Photonic crystals in biology

Poster Session, Tuesday, June 15
Theme A1 - B702
Evol ution of Nano-crys talline NiTi and Amorphous Structures in Mechanical Alloyi ng o f Ni and Ti
A. Rabiezadeh 1 and S. F. Kashani Bozorg 1 *
1 School of Metallurgy and Materials Engineering, University of Tehran, Tehran, P.O. Box: 14395-553, Iran
Abstract-Mechanical alloying of Ni and Ti was conducted in a planetary ball mill using a powder mixture with equiatomic stoichiometry for
various times. The milled products were found to decrease in size as a function of milling time as evidenced by scanning electron microscopy.
X-ray diffractometry of the milled products exhibited peak broadening and the formation of amorphous structure which became dominant after
25h of milling. Transmission electron microscopy using bright field imaging and electron diffraction pattern modes revealed that the 25h milled
product was consisted of mixture of nano-crystalline NiTi (20-40nm size) and an amorphous phase.
NiTi has unique properties that could be very useful in
surgical applications [1]. Conventionally, Ni-Ti alloys are
produced by arc or induction melting. Melting methods have
shortcomings due to gas absorption, elemental vaporization,
segregation formation, and crucible contamination absorption.
Thus, throughout the last decade solid state processing routes
have gained considerable interest for NiTi fabricat ion [2-5].
In this study, the mechanical alloying (MA) process of
elemental Ni and Ti with equiatomic ratio was carried out. The
morphology, phase composition of the milled products for
different durations were investigated. MA of the powder
mixture was conducted in a planetary ball mill at room
temperature in a stainless steel vial with chromium steel balls
under an argon atmosphere. The milling speed and the ball to
powder charge ratio were 450 rpm and 15:1, respectively. The
maximum milling time was 25 h.
During the preliminarily stage of milling (until 4h), the
specific size of powders increases which corresponds to the
formation of coarse multiphase particles. With further milling,
the mean particle size decreased; this can be related to work
hardening and/or formation of new brittle phase(s). After 10h
of milling, substantial powder was crushed and flattened by
the collisions between the powder and the milling media. No
significant change in particle size of the powders can be
observed after 10 h of milling. With further milling, the milled
products become more homogeneous while keeping its
granular morphology. The particle size was found to be
smaller than 10 h of milling.
In a first period of milling up to 6h, the intensity of the X-
ray diffraction peaks of the milled product decreases and
broadens simultaneously. The broadening of the peaks is
commonly attributed to the structural change in the samples,
such as formation of amorphous phases, large lattice strains in
grains, decrease of crystallite size, or embedding of very small
crystals in an amorphous matrix. X-ray diffraction
examination studies showed that the milled products contained
nano-crystallites of disordered NiTi (with a mean crystallite
size of ~30nm) and an amorphous phase (Figure 1a). A new
broad diffuse scattering halo at abo -44° can be
observed after 10h of milling, which implies that the milled
product becomes partially amourphized. It is found that further
milling enhances the formation of the amorphous phase. The
diffraction lines corresponding to fcc nickel (111) become
broad and shift towards low angles with increasing milling
time; this is attributed to increasing the lattice parameter of
nickel fcc structure with increasing the milling times.
In order to evaluate the crystallite size of the powders, the
powders are further examined by TEM. Bright-field image
with corresponding electron diffraction pattern for the 25h
milled product is shown in Fig. 1b. It can be seen that the
approximate grain size of the milled product are 20-40 nm.
The corresponding diffraction pattern using smallest selected
area aperture showed several even and broad rings. The rings
were found to be consistent with NiTi nano-crystalline
structure. Also, the first wide ring may be indicative of
coexistence of substantial amorphous phase.
Figure 1. (a) X-ray pattern of the milled product for 25h. (b) TEM
bright field and corresponding selected area diffraction pattern
micrographs of 25h milled product. The key of the diffraction pattern
is shown at the top left.
In summary, nano-crystalline NiTi-based powder was
achieved using mechanical alloying of elemental Ni and Ti
powder mixtures. NiTi mean crystallite size after 25h of
milling was found to be ~30 nm. In addition, an amorphous
NiTi phase was detected.
*corresponding author fkashani@ut.ac.ir
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[2] B. Y. Li, L. J. Rong, Y. Y. Li and V. E. Gjunter, Scripta Mater,
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[3] A. Kapanen, J. Ilvesaro, A. Danilov, J. Ryhanen, P. Lehenkari and
J. Tuukkanen, Biomaterials, 23, 645 (2002).
[4] D. S. Grummon, J. A. Shaw and A. Gremillet, Applied Physics
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[5] A. Takasaki, Physica Status Solidi A, 169, 183 (1998).
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