10.5005/jp-journals-10021-1239
Jitendra Bhagchandani et al
Original research
Microscopic Grain Structure Analysis of Titanium-based
Orthodontic Archwires
1
Jitendra Bhagchandani, 2Anju Loomba, 3Kapil Loomba, 4Ashish Kumar Singh
ABSTRACT
Received on: 7/6/12
Introduction: Orthodontic archwires are designed to move
teeth with light continuous forces. Great strides have been
accomplished and are continuously evolving to produce the
‘perfect’ wire through complete knowledge of their metallurgical
properties. Advent of shape memory alloys in orthodontics has
been phenomenal. This study addresses the need of basic
understanding of its micrograin structure.
Accepted after Revision: 14/10/12
Materials and methods: Nickel-titanium and titanium molyb­
denum; archwires were selected. Each of the samples was
studied using following equipments: Scanning electron
microscopy and energy dispersive X-ray spectroscopy.
Results: Surface topography of nickel-titanium alloys showed a
highly rough surface with pits along with phases of intermetallic
compounds of TiNi, Ni4Ti and Ti2Ni while titanium molybdenum
alloys showed a highly porous surface topography. Precipitates
of stabilizing elements were seen at the grain boundaries
in both the alloys. Nickel-titanium alloys showed equiatomic
concentrations of nickel and titanium with titanium to be slightly
more than nickel. High percentage of interstitial elements like
carbon, oxygen, etc. were detected; while titanium molybdenum
showed maximum percentage of titanium followed by beta
stabilizer, molybdenum and low concentrations of interstitial
elements.
Conclusion: Metallurgists are continuously experimenting with
alloying elements to improve titanium alloys with consistent and
predictable properties.
Keywords: Metallurgy, Nickel-titanium alloys, Titanium
molybdenum alloys, Scanning electron microscopy, Energy
dispersive X-ray spectroscopy.
How to cite this article: Bhagchandani J, Loomba A, Loomba
K, Singh AK. Microscopic Grain Structure Analysis of Titaniumbased Orthodontic Archwires. J Ind Orthod Soc 2014;48(3):
168-174.
Source of support: Nil
Conflict of interest: None
1
Senior Lecturer, 2,3Professor and Head, 4Reader
1,4
Department of Orthodontics and Dentofacial Orthopedics
Sardar Patel Postgraduate Institute of Dental and Medical
Sciences, Lucknow, Uttar Pradesh, India
2
Department of Orthodontics and Dentofacial Orthopedics
Saraswati Dental College, Lucknow, Uttar Pradesh, India
3
Department of Conservative and Endodontics, Saraswati
Dental College, Lucknow, Uttar Pradesh, India
Corresponding Author: Jitendra Bhagchandani, Senior Lecturer
Department of Orthodontics and Dentofacial Orthopedics
Sardar Patel Postgraduate Institute of Dental and Medical
Sciences, Lucknow, Uttar Pradesh, India, Phone: 9839289804
e-mail: [email protected]
168
INTRODUCTION
‘The beginning of knowledge is the discovery of something
we do not understand.’
Knowledge of fundamental principles governing the
relation­ships between compositions, structures and pro­
perties is central to an understanding of orthodontic mate­
rials. Because wide arrays of metallic, ceramic and poly­
meric materials are used in the profession, and new materials
are continuously being introduced, it is essential that the
scientific basis for the selection and proper use of materials
for clinical practice be thoroughly understood.1
‘The field of orthodontics has seen a gradual transition
from traditional gold wires to stainless steel wires and finally
to titanium-based wires with changes in the concept of force
delivery.’2
The never ending enigma of titanium metal and its
pro­­perties has always drawn curiosity among scientific
commu­­nity. Perhaps it dates back from early 17th century
until now and sure in future too. This has been termed by
many research scholars as active, adaptive or simply shape
memory and superelastic metal; a unique property utilized
both in aeronautical and medical field.
Endeavor to further improve titanium has always been
a matter of discussion among the researchers. Titanium
and its alloying methods have revolutionized the field of
orthodontics delivering precision movements from the
stages of initial levelling and aligning to space closure until
finishing and detailing.
Titanium combinations with nickel, molybdenum,
niobium, etc., yielded fruitful results after a series of clinical
researches involved. After all ‘Research is formalized
curiosity. It is poking and prying with a purpose.’
