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Duplex steels:
Duplex steels consist of an assembly of both
austenite and ferrite grains. Besides iron
these steels contain molybdenum and
chromium and they have lower nickel content.
This results in improved toughness and
ductility compared to ferritic steels. Yield
strength is twice that of Austenitic stainless
steels. Also they are highly corrosion
resistant.
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COBALT CHROMIUM ARCH
WIRES

. HISTROCIAL BACKGROUND :-
In 1950s Elgin watch company was developing a complex alloy whose
primary ingredients were cobalt, chromium Iron and Nickel.
Cobalt chromium alloy was marketed as ELGILOY by Rocky Mountain
Orthodontics.
II. COMPOSITION :-
Cobalt - 40% Molybdenum - 7%
Chromium - 20% Manganese - 2%
Nickel - 15% Iron - 15.8%
Carbon -0.15% Beryllium -0.04%

TYPE OF ELGILOY :-
Manufactured in 4 tempers :-
1. SOFT (Blue)
2. DUCTILE (Yellow)
3. SEMIRESILIENT (Green)
4. RESILIENT (Red)

. BLUE ELGILOY :-
- Softest of the four wire tempers.
- Bent easily with fingers or pliers.
Used when considerable bending, soldering and welding is
required .
YELLOW ELGILOY :-
- Relatively ductile and more resilient than blue.
- Bent with relative ease.
Further ↑ in resilience and spring performance can be achieved by
heat treatment

3. GREEN ELGILOY :-
- More resilient than yellow elgiloy.
- Can be shaped with pliers before heat treatment.
4. RED ELGILOY :-
- Most resilient and provides high spring qualities.
- Careful manipulation with pliers is recommended
when using this wire because it withstands only minimal
working.
- Heat treatment makes it extremely resilient.

PROPERTIES :-
1. SPRINGBACK :
With exception of red temper elgiloy non-heat treated Co-Cr
wires have smaller springback than SS wires of comparable
sizes. But this property can be improved by adequate heat
treatment.
.0045-.0065-for soft as received wire
.0054-.0074 after heat treatment
2. STIFFNESS :-
High Modulus of elasticity Deliver twice force of β - Ti and 4 times
force of Nitinol for equal amounts of activation.
160-190GPa as received
180-210GPa heat treated

3. FORMABILITY :-
- Good formability.
Modified by heat treatment.. Once appliance has been
fabricated, the practitioner no longer requires formability
instead they require resilience in order to capitalize on
inherent elasticity of material which could be achieved
by heat treatment
yield strength-830-100MPa as received
1100-1400MPa heat treated

4. JOINABILITY :-
Can be soldered and welded.
Precaution :-
- High temp (749°C) causes Annealing.
- Low fusing solder is recommended.
5. BIOCOMPATIBILITY AND ENVIRONMENTAL
STABILITY
Good

6. FRICTION :-
Although larger frictional forces have been noted
previously between brackets and cobalt
chromium wires but recent reports suggest that
resistance to tooth movement along stainless
steel and cobalt chromium wires may be
comparable.

. HEAT TREATMENT OF ELGILOY :-
Temp and Time :-
900° F (482° C) for 7 – 12 min in a dental furnance.
Temp above 1200° F (749°C) results in partial
annealing - ↓ in resistance to deformation.
Optimum levels of heat Rx are confirmed by : -
- Dark straw colored wire.
Temp indicating paste

MECHANISM :
Precipitation
Hardening Results
in
- ↑ resistance of
wire to deformation.
- Wire demonstrates
properties similar to
SS.

PROCEDURE :- Three heat treatment procedures are
recommended.
1. Oven heat treatment.
2 Electrical heat treatment – using a heat treatment unit
and temperature indicating paste to achieve proper heat
treatment temperature. Wet cotton is placed over bends
in wire to prevent overheating the wire.
3 Flame heat treatment with match or brush flame. Uniform
results are not attained.

Martin et al (1984) :-
Investigated effect of heat treatment on various
properties of Blue Elgiloy.
• Heat treatment of blue Elgiloy ↑ its yield strength and
stiffness but ↓ no. of 90° bends cycles to failure.
• Significantly higher yield strength of electrically heat
treated straight wire samples over oven heat treated
samples was noted.


