Biomedical Engineering Reference
In-Depth Information
increasing longevity of populations worldwide, and the overall advance-
ment of diagnostic and surgical procedures in medicine. Consequently, the
demand for medical grades of alloys has grown as well. In addition, metal
production techniques have evolved to support more economical produc-
tion of small lot sizes. h is has enabled the development of new alloys and
surface modii cations of existing alloys that are optimized for biomedicine.
h is chapter addresses a new class of metals that have emerged over
the past 20 years: bulk nanostructured metals [4-6]. Nanostructured met-
als are by dei nition metallic solids that have been deliberately engineered
to have nanometer scale features (grains, precipitates, etc.) within the
range between 1 nm to 100 nm that impart desirable physical, mechani-
cal, electrical, and biological properties. We focus in particular on metals
that can be produced in bulk forms such as rod, wire, sheet or plate. We
will not address thin i lm technology (<100 nm thickness), compaction of
nanosized powders that includes such techniques as hot isostatic press-
ing (HIP), and serial 3-dimensional fabrication methods such as Selective
Laser Sintering (SLS), Laser-assisted Chemical Vapor Deposition (LCVD),
and Laser-Based Additive Manufacturing (LBAM) [7, 8]. Instead, here we
are interested in bulk nanostructured metals for which the mechanical and
other properties can be customized, particularly for structural biomedical
applications. Such metals can be produced by severe plastic deformation
(SPD).
1.1.2
Brief Overview of the Evolution of Bulk
Nanostructured Metals
Since the Second World War (1939-1945), researchers recognized that
desirable characteristics such as improved strength and formability could
be achieved in metals with “i ne grain” sizes, in the range of 1 to 10 microns.
h e relationship between grain size and strength was published by E.O.
Hall in a series of papers in the Proceedings of the Royal Physical Society
in 1951 [9, 10]. In parallel, N.J. Petch from the University of Leeds inde-
pendently published the results of his experimental work from 1946-1949
showing the relationship between fracture strength and ferritic grain size
[11]. Also from 1945, researchers increasingly recognized the importance
of i ne grain size for enabling superplastic shaping and forming [12-17].
h e earliest studies of SPD processing were enabled by the development
of Bridgman's anvils to impart very large shear strains via high pressure
torsion [18]. From the mid-1970s researchers increasingly examined the
behaviors of grain boundary structures in i ne grain size metals in con-
nection with superplastic deformation [19, 20]. h is research provided the
