Biomedical Engineering Reference
In-Depth Information
and exhauster details), in the aircraft industry, in nuclear power and in
metallurgy. However, the low yield point at room temperature (what causes
difficulties in mechanical treatment) is one of the main hindrances to using
them more widely. Modification of features of the inter-metallic phase can
be made, for example, by adding ceramic particles into their structure.
Inserting ceramic particles (type: Al
2
O
3
,ZrO
2
or TiC) into the intermetallic
matrix (Ni, Al) leads to improvements in some mechanical properties like
hardness and abrasion resistance at high temperatures and a significant rise
of utility features, for example an increase in the corrosion temperature and
in oxidation and erosion resistance.
The mechanisms of metal alloy formation by mechanical treatment of
metal powders are categorized into three different systems depending on the
mechanical features: ductile-ductile, ductile-brittle and brittle-brittle. In the
first case in the early stages of milling, the components become flatted to
platelets by cold welding. In the next stage, they are cold-welded together
and form a composite lamellar geometry of the constituent metals. Further
true alloying occurs at the atomic level until the homogeneous structure of
the powders is attained.
The mechanisms of metallic-ceramic composite formation fall into the
ductile-brittle components category because the brittle oxide particles are
dispersed in a ductile metallic matrix. The ceramic reagent is closely spaced
along the interlamellar spacing. With further milling, the ductile powder of
alloy particles becomes work hardened and the lamellae become convoluted
and refined. With continued milling the lamellae are further refined and the
brittle particles are uniformly dispersed in the ductile metallic matrix. The
ductile components are flattened by a micro-forging process while the brittle
ones are fragmented. One can assume that the brittle particles of the
materials are dispersed in the ductile matrix. However,
if both milled
materials are brittle, this phenomenon is not observed.
A typical example of ductile-ductile microstructures is Cu/Al-Al
2
O
3
composite powder formed during milling of the Cu-hydroxocarbonate and
Al mixture, described in Section 12.5 (Wieczorek-Ciurowa et al., 2000,
2003a, 2003b). The microstructure of the products after the first stage of the
mechanochemical synthesis is shown in Fig. 12.1, which presents the SEM
micro-photograph. The material reveals a lamellar microstructure. Based on
quantitative energy dispersive X-ray elemental analysis (EDX) (Table 12.1)
it is possible to estimate that mechanically alloyed composite particles
consist of Cu-Al intermetallic phases as a matrix, and aluminium oxide.
The darker network corresponds to the higher amount of Al
2
O
3
and the
brighter phase to the higher amount of metallic phases. The final form of
Cu-Al/Al
2
O
3
composite particles shown in Fig. 12.2 reveals its homo-
geneous microstructure.
Reactive milling of solids relates to the process in which chemical