Awareness about intricacies involved speciality as related
to structure is as important to clinicians as the pathogenesis
to physicians and pharmacokinetics to all medicos. Hence,
it is imperative to orthodontists to at least procure deep
knowledge about the wires and the way they correct
malpositions of the teeth, so that there is an appropriate and
judicious use of these wires. Hence, with these objectives in
mind the present study was undertaken to have a deep insight
about the ‘miracle’ titanium-based orthodontic archwires.
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Microscopic Grain Structure Analysis of Titanium-based Orthodontic Archwires
The present study addresses the need of exploring the
material engineering behind titanium-based alloys; hence
an attempt is made on characterization or probing into the
internal structure and properties of the material. Series of
analytical instrumentation were performed but emphasis
in this section of the article is on preliminary or basic
understanding of their micrograin structures.
MATERIALS AND METHODS
The present study was carried out as a collaborative study
between the Department of Orthodontics and Dentofacial
Orthopedics, Saraswati Dental College, Lucknow and the
Scanning Electron and Energy Dispersive X-ray Spec­tro­
scopy Department, Birbal Sahni Institute of Palaeobotany,
Lucknow.
Nickel-titanium and titanium molybdenum alloys, the
two commercially available and widely used orthodontic
archwires were selected for the study (Table 1).
Cut wire pieces of the above alloys were used for the
purpose of evaluating their micrograin structure. Each of the
alloys was studied using following microanalysis.
Scanning Electron Microscopy
ultrasonic vibrator and stored in a dessicator to avoid any
environment exposure. Wires were then mounted over stubs
and sputtered with gold or palladium (Fig. 1). Post-sputtering
alloys were loaded in scanner – Philips 505 for microstructure
analysis of each sample (Fig. 2). Alloys were magnified from
×100 to ×15,000 at 30 kV and the size of field magnified
was up to 1 micrometer. Area of interest of scanned surfaces
were freezed and transferred over the computer interface
through Orion image grabbing software at a frequency of
32 seconds/1000 lines to avoid noise or any distortion in
image capture.
Energy Dispersive X-ray Spectroscopy—EDAX
(Coupled to SEM for Compositional Analysis)
EDAX provides further information of the composition
of surface topography and microstructures of the scanned
areas. The elements are represented on the graph as peaks
since the signals generated from any particular element
are charac­teristic of that element, and as such, can be used
to identify which elements are actually present under the
electron probe.3,5
Sample Preparation
The scanning electron microscope (SEM) is a type of
electron microscope that images the surface topography
and microstructures of the samples by scanning it with a
high-energy beam of electrons.3,4
Sample Preparation
The 8 mm wire of each alloy was cut and labelled. They were
etched with warm aqua regia and immediately cleaned in an
Each labelled, etched and coated wire immediately after
SEM was analyzed for relative proportions of elements in
the field of scanned surface. Elemental spectra for the given
field were recorded over the computer interface.
RESULTS AND OBSERVATIONS
Scanning Electron Microscopy
Nickel-Titanium Alloy
Fig. 1: Sputtered alloys over stubs
At magnification of ×503: Samples show presence of
surface porosities (a) (occupied by interstitial elements)
and superficial defects, such as scratches and pits (Fig. 3).
At magnification of ×625: The microstructure shows den­
drites with lath morphology/black plates (b) and distinct
Table 1: Titanium-based orthodontic archwires
Titanium-based orthodontic archwires
Wire cross-section
Clinical advantage
Manufacturer
Nickel-titanium alloy—Nitinol SE
Titanium-molybdenum alloy—TMA
0.017" × 0.025"
0.017" × 0.025"
Initial leveling and aligning
Space closure
3M Unitek
Ormco
The Journal of Indian Orthodontic Society, July-September 2014;48(3):168-174
169
Jitendra Bhagchandani et al
Fig. 2: Philips 505, SEM machine
Fig. 3: Magnification at ×503
Fig. 4: Magnification at ×625
Fig. 5: Magnification at ×5000
Fig. 6: Magnification at ×5000 concentrating at the
grain boundaries
Fig. 7: Magnification at ×15,400
grain boundaries (d). Sample exhibits regions of intermetallic
compound of Ti2Ni (c) (Fig. 4).
At magnification of ×15,400: Stabilizing elements that
precipitates at the grain boundaries presented with lenticular
morphology (i) (Fig. 7).
