the milling tools at the later stage of MA. However, after 1200 rain of MA, both Ni and AI contents were found to decrease. Detailed energy dispersive X-ray (EDX) study showed that contamination of Fe and Cr elements fl'om the milling tools occurred, leading to the relative decrease in Ni and AI contents. To avoid contamination, milling time should therefore be shorter than 1200 min. Particle size distribution analysis showed that three stages of particle size change can be identified: before 60 rain of MA, particle size increased due to more welding than fracturing of the powders; at the second stage, particle size continuously decreased until 300 min of MA due to more fracturing than welding; finally, after 300 rain of MA, the particle size remained constant when welding and fracturing had reached an equilibrium. However, as calculated according to Scherrer equation from X-ray pattern, tim effective crystal size continuously decreased with MA time. X-ray diffraction pattern confirmed the formation of Ni3A1 intermetallic compound after 600 rain of MA. The most suitable alloying time for Ni3A1 at a milling speed of 200 rpm was observed to span between 600 and 1200 min.
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