Biomedical Engineering Reference
In-Depth Information
coarse-grained counterparts. h e superior strength is particularly impor-
tant for medical applications such as orthopedic devices. Drivers for
additional strength in medical applications include the growing average
weight and prevalence of obesity in adults [92], the need to achieve greater
structural functionality using smaller volumes of metal, the need for
greater fracture resistance in high load applications, and the need to place
implants in coni ned spaces in the human body that cannot be adressed
otherwise [93].
In general, one can increase the strength of virtually any metal or alloy
by 20% to as much as a factor of four via SPD [94]. One can also improve
ductility via SPD processing, with increments in the elongation to failure of
up to 5 times reported [95-102]. Fracture toughness can also be increased
in most alloy families [103-106]. Resistance to fracture under cyclic load
may increase or decrease in SPD metals [107-116]. Generally, bulk nano-
structured metals have superior fatigue properties. However, cyclic sot en-
ing of SPD-induced microstructures subject to large inelastic cyclic strains
can lead to diminished low cycle fatigue resistance [117]. h reshold stress
levels for fatigue crack growth are generally higher in SPD metals, but can
be lower in some cases [118]. h is is due in part to the fact that SPD can
cause the formation of textures that are deleterious to fatigue resistance
[111, 119]. h e localized shear that occurs during SPD can also alter sec-
ond phase or precipitate morphologies so as to diminish the resistance to
fatigue crack growth [120].
h e corrosion resistance of most alloys is enhanced by nanostructuring
[27, 71, 79, 121-137]. h e improved corrosion performance of nanostruc-
tured metals has been attributed to their reduced grain size [138, 139],
greater uniformity of microstructure [139, 140], and higher polarization
resistance [141]. For alloys such as stainless steel that form tenacious
oxides the polarization resistance and stability of the oxide layer increases
with decreasing grain size [122, 142-144].
For most medical applications the range of temperatures experienced in
service is comparable to the ambient temperatures experienced by humans.
However, sterilization processes used to prepare metals for medical applica-
tions can expose metals to temperature of 100 °С. Low temperature stor-
age of medical materials, as low as -80 °С must also be considered. Within
the range of -80 °С to 100 °С, the microstructures present in bulk nano-
structured metals are highly stable. h ese microstructures possess a diverse
range of features, including for example high dislocation densities, high
angle grain boundaries, dense dislocation walls, micro- and macro- shear
bands, stacking faults, microtwins, and vacancy clusters [145-157]. Because
these structures are mechanically induced, ot en at lower temperatures than
