Alloys

Dr. Waseem Bahjat Mushtaha

Specialized in prosthodontics

Alloys

Pure metal are not suitable for dental application because they are too soft and ductile. The mechanical properties are improved by using mixtures of metals.

These are called alloys .

An alloys is a combination of two or more metals which are soluble (miscible) in the molten condition.

Classification of alloys:

1 )According to the number of alloying elements:

Binary alloys (2 constituents) .

Ternary alloys (3 constituents).

Quartinary alloys (4 constituents).

2 )According to the solubility (miscibility) of the atoms in the solid state:

When two molten metals are mixed, they are usually form a solution in the molten condition. A solution is defined as a perfectly homogenous mixture. On cooling such a solution, on of three things may happen: 1) solid solution alloy, 2) eutectic alloys. 3) intermetallic compound.

1 )A solid solution may be formed in which the atoms of the two metals are distributed randomly in a common space lattice. Because the structure is homogenous, under the microscope the grains of solid solution alloys resemble those of pure metals. The metals are said to be soluble in each other in the solid state, solid solution alloy.

2 )The two metals may be completely insoluble or partially soluble in the solid state, the solid alloys are heterogeneous containing two phases, Eutectic alloy.

3 )A new chemical compound can be formed in the solid state, intermetallic compound.

1 )Solid solution alloy:

1 )Definition: Metals which are completely soluble in each other in both liquid and solid states.

2 )Examples: Most dental restoration are solid solutions.

Gold and copper, silver and palladium, silver and gold, iron and carbon, cobalt and

chromium.…………,

3 )Types of solid solution:

Solid solutions may be of two types:

a) Substitutional solid solutions:

Where two different types of atoms occur in different positions in the same space lattice

(order, disorder)

b) Interstitial solid solutions:

Where very small atoms can be accommodated in the spaces between larger atoms e.g. carbon in iron.

4 )Conditions for substitutional solid solutions:

Conditions for substitutional solid solutions between two metals:

1 )They have the same type of space lattice.2 )Metals have the valence.

3 )Their atomic sizes differ by less than about 15%.

4 )They do not react to form intermeallic compound (no chemical reactions)

Solid solutions

1 )The greatest number of alloys which are useful as dental restorations are solid solutions.

2 )Melting point temperature of silver 960 C3 )Melting point temperature of palladium 1550 C

4 )Increase of percentage of palladium than silver is called palladium silver.

5 )When the change of composition, changing in melting range.6 )Metals and alloys at different percentage above liquidus line are

in a liquid state.7 )Metals and alloys at different percentage below solidus line are

in a solid state.8 )Alloys between liquidus and solidus are in a liquid+solid state.

Coring

From the silver- palladium phase it is evidence that the composition of the grain is not uniform. The first nucleus is rich in palladium, but as the temperature decreases, the palladium content decreases with an increase in the silver

content. It can recognized therefore that cored structure results with the core consisting of the higher melting alloy constituents and the matrix containing the lower melting components. This is called coring. A cord structure is undesirable particularly in relation to the corrosion resistance of the alloy. A cored structure is formed a) when the alloy is rapidly cooled after solidifying. b) when the rang between

the liquidus line and the solidus line is great .

Homogenization

It is the process used to eliminate coring through elimination of compositional differences. This is done by heating the cored structure below is melting temperature to allow atomic diffusion to take place.

Properties of solid solution alloys

1 )They have melting ranges.

2 )They are homogenous ( their solid is one phase) and so resist tarnish and corrosion.

3 )They are high ductility but lower strength and hardness than eutectic alloys.

Eutectic alloys

1 )Definition : metals which are completely soluble in the liquid state but either insoluble or partially soluble in the solid state.

Examples:

a) Lead and tin: used in soldering but not in dentistry.

b) Silver and copper: used in dental soldering and in amalgam which improve amalgam properties.

Composition

1 )Melting point of silver = 960 °C2 )Melting point of copper = 1083 °C

3 )Melting point of eutectic solution = 779 °C which lowered than the parent constituent.

Eutectic solution at composition of 72% silver and 28% copper.

α alloy is more resistance to tarnish and corrosion than β alloy. So we can use α alloy in dentistry.This alloy use in high copper amalgam and soldering

Properties of eutectic alloys

1 )They have a melting point.2 )They have poor tarnish and corrosion resistance

due to their heterogonous structure (two phases system)

3 )They are brittle because of the presence of insoluble phases (α and β) that inhibit dislocation movement.

4 (The strength and hardness are higher than those of constituent (parent) metals because of the composite cored nature of the alloy.

Solid state reactions

In certain solid solution system, the elements are completely soluble in the liquid state. At lower temperatures the attraction of solvent and solute atoms may be sufficient to convert the random solid solution into an ordered solid phase.

