Biomedical Engineering Reference
In-Depth Information
Ta b l e 3
.
14
Mechanical properties of protasul-21 WF after different
processing
Properties
Annealing
Hot plastic forming
Cold plastic forming
R
0,2
[MPa] 550
700
827
R
m
[MPa] 750
1000
1172
A [%]
16
12
12
3
.
2
.
3
Titanium and Titanium Alloys
Since 1965, cp titanium has found an increasing application as an
implant material in medicine (Table 3.15). Cp titanium has high
corrosion resistance and outstanding biocompatibility [1, 2, 46, 47,
71, 110]. One reason for these advantages may be a protective oxide
layer, which forms spontaneously on the implant surface as long as
oxygen is present. Variable ilm thickness can be electrochemically
created yielding various colors of implants [33].
Biomechanical properties of cp titanium depend mostly on the
amount of trace elements present. Four grades of cp titanium are
distinguished for medical applications (ISO 5832-2). The grading
depends on the amounts of nitrogen, carbon, hydrogen, iron, and
oxygen (Tables 3. 16 and 3.17). Increasing amounts of trace elements
elevate tensile strength but reduce ductility of cp titanium. Titanium
plates, used for osteosynthesis, offer less stress-shielding to bone
tissue, because of their low Young's modulus of 68 GPa [110].
Therefore, titanium is an ideal biomaterial for plates and screws
used for fracture treatment.
Titanium exists in two allotropic forms. At low temperatures,
it has a closed packed hexagonal crystal structure (cph), which is
commonly known as α, whereas above 883
°
C, it has a body centered
cubic structure (bcc) termed β. The α to β transformation tempera-
ture of pure titanium either increases or decreases based on the
nature of the alloying elements. The alloying elements (such as Al, O,
N, etc.) that tend to stabilize the α phase are called alpha stabilizers
and the addition of these elements increase the beta transus
temperature, while elements that stabilize β phase are known as
beta stabilizers (V, Mo, Nb, Fe, Cr, etc.) and addition of these elements
depress the β transus temperature. Some of the elements that do not
have marked effect on the stability of either of the phase but form
solid solutions with titanium are termed as neutral elements (Zr and
