Biomedical Engineering Reference
In-Depth Information
causes a decrease in
M
s
temperature. It also reduces the grain grown
through precipitation formation, similar reaction causes nitrogen
(see Table 8.1.) [30]. Crucial to present research is the inluence of
both elements that is additive.
Table 8.1
Schematic illustrating the effects of various melting impurities
on the resulting ingot properties [33]
Element O N H C Cu Cr Co Fe V Nb
Temperature
↓ ↓ ↓ ↓ → ↓↓ ↓↓ ↓↓ ↓ →
Strength
↑↑↑↑→↑↑↑↑↑
↓↓↓↓→↓↓↑↓→
Ductility
The substitution of Fe, Co, Cr, Al, and V strongly inluences the
decrease of the
M
s
temperature, and this behavior is additive also. The
main function of the mentioned additions is to increase the stability
and strength of the alloy [9]. Substitution of Pd or Pt in small amount
causes a decrease in
M
s
temperature, but with an increase in their
contribution, the temperature begins to rise and can reach 350°C
[8]. The substitution of Cu or Nb gives the possibility of controlling
the hysteresis shape and also strengthening the martensite phase
[7].
The next crucial fact that the reader must be aware of, is a
strong relationship of the alloy properties with the temperature,
stress, and history. A range of characteristic temperatures strongly
depends on the chemical composition and also on the applied loads
that we mentioned earlier while discussing the hysteresis shape.
Melting and fabrication of Nitinol sometimes become a
challenge, considering the material's strong sensitivity toward the
chemistry and processing conditions. Forming techniques in some
cases are not appropriate when considering a spring-back effect or
thermal recovery of a shape memory alloy. Shape change by the
reduction of thickness, for instance, during cold rolling of sheet, is
lost when the material needs to be heat treated. Therefore, it is so
important to understand the underlying mechanism.
For the primary ingot methods, the reduction of dimensions
by a hot working process is usually employed. It is breaking up
as cast microstructure that exhibits poorly mechanical properties.
Finally, the shape of the product depends on the forming technique
where for the hot working, this could be forging, rotary forging, bar
