13. Metals and Alloys 1 Flashcards
Features of metals (2)
Superior mechanical properties (strength, rigidity, elastic limit, ductility, hardness)
Poor aesthetics
Definition of metal
An aggregate of atoms in a crystalline structure
Metals are the building blocks of alloys
Definition of alloy
A combination of metal atoms in a crystalline structure
Definition of elastic limit
Maximum stress a material can withhold without plastic deformation
Definition of ductility
Amount of plastic deformation prior to fracture (measure of the extent a material can be shaped/manipulated)
Mechanical properties are based on (2)
Choice of material (each has its own inherent characteristics)
Crystalline structure depends on history (method of production) and shaping (crucial for dental applications - cold working, swaging)
Factors affecting mechanical properties (3)
Crystalline structure
Grain size
Grain imperfections
Features of crystal growth (3)
Atoms at these sites act as nuclei of crystallisation
Crystals grow to form dendrites (3D branched lattice network(
Crystals (or grains) grown until they impinge on other crystals
Definition of grain boundary
Region where grains make contact
Change in orientation of the crystal planes (impurities concentrate here)
Types of grain structure (3)
Equi-axed
Radial
Fibrous
Definition of equi-axed grains
If crystal growth was of equal dimension in each direction
Features of radial grain structure
Molten metal cooled quickly in cylindrical mould
Features of fibrous grain structure
Wire pulled through die (cold worked metal/alloy)
Features of grains (2)
Varying sizes and shapes
Contain atoms arranged in a lattice-like structure
Types of crystal growth (2)
Fast cooling (quenching) Slow cooling
Best type of crystal growth
Fast cooling (quenching)
Features of fast cooling, quenching (2)
More nuclei (more grains) Small fine grains
Features of slow cooling (2)
Few nuclei
Large coarse grains
How is crystal growth enhanced
Addition of nucleating agents
How do nucleating agents enhance crystal growth
Impurities or additives act as foci for enhanced crystal growth
Definition of grain
A single crystal (lattice) with atoms orientated in given directions (dendrites)
What type of grains are advantageous
Small grains
Why are small grains advantageous (3)
High elastic limit
Increased ultimate tensile strength
Increased hardness
Why are small grains disadvantageous
Decreased ductility
Factors for rapid cooling, quenching (4)
Small bulk
Heat metal/alloy just above melting temperature
Mould – high thermal conduction
Quench
Definition of dislocations
Imperfections/defects in the crystal lattice
Dislocations lead to an alteration of lattice structure and shape and are resistible
SLIP is due to
Propagation of dislocation
Involves rupture of only a few bonds at a time
Features of dislocations
Weak points Defect moves along the plane by SLIP Defect will move upon force Leads to a change in material shape Dislocations cannot move from one grain to another and therefore accumulate at grain boundaries
Impeded movement of dislocation increases (3)
Elastic limit
Ultimate tensile strength
Hardness
Impeded movement of dislocation decreases (2)
Ductility
Impact resistance
Factors impeding dislocation movement (3)
Grain boundaries (hence fine grains) Alloys (different atom sizes) Cold working (dislocations build-up at grain boundaries)
Features of cold working (4)
Work hardening/strain hardening
Involves work being done on the metal/alloy (bending, rolling, swaging) at low temp. (below recrystallisation temp.)
Cold working causes SLIP (dislocations collect at grain boundaries) and a stronger, harder material
Cold work results in internal stresses - may lead to appliance distortion over time
Cold work modifies grain structure increasing (3)
Elastic limit
Ultimate tensile strength
Hardness
Cold work modifies grain structure decreasing (3)
Ductility
Impact strength
Corrosion resistance
Effects of residual stress (2)
Instability in the lattice and results in distortion over time
This is undesirable and is relieved through annealing
Features of annealing
Involves heating a metal or alloy so that greater thermal vibrations allow migration of atoms (re-arrangement of atoms)
Features of stress relief annealing (2)
Eliminates stresses by allowing atoms to re-arrange within grains
Grain structure and mechanical properties are unaltered, and some further cold work is possible (final shaping)
Recrystallisation occurs when the metal/alloy is heated causing (3)
New smaller, equi-axed grains
Lower elastic limit, ultimate tensile strength and hardness
Increased ductility
Recrystallisation spoils the benefits of
Cold work and allows further cold work
Repeating cold work and recrystallisation
Cold work/recrystallisation can be repeated until the correct shape is obtained
Relationship between recrystallisation temperature and cold work (2)
The recrystallisation temperature depends on the amount of cold work
The greater the amount of cold work, the lower the recrystallisation temperature
Features of excessive temperature rises
Causes large grains to replace smaller coarse grains, yielding poorer mechanical properties
Features of grain structure (2)
Influences mechanical properties
Influenced by dislocations (SLIP)
Types of dental appliance manipulation (3)
Cold working
Stress relief annealing
Recrystallisation