CLINICAL APPLICATIONS :-
Elgiloy : Easier to bend than SS, NiTi and β-Ti in its “as
received state”.
- Preferred in techniques in which loops are used.
- Along with SS considered most ideal and economical
finishing wire.
VI ADVANTAGES OVER SS WIRES :-
- Greater resistance to fatigue and distortion.
- Longer function as a resilient spring.
- excellent corrosion resistance

In other aspects mechanical properties of Co-Cr
wires are very similar to those of stainless steel
archwires.
stainless steel wires may be used instead of Co-Cr
wires of same size in clinical situations in which
heat hardening and added torsional strength of
Co-Cr wires are not required.

DRAWBACKS :
- High elastic force delivery similar to that of SS
Lower spring back than stainless steel
AVAILABILITY :
Available commercially as
Elgiloy - Rocky Mountain orthodontics
Azurloy - Ormco Corporation
Multiphase - American Orthodontics.
Flexiloy - Unitek Corporation

 Due to its soft feel during manipulation ,operater
can mistakenly believe that as received elgiloy
blue wire has low gorce delivery,
 The value of modulus of elasticity are very
similar for blue elgiloy and stainless steel

NICKEL TITANIUM WIRES

 original work of Buehler for the Naval
Ordinance Laboratory in the early 1960s.
 . Buehler’s preliminary results led to
development of the first NiTi orthodontic
alloy55% nickel and 45% titanium by pioneers
such as Andreasen and his colleagues
 The Unitek Corporation licensed the patent
[1974] and offered a stabilized martensitic
alloy (M-NiTi) that does not exhibit any shape
memory effect (SME) under the name,
Nitinol..

 It is a stabilized form
of the alloy in which
work hardening has
abolished the phase
transformation
 This alloy has low
elastic modulus and
high range

METALLURGICAL ASPECTS OF TITANIUM
• Titanium – a metal discovered by M.H. Klaproth in 1795.
Titanium Light weight
High strength
Corrosion resistance
Titanium is obtained in its pure form by heating the titanium ore in
presence of carbon and chlorine. The titanium tetrachloride
(TiCl4
) is then reduced with sodium to produce a titanium sponge.
This sponge is fused under vaccum or in an inert argon
atmosphere and converted to ingots.

•     Pure titanium exhibits ALLOTROPY – can crystallize
into more than one structure.
•     At room temperature or below 885° C → Hexagonal
close packed (HCP) α- lattice is stable.
•      At higher temperatures i.e. above 885° C .
Rearranges into BCC or β phase. Addition of
Molybdenum / Columbium stabilize the β - phase at
room temperature.

BASIC CONCEPTS ABOUT NICKEL
TITANIUM ALLOYS
:-
1. ACTIVE :-
A term that is used to describe an alloy that is capable of undergoing its
anticipated phase transformation.
2. PASSIVE :- An alloy that is incapable of undergoing its anticipated
phase transformation because extensive plastic deformation has
suppressed the transition.
3. TWINNING :- In certain metals that crystallize in HCP
structure, deformation occurs by twinning.
It refers to a movement that divides the lattice into two symmetric
parts; these parts are no longer in the same plane but rather at a
certain angle.
e.g., :- NiTi alloys are characterized by multiple rather than single
twining throughout the metal

• When these alloys are
subjected to higher
temperature.
⇓
DETWINNING OCCURS
⇓
Alloy reverts to its
original shape.
(SHAPE
MEMORY EFFECT).

4. AUSTENITE :-
High temperature phase of Nickel titanium alloys is sometimes
called Austenite because like many ferrous alloys this austenite
can transform to Martensite. It has got BCC structure. The
stronger, higher temperature phase present in NiTi.
5. MARTENSITIC TRASFORMATION :-
Process of phase transformation which is DIFFUSIONLESS,
occuring from within and without any chemical change which
results in transformation of Austenite (parent phase) to
Martensite following rapid cooling. It has got HCP structure. The
more deformable, lower temperature phase present
in NiTi.
e.g.,:- Product of such transformation in NiTi alloys is called
MARTENSITE whether it is thermally activated or not thermally
activated.