The gold copper system

The gold copper system

The gold copper alloy system exhibits this phenomenon at certain composition. From the diagram, it could be seen that, the melting range is very narrow for all compositions, and that the liquidus and

solidus actually touch at 80.1 percent gold .

Also, it could be seen in the figure that there are two areas below the solidus. These represent conditions of temperature (below 410 C) and composition (64-88% gold) where reactions can occur in the solid state, provided that the alloy is maintained at an elevated temperature for sufficient time to permit diffusion of atoms.

The reaction occurring in this system is called ordering. If an alloy of 50% copper is cooled rapidly from 450 C, the lattice structure is random or disordered. But slow cooling permits the formation of an order substitutional solid solution. In the first area, the ordered lattice has three copper atoms (on the surfaces of the cubic lattice Au Cu3) and one gold atom on the eight corners.

In the second area there are equal numbers of atoms of the two metals in the lattice (AuCu) gold atoms are present on four faces of the cubic lattice and the copper atoms on the remaining two faces and the eight corners.

Such an ordered structure is termed a super lattice. The unit cell of the AuCu super lattice is face-centered cubic structure in that only two of the three axes of the unit cell are equal in the length.

The formation of a tetragonal lattice within a cubic structure involves contraction on one of the crystal axes, this sets up strain which interferes with the movement of dislocations. Hence the yield stress, ultimate strength and hardness are raised, and this is termed order hardening.

The larger amount of copper in the AuCu3 phase is not compatible with dental applications. Also, this superlattice AuCu3 is of little aid in changing the mechanical properties of the alloy because its lattice has essentially the same dimensions as those of the disordered lattice. The composition of most of the casting gold alloys is within the compositional limits (64-88% gold) for the AuCu phase.

If a gold-copper alloy is cooled rapidly from the solidus temperature, the disordered solid solution is retained at room temperature, and the alloy is relatively soft. Slow cooling allows diffusion to occur and the alloy would partially transform to the ordered phase and would be harder than the quenched (cooled rapidly) alloy.

Comparing between AuCu3 & AuCu

AuCu3 :1 )Contain more copper and less gold.

2 )Will formed when heat on temperature not exceed 360 °C.

3 )Face centered cubic “ gold in corners and copper in faces.

4 )It is weak. 5 (Less resistance to tarnish and corrosion when

compared to AuCu.

AuCu :1 )Contain more gold and less copper.

2 )Will formed when heat on temperature not exceed 424 ºC.3 )Face centered tetragonal . In case of copper 1 atom of

corner and 1 atom in two face.In case of gold, two atoms of gold in other 4 faces.

4 )It is stronger compared to AuCu3. 5 )More resistance to tarnish and corrosion when

compared to AuCu3.

Heat treatmentSoftening heat treat

“ Annealing”Red tem

rapid cooling“ quenching”

Disorder solid solutionFine grain.

strength, hardness and PL Ductility.

Hardening heat treatment1 )Slow cooling 2) Rapid cooling 3) softening

heat treatFrom red temp softening heat treat then heat the alloy to 450ºCNot suitable in then heating alloy for 15 minutes for Dentistry. to 450ºC rearrangement of structure.Will strength then slow cooling Then make rapid cooling.

But more in to 350ºC to allow this technique is more suitable to dentistry .

Ductility rearrangement of order structure .

then make rapid cooling

N.B. during heat treatment PL. Y.S. hardness and ductility are affected but modulus of elasticity is not affected because it depends on the chemical composition. But not on the microstructure

Method of altering the mechanical properties of alloys

1 )Work hardening:

The properties of an alloy can be altered by cold working where there are

a) Increase in hardness.

b) Greater yield stress and ultimate strength.

c) Less ductility.

2 )Solution hardening:In a substitutional solid solution, the atoms of the two metals are of different size, through the size difference is usually less than 15%. Consequently the crystal lattice of such an alloy is distorted by the presence of either smaller or larger atoms. Theses distortions stop the movement of dislocations and hence raising the strength. Thus, gold alloys containing copper or silver are harder,

stronger and less ductile than pure gold .

3 )Order hardening:

As in the gold-copper system where the ordered super lattice AuCu is formed. The formation of a tetragonal lattice within a cubic structure involves contraction of one of the crystal axes. This sets up strains which interfere with the movement of dislocation. Hence, the yield stress, ultimate strength and hardness are raised.

4 )Precipitation hardening:

As in the silver-copper system, the precipitated phase (α or β) reduce the mobility of dislocations, thus increasing strength and hardness, and reducing the ductility.

Alloys in dentistry

1 )Gold alloys (solid solution) are used for inlays, crowns and bridges. Alternatives to gold alloys include silver-palladium, and nickel-chromium alloys.

2 )Gold alloys, and cobalt-chromium alloys are used for partial denture bases.

3 )Silver-copper alloy (eutectic alloy) is used in dental soldering 4) The silver-tin alloy (intermetallic compound) is mixed with mercury to form the dental

amalgam .