Biomedical Engineering Reference
In-Depth Information
a
b
Fig. 2.8
Metallographic section through titanium (F67) (
a
) with nearly equiaxial grains; (
b
) sec-
tion through a Ti6Al4V wire parallel to the wire axis: the grains are elongated by uniaxial
deformation. Photo by P. Crabbe
chapter! A general overview of composition is given in the synoptic Table
2.3
and
properties in Table
2.6
.
˛
-alloys
: The microstructure is single-phase h.c.p. The attribute 'single-phase'
makes it on the one hand easily weldable, because the heating during welding
does not change the microstructure, on the other hand it cannot be heat-treated for
strengthening. Nearly equiaxial grains is the main feature of the crystallographic
section of CP titanium of Fig.
2.8
a. The only commonly used alloys are several
grades of commercially pure titanium (CP titanium), which are in effect Ti-O alloys.
We encounter here oxygen as an explicit alloying element with drastic effect on the
properties. Here and there some oxide will be found in the matrix but talking of oxy-
gen as alloying element means that oxygen is present as interstitial element in the
crystal lattice with consequences for its properties. Ti CP grade1 for example with
0.18% oxygen has a YS of 221 MPa, while CP grade4 with 0.4% has a YS 559 MPa.
The properties are relatively insensitive to grain size. Oxygen is one factor, texture
another as shown in Fig.
2.8
b. Texture has so its own impact on properties as demon-
stratedinFig.
2.9
: fatigue properties will be superior in a direction perpendicular to
the main orientation of the grains as shown.
ˇ
-alloys
: The crystallographic structure is b.c.c. The addition of the elements Mo,
V, Fe, Mn, Cr or Nb stabilizes the ˇ-phase below the transformation temperature to
the h.c.p. phase. Experience in hip implants learns that a modulus too much different
from the one of bone should be responsible for stress shielding, a phenomenon we
will deal with in Chap. 11. The opportunity offered by ˇ-alloys is the combination
of low modulus, closer to bone, with high strength, similar fatigue and good frac-
ture behavior, improved wear resistance, low sensitivity to corrosion, as a matter of
fact a favorable mix of mechanical, biological properties and, say, biocompatibility.
Numerous slip systems favor dislocation mobility which results in excellent forma-
bility. It is evident that this combination opens the possibility to realize a hip stem
quasi-isoelastic with bone. A comprehensive set of data was published by Mishra
