METALS & ALLOYS

give examples of metals and alloys in dentistry

• amalgam

• cobalt chromium

• stainless steel

• gold

• titanium 

dental applications of metals and alloys

• amalgams incl. Hg, Au alloys

• inlays/ onlays: Pd alloys

what is a metal

metal: a crystalline material held together by metallic bonds

what are examples of crystal structure

which crystal structure has a larger packing factor 

what does a bigger packing factor mean

bigger packing factor = denser metal

what is the correlation between grain size and material strength

smaller grains = stronger material

diagram showing metal microstructure

metals: usually polycrystalline structure consisting of grains and grain boundaries

how does metal need to be prepared to view the microstructure under the microscope 

• metal must be etched with acid 

• etching creates grain contrast depending on the crystallographic orientation 

what pure metals are used in dentistry

• gold

• platinum

• titanium

state the types of alloy structures

when metals are mixed they can form multiple different mixed structures:

• substitutional solid solutions

• interstitial solid solutions

• intermetallic compounds

outline substitutional solid solution 

• the different atoms have the same valency

• with the same crystal structure i.e. body centred plus body centred and face centred plus face centred

• their atomic size is within 15%

outline interstitial solid solution 

• solute atom ≤ solvent atom

• the solute (smaller) atoms occupy the space between the solvent (larger) atoms

• distortion of the lattice occurs to accommodate the extra atoms

outline intermetallic compound

• formed when two or more metals combine with a discrete composition or stoichiometric ratio

what is an example of an intermetallic compound

• dental amalgam

  • alloy contains a silver/ tin compound: Ag₃Sn

  • also contains a copper/ tin compound: Cu₆Sn₅

what does this image show

microscopic view of amalgam

define the term ‘phase’

phases: a homogenous physically distinct part of a system that is separated from other parts by a definite physical boundary

• the different crystal structures are called phases

images of different alloys

how many phases does an alloy consist of 

• an alloy can consist of single or multiple phases 

  • phases can have distinctly different compositions

  • phases can have different crystalline structures

metals summary

• metals are held together by metallic bonds - these can be weak or strong

• metals are polycrystalline and can take up many crystal forms

what do the phases in an alloy depend on

the phases in an alloy depend on composition and temperature

what is a phase diagram

a phase diagram is a graphical representation of the phases in an alloy with different compositions and temperatures/ a means of representing the structure of alloys as a function of composition

phase diagram for Cu and Ni 

• copper and nickel are totally soluble in each other

• either vertical line is pure copper/ nickel

• this is a binary phase diagram

• alloys can be solid and liquid over a range of temperatures

graph showing transitions in pure metals

cooling curve for a pure metal

only achieve solid + liquid at one precise temperature in a pure metal (at T_m) - unlike an alloy

cooling curve for an alloy

achieve solid + liquid over a range of temperatures - unlike a pure metal

pure metal VS alloy cooling curve

eutectic phase diagram for Ag and Cu

• in alpha and beta area, Ag and Cu are fully soluble in each other

what is the eutectic point

eutectic point: alloy acts like a pure metal

• this will only be at one specific composition

• there is no solid-liquid region, straight from liquid » solid