Metal alloys for fixed appliances

What are dislocations?

• Imperfections within metals, all metals will have these imperfections

• e.g a missing plane of atoms - line defect

• applying a shear stress above the yield stress - permanently deforming the metal through movement of a dislocation from one plane to another within the crystal until it reaches a grain boundary where it cannot move anymore - so dislocations stack up at the grain boundary -

• while deforming a metal you are increasing the density of dislocations - which makes it harder to deform the metal as you have to move extra deformations

Give examples of strengthening mechanisms? (7)

Baseline: Increasing the dislocations within the metal, making it harder as well as decreasing the ductility

• Work (strain) hardening (Cold working): application of shear stresses produces more dislocations, hinders movement, more difficult to deform

• Forging: Drawing wires/rolling sheets

• Bending: Denture clasps/orthodontic appliances

• Burnishing: Amalgam/Gold inlays

• Heating treatments:

• Solution hardening - Different atomic radii of atoms (e.g Au and Cu) within the same crystal lattice limit the movement of dislocations (dislocations cannot move as you are creating obstacles)

• Order hardening: Super lattice - heating gives energy and time for atoms to rearrange each other to give an ordered structure formation (only one phase)

• Precipitation hardening - partial solubility varying with temperature, as temp goes down another phase will precipitate so dislocation will not move beyond this (2 phases)

Requirements of metals and alloys? (6)

• Biocompatible - lab tech, dentist and pt

• Corrosion - degradation and ion release

• Tarnish resistant - change in colour

• Mechanical properties - high modulus and yield stress, not brittle

• East of casting - low Mp and high density (easier to flow and remove any air)

• Low cost

modulus means

Classification of metals used?

High noble: more than or equal to 60% noble metal, of which 40 or more needs to be gold

[Au, Pt, Pd, Rh, Ru, Os, Ir] (silver is not considered a metal alloy because it corrodes and tarnishes in the oral environment)

Noble - must contain at least 25% noble metal

Predominantly base metal - contain < 25% noble metal

What are alloys used in Fixed pros?

• Gold alloys:

• - High gold - type 1,2,3 and 4 (discussed in removable pros)

• - Medium and low gold

• Palladium alloys:

• - Silver-palladium

• - High palladium (Pd-Cu)

• Nickel (cobalt) - chrome alloys

• Titanium and its alloys

High gold alloys

what are the main (3) and minor (7) constituents?

• Main: Au, Ag, Cu

• Minor: Pt, Pd, Ir, Zn, In, Ga, Re

Why is Ag added, what can it do?(4)?

• Solution hardening

• Precipitation hardening

• Whitens

• Reduces tarnish resistance

What does Cu add? (3)

• Lowers Mp

• Solution hardening

• Order hardening (providing more than 11% Cu)

Pt/Pd functions? (4)

which is cheaper?

• Solution hardening

• Solution precipitation hardening

• Increases Mp

• Improve corrosion resistance

• Pd is considerably less expensive

Function of Zn?

• Acts as a scavenger during casting

In, Ir, Re, Ga Ru? function? (1)

Help produce fine grain size

High gold types composition?

All the additions of metal form what with Au? (gold)

What does Pt and Pd addition cause in the graph which requires what?

• All additions form solid solutions with Au

• Pt and Pd increase the solidus/liquidus gap so type 3 and 4 need homogenisation (the metal is more likely to be a mix of solid/liquid as a result)

Overall, are gold alloys easy to cast?

and why? (3)

• Easy to cast

• Low casting temperature

• Low shrinkage (1-4%)

• High density

What is the trend in properties as you increase in type

As you increase in type, higher properties

Only type 3 and 4 have Cu > 11% so order hardening is possible hence why type 1 and 2 don’t have order hardening

hard stronger than cast

Type 1 alloys:

• Yield stress?

• what strengthening mechanisms can oyu do to this?

• uses?

• Low (this means it can be deformed more easily)

• Burnished - improve marginal fit and increases hardness (cold working)

• small, well supported inlays where forces are low

Type 2 alloys:

Yield stress?

Strengthening mechanism?

Uses?

• Improved properties over type 1, higher yield stress

• can still be burnished

• used for larger inlays, not in thin sections

Type 3 alloys?

Can they be hardened?

Bonding to adhesive resins?

Can they be burnished?

Yields stress?

• Can be hardened (because of the Cu >11%)

• Difficult to burnish

• High yield stress

• Heat to 44 degrees for 10 minutes produces CuO layer which bonds to adhesive resin luting agents

Uses of type 3 alloys?

• Inlays/onlays

• Full crown and short span bridges

• Cast posts and cores

Type 4 alloys

Yield stress?

uses?

can be it used for inlays?

• Very high yield stress

• High stress situations, partial dentures and clasps, long span bridges

• Cannot be burnished so not suitable for inlays

Medium golds:

Composition: (4)

phases?