At magnification of ×5000: Prominent intermetallic phases
of TiNi (e), Ni4Ti (f) and Ti2Ni (c) were visible. Magnified
view of grain boundaries (g) (Fig. 5).
Magnification at ×5000 concentrating at the grain boun­
daries: Grain growth occured preferentially along the
dislo­cation networks and the grain boundaries. This resulted
in a heterogeneous distribution of precipitates near these
boundaries (h) (Fig. 6).
170
Titanium-Molybdenum Alloy
At magnification of ×203: Suggestive of surface porosities
(a’) with straining effects (b’) due to work hardening of the
wire during processing (Fig. 8).
At magnification of ×1010: Surface porosities or
voids occured as closed interstices occupied by extra low
interstitial elements (ELI) (c’) (Fig. 9).
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Microscopic Grain Structure Analysis of Titanium-based Orthodontic Archwires
Fig. 8: Magnification at ×203
Fig. 9: Magnification at ×1010
Fig. 10: Magnification at ×2020
Fig. 11: Magnification at ×8800
At magnification of ×2020: The solvent phase is islands
of precipitation of alpha and omega precipitates (e’) in the
porous beta matrix (solute phase) (d’) (Fig. 10).
At magnification of ×8800: The nucleation sites were
limited to grain boundary surface (f’). Traces of alpha phase
(d’) occured along the grain boundaries as an allotriomorphic
shape or continuous layer. These nucleation sites seem to
be homogenously distributed in a plate like orientation (e’)
(Fig. 11).
At magnification of ×15,400: Prominent interdendrite
spacing (f’) or the grain boundary with an area of large void
occupied by extra low interstitial elements (c’) (Fig. 12).
ENERGY DISPERSIVE X-RAY
SPECTROSCOPY (EDAX)
(Coupled to SEM for compositional analysis)
Nickel-Titanium Alloy (Table 2) (Fig. 13)
Fig. 12: Magnification at ×15,400
Table 2: Point analysis of nickel-titanium alloy
Spectrum
Ti (wt%)
Ni (wt%)
4-5.425.42—
7.20-8 —24.89
Within 0.90
—
—
Interstitial (wt%)
—
—
C = 13.87
O = 35.81
Total = 49.68
• Equiatomic titanium (25.42%) and nickel (24.89%) by
wt% with titanium to be slightly more than nickel.
• Interstitial elements C and O are found at a wt% of 2:1
ratio to Ti and Ni.
• 18.05 wt% of alloying stabilizers (Mo—8.97 wt%, Zr—
5.05 wt%, Sn—3.47 wt% and Ir—0.56 wt%)
• Interstitial elements—15.55 wt%.
Titanium-Molybdenum Alloy (Table 3) (Fig. 14)
Advances in metallurgy during the 20th century have been
considerable and the field of orthodontics is in a position
• Maximum wt% of titanium—66.41 wt%
DISCUSSION
The Journal of Indian Orthodontic Society, July-September 2014;48(3):168-174
171
Jitendra Bhagchandani et al
Table 3: Point analysis of titanium-molybdenum alloy
Spectrum Ti (wt%)
(High scan speed)
Stabilizing Interstitial (wt%)
elements (wt%)
4-5.4
66.41—
—
Between 1.8-2.7
Mo—8.97
—
Between 1.8-2.7
Zr—5.05
—
3.00 Sn—3.47
Between 1.8-2.7
Ir—0.56
—
Within 0.9
C = 15.55%
Fig. 14: EDAX pattern for titanium molybdenum alloy
Fig. 13: EDAX pattern for nickel-titanium alloy
to reap great benefits from it. Orthodontic archwires are
designed to move teeth with light continuous forces and are
being used in various situations, from initial aligning to the
final finish. Great strides have been accomplished and are
continuously evolving to produce the ‘perfect’ wire through
complete knowledge of their metallurgical properties. Hence,
even the manufacturers try to follow the standard norms of
metallurgical science to provide an orthodontist with ideal
superelastic nickel-titanium wire or a formable titanium
molybdenum wire, since a slight variation in compositional
content can alter the inherent behavior pattern of that alloy.
Scanning Electron Microscopy (SEM)
Nickel-titanium alloy shows presence of surface porosities
and superficial defects, such as scratches and pits. The
microstructure shows dendrites with lathe morphology/
black plates and distinct grain boundaries. Samples
172
exhibit prominent intermetallic phases of TiNi, Ni4Ti
and Ti2Ni. Grain growth occurs preferentially along the
dislocation networks and the grain boundaries. This results
in a heterogeneous distribution of precipitates near these
boundaries. Stabilizing elements that precipitates at the grain
boundaries presents with lenticular morphology.