: Af Temperature: The temperature at which a shape
memory alloy finishes transforming to austenite upon
heating.
Ap Temperature: The temperature at which the SMA is
about 50% transformed to Austenite upon heating.
As Temperature: The temperature at which the SMA
starts transforming to Austenite upon heating.
.

Hysteresis: The temperature difference between a
phase transformation upon heating and cooling. In
NiTi alloys, it is generally measured as the difference
between Ap and Mp.
Mf Temperature: The temperature at which a SMA
finishes transforming to Martensite upon cooling.
Mp Temperature: The temperature, at which a SMA is
about 50% transformed to Martensite upon cooling,
Ms Temperature: The temperature at which a SMA
starts transforming to Martensite upon cooling.
:

Phase Transformation: The change from one alloy phase to
another with a change in temperature, pressure, stress,
chemistry, and/or time.
R-phase: A phase intermediate between Martensite and Austenite
that can form in NiTi alloys under certain conditions.
Shape Memory: The ability of certain alloys to return to a
predetermined shape upon heating via a phase transformation.
Superelasticity: The springy, “rubber like” behaviour present in NiTi
shape Memory Alloys at temperatures above the Af
temperature. The superelasticity arises from the formation and
reversal of stress induced martensite.
Thermoelastic Martensitic Transformation: A diffusionless,
thermally reversible phase transformation characterized by a
crystal lattice distortion.

MARTENSITE AUSTENITE
Low stiffness phase
Elastic modules of 31 – 35
GPa
High stiffness phase
Elastic modulus of 84 – 98
GPa

Other features of Martensitic transformation
1. Strongly influenced both by the nature of atoms involved and
by extraneous solute elements within them.
e.g., :- interstitials such as Carbon and Nitrogen.
2. With steels Martensitic transformation results in highly stressed
structure called Martensite – Hard and brittle.
With NiTi alloys a similar martensitic transformation leads to soft
structures.
Explanation : In case of NiTi alloys interstitial elements are
carefully avoided. (even traces of O2
, C and N in alloy lead to
loss of elasticity and shape memory

Martensitic transformations do not occur at a precise
temperature but rather within a range known as
temperature transition range(TTR).
Transformation from Austenite to Marteniste and reverse
do not take place at same temperature, this difference
is known as HYSTERESIS.
Range for most binary NiTi alloys → 40° - 60° C.
Transformation from Austenite to Martensite can occur by.
→ Lowering the temperature.
→ Applying stress (Stress induced Martensite)
SIM.

Austenite and Martensite have different crystal
structure and mechanical properties the most
notable mechanical properties of Nitinol wires i.e
superelasticity and shape memory are result of
reversible nature of Martensitic transformation

Specific TTR is a function of :-
→ Composition of the alloy.
→ Processing history.
TTRS can be obtained from below room temperature upto 275°F
or higher.
e.g . Considering body temperature as reference
- TTR above that temperature – Alloy is Austentic (Rigid).
TTR below that temperature – Alloys is Martensitic (Superelastic

EFFECTS OF ADDITIONS AND IMPURITIES ON TTR :-
Adding a third metal can lower the TTR
→ to as low as - 330° F ( - 200° C).
→ Narrow the difference b/w cooling and heating (Narrow Hysteresis).
For thermally activated purposes most common third metals are Cu and Co
because.
→ Reduce the hysteresis
→ Bring TTR close to body temperature.
Dissolved interstitial elements (small atoms such as O, N and C) disrupt the
matrices which affects alloy shape memory and super elasticity.
Thermally respondent wires – designed so that composition , Annealing and cold
working match Ms to temperature of human body

Classification of wires / alloys based on TTRs
•     Those with transformation temperatures b/w room temperature
and body temperature (MARTENSITIC ACTIVE ALLOYS).
•      Those with transformation temperature below room
temperature (AUSTENITIC ACTIVE ALLOYS)
Af
temperature :-
-      Most important marker.
-      To exploit super elasticity to its fullest potential, the working
temperature of orthodontic appliances should be greater than
Af
temperature.

