Metals and Alloys Flashcards
What is an alloy?
Combination of metal atoms in a crystalline structure.
- 2 or more types of metal atoms.
What is ductility?
The amount a material will deform to stretch before it fractures.
- amount of plastic deformation prior to fracture.
What does it mean if a material has a high YM?
The material is rigid
- it will undergo less strain when a tensile force is applied.
In terms of a metal- what factors affect it’s mechanical properties?
Crystalline structure
Grain size
Grain imperfections
What stages occur in crystal growth within a metal?
Atoms within a metal act as nuclei of crystallisation.
Crystals grow to form dendrites.
They grow until they impinge on other crystals
- grain boundary.
What is the difference between fast cooling (quenching) and slow cooling?
Fast cooling- smaller grains, more nuclei
Slow cooling- large grains, few nuclei.
What is a grain?
Single crystal lattice with atoms orientated in given directions.
What is a grain boundary?
Change in orientation of the crystal planes- when grains collide.
Why are small grains advantageous?
High elastic limit
Increased fracture stress and UTS, hardness
What is a dislocation?
Imperfection or defect in the crystal structure.
When a force is applied, the defect moves along the lattice until it reaches a grain boundary.
What is Slip?
Propagation of dislocations and involves rupture of only a few bonds at a time.
What factors impede movement of dislocations?
Grain boundaries
Alloys
Cold working.
What is cold working?
Work done on a metal or alloy at a low temperature.
Causes lip, dislocations collect at grain boundaries.
End up with a stronger, harder material.
What is an advantage of cold working?
Causes the metal to be harder, increased fracture stress and elastic limit.
What is a disadvantage of cold working?
low ductility.
low Impact strength
Low corrosion resistance.
residual stress- cause instability within the attic and distortion over time.
How do we reduce the risk of residual stress?
Stress relief annealing.
- heating the metal so that greater thermal vibrations allows migration of atoms.
What is the definition of hardness?
Resistance of surface to indentation of abrasion.
What is the purpose of a metal in a MCC?
Helps eliminate defect/cracks on the porcelain surface.
- limits the strain that porcelain can undergo because it is more ductile and less rigid.
Why do you need a supporting metal element in an MCC?
Porcelain has low tensile strength, forms surface defects which leads to fracture under low stress.
What metals can be bonded to porcelain for a MCC?
High or low gold alloy
Silver Palladium
Nickel chromium
Cobalt chromium
What properties are required of a metal alloy to bond to porcelain?
Similar thermal expansion co-efficients
Porcelain forms bonds with metallic oxides on surface.
Avoid discolouration of the porcelain
High elastic modulus
Hardness
Recrystallisation temperature of alloy higeher than the fusion temperature of porcelain- creep can occur if not.
What is creep?
Gradual increase in permanent strain experience under prolonged application of stress.
What is the bonding mechanism found in porcelain-metal?
Mechanical- interlock through surface irregularities
Stressed skin effect- slight differences in thermal contraction co-efficients lead to compressive forces.
Chemical- electron sharing in oxides.
What is stainless steel used for?
Orthodontic wires
Denture base
What is the composition of stainless steel?
72% iron
18% chromium
8% nickel
1.7% titanium
0.3% carbon
What is the purpose of Chromium in stainless steel?
Lowers austenite to martensite temperatures and rate.
decreases % carbon at which eutectoid formed.
Also corrosion resistant.
What is the purpose of nickel in stainless steel?
Helps achieve the austenite structure at room temperature by lowering the critical temperatures at which the austenite structure breaks Down on cooling.
What type of stainless steel is used in orthodontic wires and partial dentures?
18-8 stainless steel.
Austenite stainless steel.
When Austenite is quenched, what is formed?
Martensite.
What components of stainless steel suppress the transition of austenite to martensite?
Chromium and Nickel.
Why is martensite not desirable to use in dentistry?
Very hard and brittle.
Distorted lattice.
What is a wrought alloy?
Manipulated/shaped by cold working.
What is springiness?
EL/YM
Ability of a material to undergo large deflections without permanent deformation.
What are the requirements of an orthodontic wire?
High springiness
High ductility
Easily joined
corrosion resistant.
What properties are desirable for a denture base?
High YM and high EL.
What is weld decay?
Occurs between 500-900 degrees.
Chromium carbide’s precipitate at grain boundaries.
This makes the metal very brittle.
What properties are desirable for a denture clasp?
Lower YM- to allow it to flex over a tooth.
High EL.
What is the composition of cobalt chromium (in a denture)?
54% cobalt
25% chromium
15% nickel
5% molybdenum
0.4% carbon