Garattini, Abatti, Santarelli, 19916 studied the surface
morphology of newly manufactured orthodontic arches
made of rectangular NiTi wires which exhibited various
patterns of superficial defects, such as scratches and pits.
Hanawa 1991, Oshida et al 1992, Shabalovskaya 1996,
Yahia et al 19967 have reported that the surface of NiTi
consists mainly of titanium oxides (TiO 2) and smaller
amounts of nickel oxides (NiO and Ni2O3) and metallic Ni,
while nickel-titanium constitutes the inner layer. Oshida
et al 19927 has revealed the surface layer of nickel-titanium
to have irregular features characterized by lengthy island
like structures, where selective dissolution of nickel may
occur. Munther Abdullah Alhammad 20088 has reported
nickel-titanium alloy to exhibit intermetallic regions of
Ti2Ni, TiNi and TiNi3. Goldstein et al 1987, Brantley 20013
has also suggested about the nickel-titanium binary phase
and has shown regions of intermetallic compound relevant
for NiTi orthodontic wires.
Buie, Clark, Alapati 20089 have revealed the micro­
structure SE wire to have colonies of lenticular features
which are supposed to be the reminiscent of a martensitic
structure.
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Microscopic Grain Structure Analysis of Titanium-based Orthodontic Archwires
Titanium-Molybdenum alloy showed surface porosities
with straining effects due to work hardening of the wire
during processing. The solvent phase is islands of preci­
pitation of alpha and omega precipitates in the porous beta
matrix (solute phase). The nucleation sites are limited to
grain boundary surface. Traces of alpha phase occur along
the grain boundaries as an allotriomorphic shape or conti­
nuous layer.
Brantley 20013 has attributed surface porosities to
adherence or cold welding of titanium to the dies or rollers
during wire processing.
The surface roughness contributes to the high values of
archwire bracket sliding friction. Hence, at present all the
efforts are being directed to various techniques by which this
sliding friction be minimized by either nitro­gen ion implan­
tation,10 diamond like carbon coatings11 or the more recent
of all with inorganic fullerene-like tungsten disulfide nano­
particles12 (IF-WS2), which are known for their excellent
dry lubrication properties.
Bein and Behcet 199613 have also shown the presence
of porous beta phase enrichment with stabilizing elements
precipitating as continuous or allotriomorphic pattern near
the grain boundaries.
Precipitation of stabilizing elements at the time of manu­
facturing or alloying with titanium has been attributed to
their weak atomic number contrast.
Titanium-molybdenum alloy revealed presence of
straining effects which were found to be prominent at the
micro­grain level indicative of an increased degree of cold
work contrary to nickel-titanium alloy which does not show
areas of straining, hence were supposed to be less cold worked.
Surface topography revealed a rougher surface with
nickel-titanium alloy than titanium-molybdenum alloy
which was better finished though surface porosities were
evident.
Energy Dispersive X-ray Spectroscopy
(EDAX)—Coupled with SEM
Nickel-titanium alloy showed equiatomic weight percentage
of nickel (24.89%) and titanium (25.42%) along with
interstitial elements (49.68%) like C and O which were found
at a weight percentage of 2:1 ratios to Ti and Ni.
Bradley, Mitchell, Brantley 199614 have also reported the
surface composition for all commercially available nickeltitanium archwires to be nearly equiatomic. Brantley 20013
stated that surface titanium concentration was found to be
more than nickel. Buehler et al 19677 has stated that variation
in nickel concentration varies the transition temperature
range (TTR). If nickel content is above 55.6 wt% NiTi looses
its properties. Addition of nickel lowers the TTR. Properties
of NiTi are largely dependent on mechanical working and
heat treatment of the alloy at the time of manufacturing.
Titanium-molybdenum alloy exhibits titanium (66.41%)
to be in maximum percentage with varying concentration
of beta stabilizers like molybdenum (8.97%) and neutral
stabilizers like Zr (5.05%), Sn (3.47%) and Ir (0.56%).
Interstitial elements occur only in concentrations of 15.55%.
