Biomedical Engineering Reference
In-Depth Information
corresponding to the NiTi alloy with CsCl structure. The method
approves the possibility of nanocrystalline NiTi material receiving
[29], although no further research was done to characterize the
shape memory properties in obtained alloy.
Figure 8.25
TEM micrographs (a,b) and electron diffraction patterns (c)
of the annealed NiTi sample; nanocrystallites of an alloy are
clearly visible in (a) and (b) [15].
The powders obtained from the above methods of fabrication
should be compacted and sintered. The successfully applied methods
include cold and hot isostatic pressing, hot and cold uniaxial die
compaction, direct powder rolling, hot extrusion, and consolidation
by atmospheric pressure.
For the elemental Ti and Ni powders, we focus on blending,
pressing, and sintering, where additional problem arises during
thermal treatment. Strong exothermic reaction for Ni + Ti = NiTi
results in a much bigger diffusion factor of Ti to Ni, causing the
Kirkendall porosity effect. The produced heat can be used to
synthesize material like in the combustion synthesis method as
shown in Fig. 8.26. Self-propagating high-temperature synthesis,
understood as a local heating of cold compacted material above
ignition temperature, leads to Ni−Ti formation, which retaliates heat
propagation from the reaction with the surrounding material, affect
the creation of the synthesis front throughout the entire capacity.
Another approach presents the method of thermal explosion
where the entire sample of material is heated up until it reaches
ignition temperature. This leads to fusion energy release and
generates enough heat to sinter the material. If the heat exceeds the
melting point, a cast structure with no visible prior powder boundaries
will be formed. To break down the received microstructure, the
alloy should be hot rolled or extruded. Methods that use elemental
powders generally fabricate highly porous materials and may contain
