Biomedical Engineering Reference
In-Depth Information
The incentive to use different methods for achieving consistent
product came from the possibility of cost reduction, improvement
in properties, and elimination of the problems that appear in
conventional methods. Powder metallurgy gives a precise control
of the composition and the achievement of a complex shape
without machining it also eliminates the castings problems such as
segregation or extensive grain growth. A great challenge for these
methods is to produce SMA with properties comparable or better
with those of the cast alloys.
The main disadvantage of PM processes is porosity and the
manner of its control that affect the mechanical properties of
manufactured products.
Near-net shape part methods that employ powder metallurgy
use both pre-alloyed and elemental powders.
For pre-alloyed powders, we can distinguish a few fabrication
methods such as gas atomization, where molten alloy composition
is blasted through inert gas under high pressure. The melt
spinning process produces a ribbon from the rapid solidiication
of liquid composition, which should be hydrated and pulverized
for conversion to the powder form next. The process of hydrating
uses the strong solubility of hydrogen in NiTi, with rapid diffusion at
40 at.% causing the full embrittlement of the material. Checked by
our team for nanocrystalline electrode purposes, the mechanical
alloying process, which relies on mechanically induced synthesis in
the solid state, could start independently from prealloyed atomized
powders or pure powders to form the amorphous form of the
material [10]. The nanocrystalline NiTi alloy was synthesized by
mechanical alloying followed by annealing [15].
The powder mixture that was milled for more than 5 h
transformed completely to the amorphous phase, without formation
of the other phase. The formation of the nanocrystalline alloy
NiTi was achieved by annealing the amorphous material in high-
purity argon atmosphere at 700
o
C for 0.5 h. All diffraction peaks
were assigned to those of CsCl-type structure with cell parameter
a
= 3.018 Å.
Microstructure and possible local ordering in the NiTi samples
was studied by TEM. The microstructure of the annealed sample
is shown in Fig. 8.25. The analysis of the high-resolution images
(Fig. 8.25a, b) revealed the presence of well-developed crystallites
with broad range of sizes from 4 to more than 30 nm. SAED pattern
obtained from a large area (200 μm) (Fig. 8.25c) contains sharp rings
