Metals and Alloys Flashcards
What are the features of dislocations in metals?
- Dislocations produce field strains which create areas of compression and tension, and these forces decrease the further away you move from the dislocation.
- The strain field from one dislocation will interact with that of another as they move close together, two like dislocations repel each other and two dislike dislocations will attract and annihilate each other.
- The strength of a metal is increased by reducing dislocation motion.
What are the different strengthening mechanisms?
Plastic deformation is caused by the motion of large numbers of dislocations, and so the ability of a material to plastically deform depends on the ability of dislocations to move.
Strengthening techniques involve restricting the dislocation motion in order to reduce plastic deformation.
- Grain size reduction
- Solid-solution strengthening
- Precipitation strengthening
- Cold work strengthening
What is grain size reduction?
Grain boundaries prevent dislocation motion, so by decreasing grain size you increase the grain boundary area, therefore increasing resistance to dislocations.
Finely grained materials and therefore stronger and harder but less ductile.
Grain size can be decreased by:
- Increasing the rate of cooling
- Cold work and heat treatments
- Rolling - you turn an isotropic metal with large grains into and anisotropic material with smaller grains of a different shape.
What is the Hall-Petch equation?
σyield = σo + ky d-1/2
σyield = yield strength (N/mm2)
σo = starting stress for dislocation motion (constant)
ky = strengthening coefficient
d = average grain diameter
What is solid solution strengthening?
- Alloys containing substitutional/interstitial impurity atoms are deliberately produced, to increase lattice strains which interact with dislocations, restricting their movement by decreasing overall strain energy.
- If a dislocation wants to move it has to tear itself from the impurity atoms which costs energy.
- Increases tensile and yield strength, with the degree of strengthening dependent on relative atomic sizes.
What is precipitation strengthening?
- Precipitates require large shear stresses to move dislocations towards them and shear them.
- Therefore, as dislocations advance the precipitates act as pinning sites, preventing it from moving the whole way through a material.
- The yield strength is inversely proportional to the spacing of the pinning sites: σy = 1 / S
What is cold work strengthening?
Cold work causes the metal to plastically deform, causing the slip planes and dislocations in interact and dislocations to entangle with each other. This impedes further dislocation motion.
This is achieved through:
- Forging - a mould squeezes a metal into shape.
- Rolling
- Drawing - metal is pulled between two die.
- Extrusion - metal is forced between two die.
This causes:
- Yield strength to increase
- Tensile strength to increase
- Ductility to decrease
How does annealing affect strengthening?
- Heating (annealing) reduces strengthening, as diffusion allows dislocations to rearrange and annihilate.
- It also reduces dislocation density and increases grain size.
- As tensile strength increases, ductility decreases.
- The material changes greatest during recrystallisation.
What is an alloy?
An alloy is a metallic substance made up of more than one element and are produced because they are:
- Stronger
- Easier to cast
- Have good electrical/magnetic properties
How can you control the properties of an alloy?
The properties can be controlled by:
- The type of bonding
- It’s crystal structure
- Presence of defects
What are components?
The elements or compounds which make up an alloy.
What are phases?
The resulting physically and chemically distinct material regions.
What is the solubility limit?
- The maximum concentration for which an allow is only a solution.
- As the temperature increases, the solubility limit also increases.
What are ferrous alloys?
Ferrous alloys contain iron which is used because it is:
- Abundant
- Economical
- Versatile
- Stiff, strong and ductile
What are steels?
Steels are iron / carbon alloys.
How is cast iron produced?
- Cast iron is produced by re-melting iron without refinement.
It is frequently known as pig iron.
What is the iron-carbon system?
What are the different phases in the iron - carbon system?
Ferrite (α):
- The maximum solubility of carbon in ferrite is 0.022 wt% at 727 deg.
- Ferrite is soft, ductile and magnetic.
- BCC.
Austenite (γ):
- The maximum solubility of carbon in ferrite is 2.11 wt% at 1148 deg.
- Austenite is non-magnetic.
- FCC.
Cementite (Fe3C):
- Above 6.7 wt% cementite is formed.
- Cementite is hard and brittle.
- Cementite is not in equilibrium.
What are the different phase reactions?
Eutectic:
- Occurs at a solubility of 4.3 wt%
Eutectoid:
- Occurs at a solubility of 0.77 wt%
How are pearlite and martensite formed?
- If at the eutectoid phase, the temperature falls slowly, alternating layers of ferrite and cementite are formed, known as pearlite.
- If at the eutectoid phase, the temperature falls quickly, a formation of ferrite and cementite known as martensite is formed.
What are the features of pearlite?
- Pearlite is made up of hard, brittle cementite as well as softer, ductile ferrite making it very tough.
- It has a BCC structure.
- Slow diffusion of carbon is required for pearlite growth.
- Carbon atoms slowly diffuse out of the area where ferrite is being produced and to the area where cementite is being produced, as ferrite has less carbon than cementite. This forms pearlite.
What are the features of martensite?
- Martensite is formed when the quenching rate is high, meaning carbon diffusion cannot occur making it very hard and brittle.
- It has a BCT structure.
Which factors affect the hardness of steels?
- Quenching (heat treatment) can be used to improve strength and hardness of steels by preventing martensite formation.
- Martensite formation during welding causes the heat affected zone to become weak.
What are the features of pure iron steels?
0 to 0.008% carbon
