Phase Evolution During Ball Milling of Al In NH 3 and Subsequent Annealing
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Mat. Res. Soc. Symp. Proc. Vol. 481 01998 Materials Research Society
appropriate for a soft metal. Prior to insertion in the milling vial, about 3gm of Al was freshly cut from 99.999% pure sheet. The milling vial was sealed and then evacuated to a pressure of about l0-5 torr and then filled with N2 or NH 3 gas at a pressure of about 400 kPa. The pressure of the mill was monitored with an attached pressure gauge during milling for various times up to 800h. After milling the powder was handled in an oxygen-free glove box and, depending on the subsequent analysis, pressed into small pellets to minimise oxidation. Annealing was carried out either in a tube furnace up to 1000*C or in differential thermal analysis (DTA) equipment up to 1 100°C at a ramp rate of 20 0C/min. In both cases, annealing occurred under flowing Ar (or N2) gas. Following milling and annealing, the samples were analysed structurally using X-ray (powder) diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and DTA. Composition was measured using combustion analysis and energy-dispersive X-ray analysis (EDXA). RESULTS AND DISCUSSION Ball Milling in N2 gas did not result in significant N2 uptake as monitored by pressure change in the vial during milling up to 800h. Indeed, milling with low energy (shearing action) in N 2 was found to result in a large number of shiny Al balls with a diameter ranging from about 0.5 to 4mm. Fig. 1 shows an SEM micrograph of a typical cross section of a hollow Al sphere formed during milling in N2. Whereas the outer surfaces of the spheres are smooth as a result of rolling and polishing action during milling, the inner surfaces are usually quite rough and contain considerable attached debris. Indeed, some balls contain smaller balls trapped inside, somewhat like "Russian dolls". The formation of such hollow spheres during milling is intriguing and we suggest that the mechanism may first involve plastic deformation of Al between the steel balls and the vial wall to form sheets of hemispherical material. This material may then be shaped by rolling between the steel ball to form hollow spheres, sometimes trapping smaller spheres as they form. Further details of this process are given elsewhere [7]. For low energy milling of Al in NH 3, gas was clearly absorbed during milling as indicated by a reduction in vial pressure over the first 100h or so of milling. However, the pressure
Fig. 1 SEM micrograph showing the cross-section of an Al ball formed during milling of Al in N 2 gas.
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increased again at longer milling times and an analysis of the nitrogen and hydrogen content in the powder as a function of milling time was needed to understand the behaviour. The results of combustion analysis are shown in Fig. 2: ammonia is clearly absorbed with increasing milling time but dissociates to liberate hydrogen at longer milling times. By 800h milling, the nitrogen content is around 35% and there is almost no hydrogen left in the powder, thus explaining the increas
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