Exam 3 (Chapters 8, 10-14) Flashcards
Define slip.
movement of dislocations
Describe the planes with which slip occurs most easily in.
close-packed
What happens to the crystals as a result of a slip in the material?
they become plastically deformed
Name three things that can increase the strength of a crystalline material.
impeding dislocation motion with:
- adding more dislocations (work-hardening / strain-hardening)
- adding interstitials, substitutional atoms, or second phases
- decreasing grain size (more grain boundaries)
When strength increases, what happens to the material’s ductility and toughness?
they decrease
What happens when you apply stress on a metal that exceeds its yield stress?
elastic and plastic deformation / strain
What happens when you remove an applied load from a metal that was greater than its yield strength?
springback (the elastic strain that is recovered after a material has been plastically deformed) remains (permanent bend/dent/stretch)
What happens when you reapply stress on a metal with a formerly removed applied load that was greater than its yield strength?
the flow stress (stress needed to initiate plastic flow in previously deformed materials) increases, a new higher yield strength is established, and ductility decreases (until flow stress, tensile strength, and breaking strength are equal, then no ductility)
Name and define some common methods of cold (and hot) working (deformation processes)
- rolling: used to produce a metal plate, sheet, or foil
- forging: deforms metal into die cavity producing relatively complex shapes
- (wire) drawing: metallic rod is pushed through a die producing a wire/fiber
- (deep) drawing: used to form the body of beverage aluminum cans
- extrusion: material is pushed though a die forming products of uniform cross-sections
- stamping
- stretch forming and bending
Describe a material’s microstructure during cold working.
the grains and second phases tend to elongate in the direction of stress and the pores are pressed closed
Define crystallographic textures and describe them during the cold working of a material.
- crystallographic textures are the preferred orientations/directions of the grains that can develop
- certain crystallographic directions tend to elongate along the direction of stress
- makes the material highly anisotrophic
Power law behavior: σ = kεⁿ
What do “k” and “n” represent?
- k: strength coefficient
- n: strain-hardening exponent
Strain-rate sensitivity: m = [ծ(lnσ);ծ(lnέ)]
What does “έ” represent?
- έ: strain rate
Define strain-hardening / cold working.
applying a stress that is greater than the original yield strength of the metallic material while simultaneously deforming it causing a decrease in ductility
Describe residual stress, its effects, and how to relieve it.
- a stress that stores some of the applied stress as a tangled network of dislocations
- can cause distortion of the part during machining or other processes, affects ability of a part to carry a load, cold working increases, and the total internal energy increases
- can be relieved by heat treatment
Describe shot peening and its effects.
- the bombarding of a surface with steel shot at a high velocity
- leads to compressive residual stresses at the surface, an increase in resistance of the metal surface to fatigue failure, and an increase in strength
What are the advantages of cold working?
- strengthen and shape at the same time
- cheap
- surface finish is excellent
- dimension tolerances are excellent
What are the disadvantages of cold working?
- loss in ductility
- reduction in electrical conductivity
- reduction in corrosion resistance
Describe annealing, its effects, and how to create a stronger effect.
- the heating and holding of a material at high temperatures, then the material is then cooled slowly so that the inside and outside cool at about the same rate
- eliminates some or all of the effects of cold working, reduces strength, and increases ductility
- stronger effect by holding longer, using higher temperatures, increasing the duration to decrease the change in temperature per unit of time
Describe tempering, its effects, and name the resultant material.
- conducting heat treatment that leads to compressive stresses on the surface of a glass and the material’s surface is cooled faster than its center
- increases strength
- resultant glass is known as “tempered glass” and has a lot of surface area due to having a lot of stored energy
Name the three stages of annealing.
- Recovery
- Recrystallization
- Grain Growth
Describe the first stage of annealing.
(recovery)
- a low temp treatment that removes the residual stresses due to cold working without causing a change in the number of dislocations
- energy from heat allows the dislocations to move and form boundaries of a “polygonised subgrain structure” (align with each other to lower the strain - called “polygonization”)
- restores high electrical conductivity
- improves corrosion resistance
- mechanical properties don’t change
Describe the second stage of annealing.
(recrystallization)
- forms new grains with low dislocation densities by heat treating a cold-worked material
- eliminates residual stresses around dislocation with rapid recovery
- decreases strength
- increases ductility
Define recrystallization temperature.
- temperature at which recrystallization is complete in one hour (not a fixed value); decreases when amount of cold working increases, when the initial cold-worked grain size is small, and when annealing time increases; and increases with higher melting points
- defines the boundary between cold working and hot working of a metallic material
Describe laminated safety glass.
two annealed glass pieces laminated using plastic known as polyvinyl butyral (PVB)
Describe the third stage of annealing.
(grain growth)
- uses higher annealing temperatures
- has a fine recrystallized grain structure
- large grains grow and small grains shrink
- reduces grain boundary area
- material doesn’t need to go through the first two stages of annealing to go through this stage
Describe hot working and its effects.
- the plastic deforming of a metal above the recrystallization temperature so that it remains ductile
- metallic material is being continually recrystallized
- almost unlimited plastic deformation because there is no strengthening
- some imperfections are eliminated (pores close up)
- still have anisotropic behavior
- residual stress because the surface cools faster than center
- oxidizes the surface, so has a poor surface finish compared to cold working
- dimensional accuracy is more difficult to control
Describe cold working.
the shaping of a material below the recrystallization temperature
When a material cools down, the density usually goes (up/down).
up
When a material cools down, the specific volume usually goes (up/down).
down
When a viscous liquid cools faster, the specific volume goes down (more/less).
less
Describe what happens when glass is being annealed.
because density / specific volume depends on cooling rate, the glass’s outside cools faster, thin regions cool faster, and residual stress is left
Describe strain hardening in thermoplastics.
- no dislocation movement
- their long, chainlike molecules align in the necked region
- increases strength and hardness
Define a phase.
a portion of a material with:
- uniform structure (including crystal structure)
- homogeneous (approximately same composition and properties throughout)
- clear boundary between phase and surrounding phase(s)
