NNR IN RAPIDLY ROTATED METALS By - Nottingham eTheses ...

GRAIN SIZE
- um
- 73 -
% OF TOTAL WEIGHT
> 76 10
44-76 40
37-44
15
< 37 35
These powders were prepared from two grades of aluminium wire. The
purer grade was obtained from Johnson Matthey Chemicals and had a
quoted maximum impurity content of 50 ppm, the majority constituent
of which was magnesium. The other grade of wire was as supplied
directly by Metallisation Limited for industrial use and had a
purity of approximately 99.5%.
In a second method of sample preparation a tungsten carbide
coated file was used to file powder from a5 9's pure aluminium
ingot obtained from Koch-Light Laboratories. After sieving, the
powder was passed several times through a high inhomogeneous
magnetic field in an attempt to remove all ferromagnetic impurities.
The aluminium specimens were annealed by subjecting them to
a temperature of 250°C for a period of two hours, under a vacuum
of better than 2x 10-6 torr. No problems of sintering were ßxper-
ienced, so the powders were annealed loose in small glass boats
and then allowed to cool down overnight.
Surface oxidation of the metals afforded electrical insulation
between neighbouring crystallites in the loose powders. However,
with the exception of aluminium, the stresses induced by rapid
rotation of the powder samples resulted in a welding together of
the individual grains. To overcome this effect the powders were