Biomedical Engineering Reference
In-Depth Information
2008). Lengthening the milling time results in the disappearance of both
intermetallics, although with different kinetics. The first, CuAl
2
, decays after
1 hour of milling, while Cu
9
Al
4
remains in the system up to 2 hours of
milling. Further milling caused the consumption of aluminium from these
phases to form a Cu(Al) solid solution. This is confirmed by the shift of the
Cu(1 1 1) peak from 2
observed in the system treated for
20 hours. The calculated lattice parameter of this phase is equal to 3.6564 A
˚
.
Comparison of this parameter with its value for pure Cu suggests expansion
of the copper lattice by aluminium substitution, forming a solid solution of
Al in the Cu matrix. In fact, the content of the Al solute was estimated to be
at a level close to 10%. Conventionally Al dissolves in Cu at the temperature
of eutectoid transformation (565
θ
= 42.57
8
to 43.33
8
C) at 9.4%.
An increase in the temperature in the milling vial registered by the GTM
system indicates that the combustion process occurs during the first hour of
milling (see Fig. 12.4). In this case, the reaction between CuO and Al is
completed, confirmed by the absence of peaks corresponding to the initial
components. Considering phase evolution in the CuO-Al mixed powders,
one can suggest that intermetallic phases are formed only as intermediate
products under the applied conditions. During further milling they trans-
form to the Cu(Al) solid solution, that is the expected Cu
9
Al
4
phase was not
formed in final products.
The characteristics of the milling product microstructure were provided
using a scanning electron microscopy. Figure 12.10 shows the typical
morphology of composite powder after milling for 20 hours. EDS analysis
showed that the dark grey phase is Al
2
O
3
while the bright grey one is a Cu
(Al) matrix. The microphotograph confirmed that alumina particles are
evenly embedded in the Cu-Al matrix, in size range from 100 nm to 500 nm.
This indicates that milling the CuO-Al system with a stoichiometry close to
Cu
9
Al
4
brings about formation of metal matrix composite powder particles
in which Cu(Al)
8
forms a matrix while alumina grains act as
its
reinforcement.
12.6 Nickel-based composite powders with Al
2
O
3
Formation in situ of composite powders of nickel-aluminium metals
strengthened by alumina is very important from both a theoretical and a
practical point of view. Analogous to the mechanochemical synthesis of Cu-
Al/Al
2
O
3
composite described in Section 12.5, which is based on the type of
reactants, three modifications of the process are possible. Nickel in the form
of its salt, oxide or simply elemental metal with aluminium can be used.
Because nickel and its alloys have good plastic fatigue features but are not
sufficiently resistant to high temperature oxidation and abrasive wear,
intermetallics such as NiAl and Ni
3
Al are a better choice for a composite
