Biomedical Engineering Reference
In-Depth Information
13.1
Bulk Nanostructured Titanium
Till now, a number of SPD methods for producing in bulk, ultra
ine grain metals/alloys have been developed [48−54]. Valiev
and co-workers apply a process known as equal channel angular
pressing (ECAP), which is a viable processing route to grain
reinement and property improvement [53]. Their study reports of
nanostructured titanium (n-Ti), produced as long-sized rods with
superior mechanical and biomedical properties and demonstrate its
applicability for dental implants. It turns out that the extreme grain
reinement of the bulk of the metal, down to nanoscale transpires
to surface morphology that turns out to be conducive for enhanced
adhesion and growth of living cells.
Commercially pure titanium (Grade 4) of the following
composition was used: 0.052% C, 0.34% O
2
, 0.3% Fe, 0.015% N, base
material Ti (wt%). In the as-received condition, billets produced by
hot rolling had a diameter of 40 mm with an average grain size of
25 μm. Nanostructuring was performed using SPD by equal-channel
angular pressing (ECAP) with subsequent thermomechanical
processing (TMP), which made it possible to manufacture rod
semiproducts with a length of 3 m and a diameter of 7 mm [48−54].
This processing resulted in a large reduction in grain size, from
the 25 μm equiaxed grain structure of the initial titanium rods to
150 nm after combined SPD and TMT processing, as shown in
Fig. 13.1. The selected area electron diffraction pattern, Fig. 13.1c,
further suggests that the ultra ine grains contained predominantly
high-angle non-equilibrium grain boundaries with increased grain-
to-grain internal stresses. It is important to note, that a similar
structure for cp Ti can be produced in small discs using other SPD
method such as high-pressure torsion (HPT) [49].
(a)
(b)
(c)
Figure 13.1
Microstructure of Grade 4 cp Ti: (a) the initial coarse
grained rod; (b), (c) after ECAP + TMT (optical and electron
photomicrographs) [53].