. SHAPE MEMORY AND SUPERELESTICITY
Both shape memory and super elasticity are related to
phase transitions within the NiTi alloy b/w Martensitic
and Austenitic forms
SHAPE MEMORY :-
It refers to the ability of the material to “remember” its
original shape after being plastically deformed while in
martensitic form.

Training of the wire :-
To get wire to “Memorize” a certain form it must first be set into
the desired shape and held tightly while undergoing a high
temperature heat treatment. [heating wires in a mold for 10 min
at 500°F (280° C)].
After the wire is cooled to room temperature it can easily be
deformed below the TTR because nitinol alloy is highly ductile.
When the wire is heated above the TTR, the alloy will return to its
original shape.
In the austenite phase, the memory metal “remembers” the
shape it had before it was deformed

 Shape Memory is a Combination of
Thermoelasticity and Pseudoelasticity

Using these alloys, major medical advances have
been made in the People’s Republic of China for
the treatment of scoliosis. In this procedure a
patient has precooled shape memory rods
implanted in his or her back, following which the
body heat warms the rods over a period of a few
hours, and the spine is gradually straightened.

SUPERELASTICITY :-
A remarkable property of some alloys that exhibit a
reversible elastic deformation characterized by a distinct
non-linear relationship b/w load and deflection.
⇒ Seen as a characteristic plateau like appearance
of the stress/strain curve during loading and unloading.

SUPERELASTIC BEHAVIOUR – UNDERLYING
MECHASNISM :-
Martensitic crystal structure can be increasingly sheared with
only gradually increasing force to approx 10 times the strain of
normal alloys without exhibiting permanent deformation.
Twinning – Detwinning Mechanism :-
During detwinning - stress strain curve takes shape of plateau
because even a minor ↑ in stress produces as much as 8% of
deformation.
Within the plateau the rearrangement of atoms is reversible.
However if material is stressed beyond 2nd
yield point permanent
deformation results. In first stage Austenite deforms elastically
from 0-2%. Above this level Martensitic superelastic
transformation occurs that is completed at 8%-10% strain levels.
The structure becomes detwinned Martensite.

INFLUENCE OF TREATMENT :-
Memory effects lasts only as long as twinning -
detwinning phenomenon can take place.
When atoms slide against each other with a full
lattice unit – Irreversible transformation
(permanent set) takes place.
Consequently cold worked wires do not transform
b’ coz of their high elasticity

1. The martensitic-stabilized alloys do not possess
shape memory or super elasticity, because the
processing of the wire creates a stable martensitic
structure.
2. The martensitic-active alloys employ the
thermoelastic effect to achieve shape memory; the
oral environment raises the temperature of the
deformed arch wire with the martensitic structure
so that it transforms back to the austenitic structure
and returns to the starting shape. • Neo
Sentalloy and Copper Ni-Ti.

3. The austenitic-active alloys undergo a stress-induced
martensitic (SIM) transformation when activated.
These alloys display superelastic behavior (termed
pseudoelastic in the materials science literature),
which is the mechanical analogue of the
thermoelastic shape-memory effect (SME). An
austenitic-active alloy does not exhibit thermoelastic
behavior when a deformed wire segment is warmed
in the hands. These alloys are the superelastic wires
that do not possess thermoelastic shape memory at
the temperature of the oral environment, ⇒ Reversible
stress induced martensitc transformation is exhibited when Af
is
less than oral temperature.
nitinol SE.

Conventional Nitinol
. COMPOSITION:
Original alloy - 55% Nickel.
- 45% Titanium ratio of elements.
To modify mechanical properties and transition
temp. 1.6% Cobalt was added to it

CRYSTAL STRUCTURE:
- Stabilized Martensitic form.
- No application of phase transition effects.
The family of Stabilized Martensitic alloys now
commercially available are referred to as M –
NiTi.