Interstitial elements greatly influence physical properties,
density and behavior pattern of the alloy for, e.g. oxygen
content less than 30%, in solution with titanium imparts
thermodynamic stability. They do not fit properly and cause
changes in the lattice parameters. Body centred cubic (beta)
titanium has three octahedral interstices per atom whereas
closed packed hexagonal titanium (alpha) has one interstices
per atom which is occupied by oxygen atom (dissolved in
titanium). In doing so large amount of energy is released
to form dilute solid solution. Theoretically this means the
alloy must be brittle but the limit of solid solution is directly
dependent on content of oxygen (less than 30%) of which
the manufacturers take utmost care though it is impossible to
eliminate interstitial elements but are kept within limits. The
surface adsorbed oxide layer forms approximately 2 to 5 nm
thickness which is believed to be formed in nanoseconds and
further oxygen diffusion is difficult due to tenacious TiO2.
Since, closed packed hexagonal titanium has only one
inter­stice per atom therefore solubility of interstitial elements
like O, N and C was much higher in alpha or austenitic
phase.15,16
The hydrogen (H) capable of entering interstitials of
tetrahedral walls in beta phase is up to 60%. This causes
considerable volume expansions which in turn can imbrittle
the alloy.16 The manufacturers have taken care to remove this
by annealing in vacuum. Hydrogen therefore goes undetected
in both the nickel-titanium and titanium-molybdenum alloy.
Nickel-titanium alloy having the highest concentration of
interstitial elements is supposed to exist in austenitic phase
while titanium-molybdenum alloy with lesser concentration
of interstitial elements exists as beta or martensitic phase.
Nickel-titanium alloy revealed equiatomicity of nickel
and titanium with titanium concentration to be slightly more
than nickel. Titanium-molybdenum alloy showed titanium
concentration to be in maximum followed by molybdenum,
zirconium and tin.
CONCLUSION
1. Surface topography of nickel-titanium alloys showed
a highly rough surface with pits along with phases of
intermetallic compounds of TiNi, Ni4Ti and Ti2Ni while
titanium-molybdenum alloys showed a highly porous
surface topography. Precipitates of stabilizing elements
are seen at the grain boundaries in both the alloys.
2. Nickel-titanium alloys show equiatomic concentrations of
nickel and titanium with titanium to be slightly more than
nickel. High percentage of interstitial elements like carbon,
oxygen, etc. were detected; while titanium molybdenum
showed maximum percentage of titanium followed by
The Journal of Indian Orthodontic Society, July-September 2014;48(3):168-174
173
Jitendra Bhagchandani et al
beta stabilizer, molybdenum and low concen­trations of
interstitial elements.
Metallurgists are continuously experimenting with
alloying elements to improve titanium alloys with
consis­tent and predictable properties. The possibility of
further improve­ment of titanium could be possible in the
forthcoming years.
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on orthodontic archwires. Faculty of Science and Engineering,
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11. Kula K, Gibilaro A, Profitt WR. Effect of ion implantation of
TMA archwires on the rate of orthodontic sliding space closure.
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12. Redlich M, Katz A, Rapoport L, Wagner H, Feldman Y, Tenne R.
Improved orthodontic wires coated with inorganic fullerene-like
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14. Bradley TG, Mitchell JC, Brantley WA. Surface composition
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metals.com/article120.html.
RETRACTION NOTICE
Article: Madhav MK, Roopesh R, Purushothaman B, Manjusha KK, Sanal KM. Bolton ratio: A proposed norm for
Kerala population. J Ind Orthod Soc 2014;48(2):88-91.
As the editorial board has found out upon inquiry that there is plagiarism by copying the text as well as tables from multiple
sources, with the above mentioned article, the article is withdrawn from the Journal of Indian Orthodontic Society and
the link to its abstract, PDF as well full text html version have been removed from the website also. The editorial board
considers this as a serious violation of the copyright statement, which the authors have submitted to the journal at the
time of submitting the manuscript for consideration for possible publication. The matter of plagiarised publication from
the members of the Indian Orthodontic Society (MK Madhav, R Roopesh, B Purushothaman, KK Manjush, KM Sanal)
has been informed to the IOS Head Office/Dental Council of India/the authors affiliated institutions for necessary action.
This retraction notice hereby informs that the article, which appeared in Journal of Indian Orthodontic Society,
cannot be considered as publication from the side of author/s and thus cannot be accepted for career advancement
or to fulfil eligibility criteria for any job that the authors might be applying for.
Editorial Board
Journal of Indian Orthodontic Society
174