Wrought AlloysWrought Alloys

By By Dr.Reham Mohammed Abdallah

Production & Uses of wrought alloys Requirements of wrought wiresTypes of wrought alloys

1. Stainless steel alloyكله

2. Wrought cobalt-chromium-nickel alloy بس كمقارنة

3. Wrought Nickel- titanium alloyكله

4. Beta-Titanium alloy بس كمقارنة

Production of wrought alloys

• Wrought alloys are prepared by altering the casting alloy mechanically into a prefabricated shape.

• Wrought alloys have a fibrous grain structure compared with crystalline structure of casting alloys.

Uses of wrought alloys

1. Orthodontic wires and brackets

Requirements of wrought wires Requirements of wrought wires used in dentistryused in dentistry

1. Springiness (flexibility); It is the ability of the wire to undergo large deflection without permanent deformation.

• Spring back potential = yield strength/Elastic modulus• Spring back should be high which results

in an increase in its range of action. • Spring back is the measure of how far a

wire can be deflected without causing permanent deformation.

Requirements of wrought wires Requirements of wrought wires used in dentistryused in dentistry

2. Stiffness; certain stiffness is required to apply a

suitable force for tooth movement during orthodontic treatment.

• Clinically, it represents the magnitude of the force required to deflect or bend a wire.

3. Least friction at bracket – wire interface. Otherwise it limits the tooth movement

Requirements of wrought wires Requirements of wrought wires used in dentistryused in dentistry

4. Formability (ductility);It is the ability to be bent without fracture.

5. Corrosion resistance.6. Ability to be joined by soldering or

welding.7. Resilience is a property of arch wires

whereby they store energy when deformed elastically, and release energy when unloaded.

It represents the stored energy available in the wire to move teeth during deactivation.

Types of wrought alloys used in Types of wrought alloys used in dentistrydentistry

1.1. Stainless steel alloyStainless steel alloy

• St.st alloy are iron- carbon- chromium alloy.

• Chromium ranges from 13-28 %

• Chromium improve corrosion resistance

Mechanism of passivationMechanism of passivation

• The chromium exposed at the surface of the alloy is readily oxidized to form a tenacious layer of chromium oxide,

• Resists further attack from aqueous media

• Prevents corrosion

Properties of St-St wiresProperties of St-St wires

• 1. Lower spring-back action and also store less energy compared to those of beta-titanium or nickel titanium.

• 2. High stiffness ( modulus of elasticity 180 Mpa). So, they produce high force applied on tooth during shorter time periods.

3. High ductility.4. Good corrosion resistance.

Properties of St-St wiresProperties of St-St wires

5. Greater ease of welding and soldering

6. Low resilience

7. Lowest cost

8. Low friction(advantage) with brackets.

Types of St-St alloysTypes of St-St alloys

Sensitization of 18-8 st.stSensitization of 18-8 st.st

Definition•Loss of corrosion resistance if st.st is heated between 400-900oC

Mechanism and effectMechanism and effect• Carbon reacts with Cr which

precipitates chromium carbide at the grain boundaries this occurs very rapidly at 650 oC and leads to: a. Brittleness of the alloy b. Reduction of corrosion

resistance

Methods of preventing Methods of preventing sensitizationsensitization

a. Avoid heating too highb. Carbon content in the alloyc. Severe cold working of the alloy distribute the carbides evenly decrease the areas of chromium

deficiency

d.d. Stabilization of stainless steel;Stabilization of stainless steel;

• Addition of a stabilizing element as titanium, niobium or tantalum• This forms titanium carbide instead of chromium carbide such that chromium remains in its place

“ “stabilized austenitic stainless stabilized austenitic stainless steel”steel”

3. Wrought Nickel- titanium alloyWrought Nickel- titanium alloy

The most commonly used one is; nitinolnitinol

Ni ti n o lNi ti n o l

55% Ni 45% Ti

Naval Ordnance Laboratory

Characters of NiTi wiresCharacters of NiTi wires

1. Good spring back action and flexibility2. Low stiffness low

force3. Excellent corrosion resistance4. Larger stored energy than st-st, so

fewer activation or changes are required

5. Nitinol is characterized by phase phase transformation transformation by changing the

temperature temperature

or the stresses;or the stresses;

• At high temperature and low stress austenitic (BCC, ordered)

• At low temperature and high stress martensitic ( HCP, disordered)

This transformation is associated This transformation is associated with volumetric changes.with volumetric changes.

Schematic presentation of lattice structure changes caused by outer stress in stainless steel or superelastic NiTi alloy

According to transformation, Ni-Ti wire has According to transformation, Ni-Ti wire has 2 unique properties2 unique properties

1. Shape memory Definition: It describes the effect of restoring

the original shape of a plastically deformed sample by heating it.

• This phenomenon results from a crystalline phase change known as "thermoelastic martensitic transformation".

• At temperatures below the transformation temperature, shape memory alloys are martensitic.

• In this condition. their microstructure is characterized by "self-accommodating twins“.

• The manensite is soft and can be deformed quite easily by de-twinning.

• Heating above the transformation temperature recovers the original shape and converts the material to its high strength austenitic condition.

• Shape memory is achieved by first establishing a shape at temperatures near 482° C .

• If the appliance wire is then cooled and formed into a second shape and heated through a lower transition temperature range (TTR), the wire will return to its original shape.

• The cobalt content is used to control the transition temperature range, which can be near mouth temperature.

The temperature range over which the phase transformation occurs is adjusted to correspond to the oral environment.

Within the upper super elastic plateau region, the markedly increased strain for a small increase in bending stress facilitates adaptation of the instrument along a sharply curving root canal, thereby minimizing the risk for root perforation.

Clinical significance In orthodontic treatment, the wire is shaped at high temperature into a ready made arch shape. After adaptation by clinician into brackets bonded to the mal-posed teeth, the wire returns to its original form which promotes tooth movement.

Clinical significance In orthodontic treatment, it is desirable clinically because very low and nearly constant forces or tooth movement are provided by wire during deactivation. In endodontic treatment, perforation of curved canals is avoided due to the increased stress decrease stiffness.

2. Superelasticity (pseudoelastic nitinol)• It is a stress induced crystalgraphic

transformation from austinitic to martensitic.

• Within the upper super elastic plateau region, the markedly increased strain for a small increase in bending stress facilitates adaptation of the instrument along a sharply curved root canal, thereby minimizing the risk for root perforations.

Super elasticity of nitinolPseudo-elasticity

The temperature range over which the phase transformation occurs is adjusted to correspond to the oral environment.

Within the upper super elastic plateau region, the markedly increased strain for a small increase in bending stress facilitates adaptation of the instrument along a sharply curving root canal, thereby minimizing the risk for root perforation.

Clinical significance In orthodontic treatment, the wire is shaped at high temperature into a ready made arch shape. After adaptation by clinician into brackets bonded to the mal-posed teeth, the wire returns to its original form which promotes tooth movement.

Clinical significance In orthodontic treatment, it is desirable clinically because very low and nearly constant forces or tooth movement are provided by wire during deactivation. In endodontic treatment, perforation of curved canals is avoided due to the increased stress decrease stiffness.

Uses of NitinolUses of Nitinol

1. Good for orthodontic wires when low force and large working range are

needed.

2. Good for endodontic files in curved root canalto avoid perforation.

Disadvantages of NitinolDisadvantages of Nitinol

1. Limited formability2. Higher friction than st-st and lower than ß Titanium3. Difficulty in soldering4. Expensive5. Ni has got some hazardous and

allergic effect

No Nickel