The overall properties are similar in any direction. The material is ISOTROPIC.
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What is strong texture?
The properties differ in different directions. The material is AN-ISOTROPIC. (this is useful when a property needs to be maximised in one direction)
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Describe annealing
Cold worked metal +elevated temperature = annealing
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What is the total input energy (of cold worked deformation) equal to?
Heat (lost to rollers and air) ,Metallurgical stored energy
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What proportion of the energy is stored in the material and where is it stored?
1 % which is stored in the defects,dislocations and grain boundaries.
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Define annealing
The application of heat treatment to a material to change its properties.
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What are the important parameters involved in annealing?
Heating and cooling rate, Hold temperature and hold time.
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What do the heating and cooling rates control?
Internal stresses
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What is dependent on hold temperature and time?
Microstructure and dislocation density.
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How can stress induced cracking occur during annealing?
If cooling/heating rates are too large different parts of the component can expand/contract at different rates leading to large stresses and thus cracking.
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What does annealing allow?
Dislocations to move more easily and to rearrange themselves. As heat gives energy to the system.
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What and how are properties affected by annealing?
There is a reduction in hardness and strength but an increase in ductility.
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Why/how do annealing processes occur?
The system is pushed by a driving force --> that is to reduce stored energy as a system is most stable when it has the least amount of energy.
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What happens during recovery?
1)The microstructure and thus properties are partially restored. 2)Annihilation and rearrangement of dislocations occur.
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Define Dynamic recovery
Deformation at elevated temperatures (dislocations have energy to move around so the material deform more
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How does annihilation occur?
When dislocations of 'opposite sign' interact with each other annihilation occurs.
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How does the removal of defects increase the ductility of a material?
It makes t easier for other dislocations to move thus plastic deformation become easier.
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Name the stage of recovery
a) Dislocations tangle b)Cell formation c)Annihilation of dislocations d) sub-grain formation e) sub- grain growth
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How do 'cells' form?
When dislocations of a similar sign meet at low temperatures they form a 'pile up or 'cells'.
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What are sub-grains?
At higher temperatures dislocations have more energy to move which allows more regular dislocation arrays to form which are called SUB-GRAINS
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Define 're-crystallisation'
A further restoration process where new grains grow and consume old grains. A new grain structure arises with a low dislocation density.
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Where does recovery take place?
Within deformed grains
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What is 'dynamic re-crystallisation' ?
Re-crystallisation at high temperatures.
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Name the two stages of re-crystallisation
1) Nucleation 2) Grain growth
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Define incubation time
The time before there are signs of nucleation.
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Where does nucleation occur?
Regions of large orientation gradients: grain boundaries, second phase particles, deformation bands.
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How does impingement occur?
As the grains get bigger there is less space for them to grow thus the growth rate decreases. ( the grains impinge on one another)
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Why do we re-crystallise?
To restore ductility, gives a more isotropic micro-structure and helps to control grain size. The grains are dislocation free.
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What does further annealing result to?
Smaller grains are eliminated and larger grains are left.
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Define 'normal grain growth'
When the grain boundaries assume a lower energy configuration.
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Define 'abnormal grain growth
The selective growth of a few large grains.
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Describe the relationship between grain size and strength of a material
Grain boundaries are obstacles to dislocation motion so the smaller the grain size the greater the strength and toughness of a material.
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Give an advantage of a structure with large grains
Large grains reduce creep rates in Ni based super- alloy at high temperatures.
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What is the Hall -Petch relationship?
Quantifies the relationship between grain size and strength: sigma_y =sigma_0 +kd^(-1/2)
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Other cards in this set
Card 2
Front
What is strong texture?
Back
The properties differ in different directions. The material is AN-ISOTROPIC. (this is useful when a property needs to be maximised in one direction)
Card 3
Front
Describe annealing
Back
Card 4
Front
What is the total input energy (of cold worked deformation) equal to?
Back
Card 5
Front
What proportion of the energy is stored in the material and where is it stored?
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