Biomedical Engineering Reference
In-Depth Information
The
α
and
β
phases also form the basis for normally accepted
classiication of titanium alloys. Alloys having only
α
stabilizer and
consisting entirely of a phase are known as
α
alloys. Alloys containing
1-2% of
α
stabilizers and about 5-10% of
β
phase are termed as near
α
alloys. Alloys containing higher amounts of
α
stabilizers, which
results in 10-30% of
β
phase in the microstructure, are known as
α
+
β
alloys. Alloys with higher
β
stabilizers where
β
phase can be
retained by fast cooling are known as metastable
β
alloys. These
alloys decompose to
α
+
β
on aging. Most of the biomedical titanium
alloys belong to
α
+
β
or metastable
β
class.
An alternative and potentially more attractive method for
enhancing the mechanical performance of commercial-purity titan-
ium has recently been reported by Valiev and coworkers [131].
These investigators have investigated the strengthening of grade 2
commercial-purity titanium utilizing equal channel angular press-
ing (ECAP) in combination with other deformation processes.
Procedures examined include ECAP (8 passes) at 400°C (#1),
ECAP + 65% cold rolling (#2), and ECAP
+
rolling followed by
annealing at 300°C, for 1 h (#3). Ultraine grained (UFG) structures
can range from an equiaxed cellular microstructure to a sub-grain
structure with a deined boundary structure. In all cases, the micro
hardness of severely deformed commercial-purity titanium was
superior to that of the original coarse-grained commercial-purity
titanium (Table 3.18). The yield and ultimate tensile strengths
also exhibit this enhancement, a 140% increase in ultimate tensile
strength vis-à-vis coarse-grained commercial-purity titanium being
observed. Notably this increase was achieved while maintaining an
elongation to failure of 9%.
Ta b l e 3
.
18
Microhardness, tensile mechanical properties and fatigue limit
of grade 2 Cp Ti in different states in comparison to Ti-6Al-4V
ELI [131]
HV
(MPa)
UTS
(MPa)
YS
(MPa)
EL
.
(%)
RA
(%)
Fatigue limit
(MPa)
Structure type
Coarse-grained
1800 460 380 26
60
238 ± 10
UFG #1 (Equiaxed,
submicron-grained)
2700 710 625 14
60
403 ± 8
UFG #2 (Fibrous,
with high dislocation
density)
2821 960 725 10
45
434 ± 5
UFG #3 (subgrained
with internal cells)
2850 1100 915 9
40
500 ± 8
(
Contd
)
