Ch 8 Flashcards
Discuss how plastic deformation occurs. Connect this to the structure of metals.
plastic deformation occurs by motion of dislocations (edge, screw, mixed).
Metals have metallic bonding (Non-directional) and also are closed packed in certain directions/planes. This means that atoms are relatively close requiring less energy to break and reform bonds. Also, lots of directions/options for slip.
Discuss how plastic deformation occurs. Connect this to the structure of metals.
plastic deformation occurs by motion of dislocations (edge, screw, mixed).
Metals have metallic bonding (Non-directional) and also are closed packed in certain directions/planes. This means that atoms are relatively close requiring less energy to break and reform bonds. Also, lots of directions/options for slip.
Compare the plastic deformation of metals to that of ceramics, Give reasons for the differences.
Ceramics either have covalent bonding or ionic bonding.
Covalent bonding is highly directional and much stronger than metallic bonding.
Similarily, ionic bonding is much much stronger AND while it isn’t directional, the motion of nearby ions of LIKE charge is restricted by electrostatic repulsive forces
All reasons why ceramics are so brittle and don’t deform well
Slip systems.
What is a slip system, list the most preferred slip systems for SC, BCC, FCC
Slip system: preferred direction and plan that a material will deform along. High planar density in the high linear density direction
What is the applied tensile stress?
Shear stress component when slip plane is oriented neither perpendicular nor parallel to stress direction
Why is Dislocation Motion & Plastic Deformation in Metals & Ceramics important?
Dislocations are intimately involved in plastic deformation. To control the mechanical properties (e.g. yield strength, UTS, toughness, high temperature properties) you control the ease with which dislocations move in a material. Understanding dislocation interactions with obstacles such as solutes, grain boundaries, and other dislocations, helps engineers to design better alloys and to achieve desired performance.
Why is this important Interaction of Dislocations with Obstacles & Annealing?
Pure metals are soft, so to increase the strength of metals, engineers can use dislocation interactions with obstacles to increase yield strength: by solid solution hardening (alloying), grain boundary refinement (decreasing grain size), and increasing dislocation density (cold work). These design schemes form a key basis for alloy design everywhere throughout technology.
During plastic deformation strength increases (which is generally good), yet ductility decreases (which is generally bad). Annealing is a controlled heat treating process by which ductility is restored at the expense of yield strength. This is an important manufacturing process, which engineers can use to broaden the range of useful properties in alloys, and tailor their properties to specific applications.
What angles is Tr the highest?
1/2 yield strength
Discuss how grain boundaries play a role in polycrystalline materials
polycrystalline materials have many grains, and often random crystallographic orientations and therefore slip directions vary from grain to grain
There will be one (at least) plane that is oriented favorably and that is where the slip with begin.
slip first fail first
What happens when you roll a material?
rolling of grains takes a random orientation and results in one direction
Define anisotropic
How are dislocations used as a strengthening mechanism?
If you want a material to not deform, you need to stop dislocations from moving
What are the three mechanisms used to stop dislocations from moving around?
- grain size reduction
- solid solution strengthening
- strain hardening (cold working)
How do you use grain size to stop dislocations from moving around?
because grain boundaries act as barriers to dislocation motion, an increase in boundaries (decrease in grain size) will increase strength of material
What are the consequences of reducing grain size? (or more generally any of the three mechanisms)
increase yield strength
increase tensile strength
increase hardness
What is solid solution strengthening? Alloying?
When dislocations are present, the result is areas of compression and tension.
You can add a smaller atom to an area of compression to pin the line dislocation. This is favorable because the line dislocation has a ‘choice’ between well fitting (eg small atoms) and an area of compression
OR
You can add a larger atom to an area of tension. Similarily, edge location ‘preferrs’ to not move on
What is strain hardening?
Plastically deforming most metals at room temperature makes them harder and stronger (because you are generating LOTs and LOTS os dislocations) which in turn get closer together which in turn get tangled up
As %CW increases what other properties are affected?
Increase yield strength
Increase tensile strength
DECREASES %EL or %AR ductility
so, now that you have generated all kinds of dislocations, what happens if you heat treat a metal?
heat treating will fix the defects by moving them around. Defects are not the lowest energy state and added energy through heat ‘allows’ defects to be reduced
How does annealing (increase in temp) affect other properties?
Decreases tensile strength
Increases %EL ductility
What are the three stages of annealing?
Recovery: dislocation annihilation
Recrystallization: create new small, perfect grains with few defects
Grain Growth: grain size grows
What is the ideal temperature for recrystallization?
Tr = recrystalization temp
0.3Tm < Tr < 0.6Tm
in kelvin
What does the recrystalization temp depend on?
%CW – TR decreases with increasing %CW
Purity of metal – TR decreases with increasing purity
What is the relationship between cold working and hot working?
Cold working: strain hardening (deformation below Tr)
Hot working: minimal strain hardening (deformation above Tr)
What is strain hardening
plastically deforming most metals at room temperature makes them harder and stronger
generates lots and lots of dislocations during this process
What is the relationship between grain size and other properties?
Small grains (strong, tough, NOT DUCTILE)
Large grains (good cree resistance at hight Ts, DUCTILE)
How can annealing treatment be used to change grain size and affect yield strength and ductility?
heat treating will ‘fix’ defects by moving them around
Do brittle polymers deform plastically?
Give examples of specific polymer types.
DO NOT plastically deform
cross-linked and network polymbers
Do plastic polymers deform plastically?
behave like metals b/c of secondary bonding between chains which are easy to break, easy to form.
Chains will align in the direction of the pull
What properties of polymers affect tensile strength? Why?
chain length (long = more entanglement increases TS)
degree of polymerization
mol. weight
What is one way to strengthen a polymer?
Drawing: form polymer with alighned chains results in
Increase E
Increase TS
decrease ductility
After drawing a polymer, if you anneal the same polymber, what are the consequences?
decrease chain alignment
reverses the effects of drawing
decrease E
decrease TS
increase %EL
Small interstitial atoms go in the
[ Select ]
strain region of an edge dislocation.
tensile
