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