• Au, Pd, Ag, Cu

• The added metals all form solid solutions with Au

• Single phase structure

Low gold:

Composition: (5)

What colour can they be depending on metal composition?

Phases?

• Au, Pd, Ag, Cu, In

• Low gold are white

• Cu-free low golds containing In are yellow

• 2 Phases

• Ag-Au and Pd-In

Medium/Low golds alloys

price and properties compared to High gold alloys?

• Cheaper

• Enough Cu to heat harden

• Similar properties to type 3 gold

Silver-palladium alloys can be known as?

Composition (5)

Properties?

• White gold

• Pd, Ag, some Cu, Zn, In

• difficult to cast, low density and high Mpt

• Tarnish (silver, Pd increases corrosion resistance?)

• Cheaper than gold alloys

Nickel chrome alloys

Composition: (9)

• Ni, Cr, Mo, Be

• Small amounts of: Al, C, Mn, Cu, Co

Ni adds what?

Cr adds what?

C?

• Ni - strength and hardness

• Cr - hardens by solution hardening, corrosion resistance through a passive oxide layer

• Must limit C/ carbide precipitation for strength

Downsides of Ni and Be?

• Ni - an allergen so could use Co

• Be - toxic

Properties of Ni-Cr alloys are comparable to what type of high gold alloy?

• Type 3 gold

Disadvantage of Ni-Cr alloys?

• High casting shrinkage

Titanium:

2 types?

Commercially pure and alloy TiAl4V

Titanium and alloys properties? (7)

• Biocompatible, can be made using CAD/CAM techniques

• Low density, high Mpt, difficult to caste, high casting shrinkage, require special casting equipment

Other elements found in commercially pure Titanium and grades

Commercially pure Titanium exists in what 2 forms

Alpha - low temp

Beta - high temp

In Titanium alloy, what stabilises either phase?

• Titanium alloys usually have a mix of alpha and beta

• V is toxic in elemental forms and can stabilise

• Or V can be replaced by Nb which can also stabilise

Hardness of cpTi and Alloys comapred

PFM?

Porcelain fused metal alloys

Additional requirements of metals in PFM?

• Good bond to porcelain

• Must not act adversely with the porcelain

• Melting range > firing temperature of porcelain

• Coefficient of thermal expansion slightly greater than of the porcelain

• Low creep or sag (high temp and weight - the metal will permanently deform)

Explain the important of thermal coefficient?

• Metal will contract a little more than ceramic, so all forces inwards so ceramic is under slight residual compression (one of the bonding mechanisms of p to m)

• Ceramic is under tensile forces, forces are outwards, ceramics are more sensitive to applied tensile forces

High golds metals used in PFM

What do Pt/Pd DO?

In and Sn?

Cu?

bond to porcelain?

Melting range still causes what?

minimum thickness of coping?

• Pt/Pd increase Mpt

• Form oxide layer to bond to porcelain

• Cu is not include as it reduces Mpt and can cause greening of the porcelain

• Strong bond to porcelain

• Melting range is still low enough to cause sag

• Modulus is low to a minimum coping thickness of 0.5mm

What is a metal coping?

Low gold alloys (Gold, palladium silver) alloys:

What 2 elements allow for bonding to porcelain?

corrosion resistance, similar to?

cost?

Mpt and what element?

• In and Sn to bond to porcelain

• High corrosion resistance similar to high gold alloy

• Cheaper

• Melting point is higher because of increased Pd content

Palladium silver alloys

Cost compared to high gold alloys?

castability?

High silver content can cause what?

• Offer an alt to high gold alloys at lower cost

• Difficult to cast - low density

• High Ag content can discolour the porcelain

High palladium (Pu-Cu)

is what free?

Cu in this acts differently to when it is in gold how?

creep?

• Silver free

• Unlike in gold, the Cu does not discolour the porcelain

• have poor sag resistance due to creep

Ni-Cr

• Has the highest what compared to other PFM alloys?

• coping thickness

• melting point

• sag

• castability?

• bonding to porcelain

• problem?

• alt?

• Highest modulus of all PFM alloys

• 0.3 mm

• Highest

• No sag

• High casting shrinkage, poor

• Poor bonding to porcelain - adhesive failure

• Ni allergy

• Co-Cr alloys can also be used for PFM - stronger and harder than Ni-Cr but still have similar problems with casting

Tilite alloys from Talladium Inc?

manufacturer claims?

• Intermetallic compound renders the Ni and Be inert so no allergy or toxicity

• Good biocompatibility

• good bonding to porcelain

• cheaper to gold alloys

Titanium alloys - cpTi and Ti6Al4V?

Mpt?

sag?

bonding to porcelain?

how can cores be made?

• Highest Mpt so no sag

• Passive oxide layer for bonding to porcelain

• cores can be made by CAD/CAM