Biomedical Engineering Reference
In-Depth Information
worldwide because of the increasing demands for fuel savings through
weight reduction in motor vehicles. At the same time, the cost of magnesium
has declined to become comparable to aluminum, partly because of the avail-
ability of low priced alloys from China and improvements in the ei ciency of
primary magnesium production [279]. Global production of primary mag-
nesium has increased from 260,800 tons in 1990, to 479,000 tons in 2000,
to 809,000 tons in 2010 [281]. Magnesium and its alloys of er high specii c
strength, but has been limited in its use because of challenges associated with
poor low room temperature workability and poor elevated temperature prop-
erties [282]. However, magnesium alloys have good machinability, weldabil-
ity, castability, and formability (at least at high temperature), supporting their
commercial use in a wide range of applications [279].
Compared to the major classes of alloys in medical use (stainless steel,
titanium, Co-Cr) magnesium has by far the lowest strength. h e ultimate
tensile strength of magnesium alloys falls in the range 160 MPa to 380
MPa, while austenitic stainless steels range from 515 MPa to 1275 MPa,
titanium alloys range from 240 MPa to 1380 MPa, and Co-Cr alloys range
from 600 MPa to 2025 MPa [221]. h e highest strength medical alloys,
such as Co-Cr superalloys MP35N and MP159 have tensile elongations to
failure of 8% - 10% at room temperature. h ough much lower in strength,
the elongation to failure for all magnesium alloys is low, ranging between
1% and 16%. h is is due in part to the limited number of slip systems avail-
able in the hexagonal close packed crystal structure of magnesium. For
orthopedic applications, magnesium has the distinct advantage of having
an elastic modulus of 45 GPa, closer than any other biomedical alloy to the
elastic modulus of bone, which ranges from 2 GPa to 35 GPa.
h e prospect of nanostructuring magnesium and its alloys to achieve
novel properties was recognized over 30 years ago [283-285]. Consequently,
a substantial body of knowledge exists on processing magnesium to rei ne its
structure at the nanoscale. Grain size rei nement has been regarded as one
of the most attractive methods to enhance the performance of magnesium
alloys [282]. Results of nanostructuring by SPD are reported most com-
monly for alloys AZ31 [286-292], AZ61 [54, 293-296], AZ80 [297-299],
AZ91 [300-306], and ZK60 [129, 307-310]. Severe plastic deformation
rei nes grain structure, increases strength, increases ductility, and imparts
crystallographic texture. h e textural ef ects in hcp magnesium alloys are
sometimes sui ciently large to introduce substantial mechanical anisot-
ropy, large enough in magnitude to cause net sot ening at er ECAP [311,
312]. However, tensile ductility is generally improved by ECAP processing
[313-316]. Examples of room temperature properties for some bioabsorb-
able magnesium alloys processed by ECAP appear in Table 1.3.
