Biomedical Engineering Reference
In-Depth Information
foundation for subsequent work on the processing to produce even smaller
ultrai ne grain sizes and focusing on the nanoscale structural characteris-
tics of grain boundaries.
In 1981 Vladimir Segal patented and published an original method for
imposing very large plastic deformations to bulk metals by simple shear
[21]. h e method entailed pushing a cylindrical or rectangular billet
through a die built with an entrance channel and exit channel with essen-
tially identical cross sectional dimensions, but dif ering in orientation by
a i xed angle. Intense shear and accompanying rotations occur in the billet
material as it passes through the channel intersection. Today this method,
known by the label Equal Channel Angular Pressing (ECAP) or Equal
Channel Angular Extrusion (ECAE) is one of the most popular techniques
developed for imposing severe plastic deformation.
In the early 1990s, Valiev and co-workers made the i rst demonstra-
tions of how severe plastic deformation leads to the continuous rei nement
of grain size and formation of ultrai ne grain structure [22-24]. By 1999
there was enough interest in grain rei nement by severe plastic deforma-
tion within the scientii c community that Lowe and Valiev organized the
i rst international workshop through NATO on the subject [25].
Of the various approaches for fabricating bulk nanostructured met-
als, methods involving severe plastic deformation have become among
the most widely recognized. h is is due in part to the fact that while SPD
of ers a cost ef ective means for grain rei nement, it also enhances other
properties of metals as well. For example, physical properties such as solid
state dif usivity, radiation damage resistance, and acoustic dampening are
enhanced [4, 26]. Within the context of biomaterials, one particularly dis-
tinctive property of SPD-processed metals stands out: living cells readily
attach and proliferate on their nanostructured surfaces [27-37]. h e rate
of proliferation of osteoblast cells on nanostructured titanium has been
reported to be as much as 19 times greater than on conventional titanium
[38]. We will explore this and other distinctive properties of bulk nano-
structured metals in the sections that follow.
1.1.3
Desirable Characteristics of Nanostructured Metals for
Medical Applications
1.1.3.1 Good Manufacturability
h e intrinsic advantage of producing bulk nanostructured metals by severe
plastic deformation is that the process is predominantly mechanical and
can be economically implemented in a manner that is fundamentally
