Biomedical Engineering Reference
In-Depth Information
Mechanical properties
Today, the most widely used titanium alloy in hip replacements is titanium
alloyed with aluminum and vanadium. This is because typically alpha±beta
alloys can achieve optimum properties necessary for demanding loading-bearing
applications (i.e., hip replacement) by balancing the amounts of alpha and beta
stabilizers. Some of the mechanical properties of two alpha±beta alloys (Ti-6Al-
4V and Ti-6Al-7Nb) are shown in Table 2.3.
Mechanical and thermal treatment
The mechanical properties of titanium alloys are principally determined by their
microstructure assuming the same composition, which in turn can be defined by
manufacturing processes (e.g., casting, forging, powder metallurgy) and sub-
sequent thermal treatment (e.g., solution heat treatment, hot isostatic pressing
(HIP), sintering). Mechanical processes can result in elongated grains in the
working direction while the final heat treatment can recrystallize the elongated
structure into an equiaxed structure. The as-worked elongated structures exhibit
higher anisotropic tensile strength and lower ductility while the recrystallized
equiaxed structures exhibit lower non-directional strength but have higher
ductility. For example, the ultimate tensile strength of commercially pure
titanium (grade 1) increased from 410 to 560MPa after 40% cold deformation
while at the same time its elongation at fracture decreased from 30% to 16%
(Black and Hastings, 1998).
Generally, cast materials may have microscopic voids that can lead to
reduced mechanical properties. In comparison, forging can achieve the best
mechanical properties for metallic prostheses. Specifically, both Ti-6Al-4V and
Table 2.3 Mechanical properties of selected titanium, titanium alloys and bone
(Niinomi, 1998)
Material and designation
Young's
Tensile
Yield
(or condition)
modulus
strength
strength
(GPa)
(MPa)
(MPa)
Ti ASTMGrade 1 (alpha)
102.7
240
170
Ti ASTMGrade 2 (alpha)
102.7
345
275
Ti ASTMGrade 3 (alpha)
103.4
450
380
Ti ASTMGrade 4 (alpha)
104.1
550
485
Ti-6Al-4V (alpha/beta; annealed)
110±114
895±930
825±869
Ti-6Al-7Nb (alpha/beta)
105±123
900±1050
880±950
Ti-6Al-4V ELI (alpha/beta)
101±110
860±965
795±875
Ti-13Nb-13Zr (beta; aged)
79±84
973±1073
836±908
Ti-12Mo-6Zr-2Fe (beta; annealed)
74±85
1060±1100
1000±1060
Ti-35.3Nb-5.1Ta-7.1Zr
55
597
547
Cortical bone
17±28
130
±
