Biomedical Engineering Reference
In-Depth Information
M
s
A
f
M
d
100
Super-elasticity in this temperature range
M
= Martensite
A
= Austenite
s = Start temperature
f = Finish temperature
M
d
= Te mperature in which transition
to Martensite is not possible
0
A
s
M
f
Te mperature
FIGUre 7.7 (See color insert.)
Phase transformation and hysteresis in
a shape memory alloy.
Austenite
Reverse
transformation
Transformation
Tw inned martensiteDetwinned, deformed martensite
Deformation
FIGUre 7.8 (See color insert.)
atomic structure in a shape memory
alloy through transformation and reverse transformation.
for Nitinol are guided by ASTM F2063-05. In its austenitic phase, the
mechanical behavior of Nitinol is similar to a β-titanium alloy with
low work hardening and ductility. In its martensitic phase, deforma-
tion occurs via the movement of twin boundaries, accommodating
large strains with low stress. At the end of its superelastic range, mar-
tensitic NiTi will deform in a conventional manner. Fatigue life pre-
diction for Nitinol alloys is extremely difficult owing to the nonlinear
stress-strain response and the stress-induced phase transformations.
Nickel release owing to corrosion may induce allergic response, cel-
lular hypersensitivity, cytotoxicity, and genotoxicity. Nitinol forms a
passive titanium oxide layer that resists corrosion, and further surface
modifications can reduce the nickel release rate to below normal daily Ni
intake. Clinical cytotoxicity has been shown to be comparable to other
implantable alloys. Some current orthopaedic uses of Nitinol include
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