PROPERTIES
1. Springback and Flexibility
Most advantageous properties of Nitinol are Good
Springback and Flexibility.
Low force per unit of deactivation – that is low stiffness.
Nitinol wires have greater springback and larger
recoverable energy than SS or β-Ti when activated to
same extent. High spring back is useful in
circumstances that require large deflections but low
forces.
Delivers 1/5th
– 1/6th
force per unit of deactivation

2. Spring Rate / Load Deflection Rate:
Load deflection rate of SS is twice that of Nitinol.
Clinically this means that for any given
malocclusion nitinol wire will produce a lower,
more constant and continuous force on teeth
than would a stainless steel wire of equivalent
size

3. Formability : Nitinol has poor formability.
Therefore best suited for preadjusted systems.
-      Fractures rapidly when bent over a sharp edge.
-      Bending also adversely effects springback property of this wire.
-      Bending of loops and stops in nitinol is not recommended.
-       Any 1st
, 2nd
and 3rd
order bends have to be overprescribed to
obtain desired permanent bend.
Cinch backs distal to molar tubes can be obtained by flame
annealing the end of wire. This makes the wire dead soft and it
can be bent into the preferred configuration. A dark blue color
indicates the desired annealing temperature. Care should be
taken not to overheat the wire because this makes it brittle.

When permanently deformed and activated in
opposite direction, nitinol actually undergoes
more permanent deformation than stainless
steel for activations less than 40 degrees
(Lopez, Goldberg and Burstone, 1979).
Clinically this means that nitinol should be
overbent and permanently deformed in direction
that the appliance will ultimately be activated.

4. Shape Memory:
Andreasen and Morrow described the “shape memory”
phenomenon as capability of wire to return to a
previously manufactured shape when it is heated
through TTR.
Ironically the first 50 : 50 composition of Ni and Ti was
shape memory alloy (SMA) in composition only.
Nitinol alloy is passive.
SME had been suppressed by cold working the wire
during drawing to more than 8 – 10%.

5. Biocompatibility and Environmental Stability:
Corrosion Resistance:
Corrosion resistance feature of NiTi alloys is due to
presence of large proportion of Titanium (48% - 54%).
Titanium alloys are covered with oxides (mainly TiO2
) –
forms a thin film that protects the metal in same way
that Cr2
O3
and Al2
O3
protect stainless steel and
aluminium respectively.
Findings on resistance to corrosion of Nitinol wires have
been inconsistent.

Schwaninger, Sarkar and Foster (1982):
Studied long term immersion corrosion on flexural
properties of Nitinol. Noted that
• Irregularities on surface of Nitinol wires – produced by
manufacturing process which predisposes wire to
corrosive attack in mouth and not due to corrosion of
alloy.
• Corrosion doesn’t affect flexural properties of Nitinol
wires.

6. Joinability:
Not joinable
Since hooks cannot be bent or attached to Nitinol, crimpable hooks and stops are
recommended for use.
7. Friction:
Garner, Allai and Moore (1986) and Kapila et al (1990):
Noted that bracket wire frictional forces with nitinol wires are higher than those
with SS wires and lower than those with β-Ti, in 0.018 slot.
In 0.022 slot – NiTi and β-Ti wires demonstrated similar levels of friction.
Although NiTi has greater surface roughness Beta –Ti has greater frictional
resistance

V. CLINICAL APPLICATIONS:
Levelling and Aligning:
Nitinol wire is much more difficult to deform during handling and
seating into bracket slots than SS arch wires.
-      Reduces loops formerly needed to level dentition.
-      Can be used for longer periods of time without changing.
-       Torque can be controlled early in treatment because
successive arch wires fit with precision and case.
-       Rectangular Nitinol inserted early in Rx
– accomplishes
simultaneous leveling, torquing and correction of rotations.
-      Bite opening using RCS. (Reverse Curve of Spee)

VI. ADVANTAGES:
-      Fewer arch wire changes.
-      Less chair side time.
-      Less patient discomfort.
Reduction in time to accomplish rotations.

LIMITATIONS:
-      Poor formability.
-      Poor joinability.
-       By its very nature nitinol is not a stiff wire which
means that it can easily be deflected. Low stiffness of
nitinol provides inadequate stability at completion of
treatment. Such stability is often best maintained by
using stiffer SS wires tailored to the desired finished
occlusion.
-      Tendency for dentoalveolar expansion.
-      Expensive.

. AVAILABILITY:
Conventional Nitinol is available as
- Nitinol classic - Unitek corporation.
- Titanal - Lancer pacific.
- Orthonol - Rocky mountain
orthodontics.

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