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)
Which would have a higher yield strength, an alloy that was cold worked 25% or the same alloy that was cold-worked 35%?
- 25% cold worked
- same yield strength
- 35% cold worked
- impossible to know
35% cold worked
True or false: When you anneal a metal, you re-melt it to ensure that all evidence of cold working is removed.
false
The (tensile strength/yield strength/percent elongation) of a 1040 steel, cold worked X% is ________________ . Note: report answer to the nearest 5 ksi. (Describe how to answer the question.)
find X% cold work on x-axis, then follow up until you reach the (tensile strength/yield strength/percent elongation) line and report its location on the y-axis
Cold working causes ductility to…
- remain about the same
- increase
- decrease
decrease
Corrosion resistance is ___________ in a cold worked metal.
- increased
- decreased
- about the same
decreased
Cold working causes the electrical conductivity to…
- increase
- decrease
- remain about the same
decrease
True or false: The ductility of a cold worked metal is higher than the ductility of the metal before cold working.
false
The yield strength is ___________ by cold working.
- unchanged
- decreased
- increased
increased
Select the statements below that are true for grain growth:
- some grains grow at the expense of others
- it can only occur after recrystallization
- makes the metal shinier
some grains grow at the expense of others
You are planning to reduce the thickness of a fully annealed metal plate by hot working. The ductility of the plate would be expected to…
- increase
- stay about the same
- decrease
stay about the same
Which would have a higher yield strength an alloy that was cold-rolled 25% or the same alloy that was hot-rolled 25%?
- same yield strength
- 25% hot rolled
- impossible to know
- 25% cold rolled
25% cold rolled
During cold work, pores are ________________.
- closed up
- opened up
- remain about the same
closed up
True or false: Dimensional tolerances are better in hot rolled products than in cold rolled products.
false
The three stages of annealing are…
- recovery, recrystallization and grain growth
- rolling, forging and drawing
- fatigue, creep and stress corrosion cracking
recovery, recrystallization and grain growth
Select the following that are true for recrystallization.
- strengths are decreased
- ductility is increased
- new grains with low dislocation densities nucleate and grow
- the metal becomes shinier
- strengths are decreased
- ductility is increased
- new grains with low dislocation densities nucleate and grow
Which has a stronger effect during annealing - temperature or time?
Temperature
Cold working leads to __________ in the tensile strength of a metal.
- a decrease
- no change
- an increase
an increase
True or false: The surface finish is better in cold rolled products than in hot rolled products.
true
Select all the answers below which are true for recovery.
- electrical conductivity is improved
- the hardness drops significantly
- corrosion resistance is improved
- the residual stress is reduced
- the dislocation density remains about the same
- electrical conductivity is improved
- corrosion resistance is improved
- the residual stress is reduced
- the dislocation density remains about the same
Match the following:
- rolling
- forging
- drawing
- extruding
- deforms metal into a die cavity
- push metal through a die to form rods or tubes
- pull rod through a die to produce wire
- reduce thickness by forcing through rolls
- rolling: reduce thickness by forcing through rolls
- forging: deforms metal into a die cavity
- drawing: pull rod through a die to produce wire
- extruding: push metal through a die to form rods or tubes
Cold working causes hardness to…
- decrease
- remain about the same xxx
- increase
increase
The following are all true for cold working except…
- the ductility decreases
- the yield strength increases
- the corrosion resistance decreases
- electrical conductivity increases
electrical conductivity increases
Select the statements below that are true for hot working.
- no residual stresses are generated
- ductility is not decreased
- strengthening does not occur
- pores are closed up
- is done at 100C below the recrystallization temperature
- ductility is not decreased
- strengthening does not occur
- pores are closed up
Which graph would have a lower percent cold work?
- graph with a slope of 1 for most of the line
- graph with a slope of 1 for about half of the line
graph with a slope of 1 for most of the line
Choose the annealing stage the leads to the biggest change in mechanical properties.
- recovery
- grain growth
- recrystallization
recrystallization
Cold working causes the surface finish to…
- get worse
- remain about the same
- get better
remain about the same
Select the statements that are true below.
- the recrystallization temperature is higher in pure metals than in alloys
- the recrystallization temperature decreases with increasing cold work
- the recrystallization temperature decreases with a smaller original cold-worked grain size
- the recrystallization temperature is the temp at which recrystallization starts in one hour
- the recrystallization temperature decreases with increasing cold work
- the recrystallization temperature decreases with a smaller original cold-worked grain size
What does boiling temperature depend on?
pressure
Name four ways to strengthen a metallic material.
- grain size strengthening (Hall-Patch equation)
- cold working / strain hardening
- formation of small particles of second phases
- additions of small amounts of elements
Describe a sold solution and when it forms.
- contains two or more types of atoms/ions that are dispersed uniformly throughout the material
- forms when small amounts of elements are added
Define an alloy.
a material that exhibits properties of a metallic material and is made from multiple elements
Describe the Unary (T-P) Phase Diagram.
- melting: solid to liquid
- freezing: liquid to solid
- vaporization: liquid to gas
- condensation: gas to liquid
- deposition: gas to solid
- sublimation: solid to gas
- critical point: between liquid and gas where the two phases cannot be distinguished (located at very high pressure)
- triple point - point at which the three phases meet
Describe the Unary (T-P) Phase Diagram.
- its lines divide the liquid, solid, and gas phases
- the intersection between 1 atm and the lines give the usual melting and boiling temperatures
- melting: solid to liquid
- freezing: liquid to solid
- vaporization: liquid to gas
- condensation: gas to liquid
- deposition: gas to solid
- sublimation: solid to gas
- critical point: between liquid and gas where the two phases cannot be distinguished (located at very high pressure)
- triple point - point at which the three phases coexist under equilibrium conditions
Name the variables for the general form of the phase rule: 2 + C = F + P
- 2: implies that both the temp and pressure can change
- C: number of chemically independent compounds/elements in the system
- F: number of degrees of freedom / number of variables that are allowed to change independently without changing the number of phases in equilibrium
- P: number of phases present
Describe unlimited solubility.
when two solutions are completely soluble in each other (creates one phase) (i.e. the solutions are fully miscible)
Describe limited solubility.
the solutions are not fully miscible (there is a max amount of one or both of the solutions before excess of that solution builds up)
An inhomogeneous solution becomes more homogeneous via…
diffusion
Describe solid-solution strengthening and its effects.
- caused by increased resistance to dislocation motion
- single, homogeneous phase
- doesn’t have strong effect on mechanical properties
- dislocations get “caught” on point defects, which slows down slip
- increase in yield strength and tensile strength
- increase in hardness
- decrease in ductility
- decrease in electrical conductivity
What two factors does the degree of solid solution strengthening depend on?
- ratio of atomic radii
- amount of solute added (up to solubility limit)
Describe the difference among stable, metastable, and unstable.
- stable: equilibrium, lowest energy state, and depends on temperature, pressure, and composition
- metastable: not the lowest energy but doesn’t change for a long time and is kinetically constrained
unstable: has a strong tendency to change
Describe the Isomorphous Phase Diagram.
- shows one liquid phase, one solid phase, and one complete liquid and solid solubility phase
- contains the liquidus, solidus, and freezing range
Define liquidus.
temp at which first solid forms
Define solidus.
temp at which last liquid freezes
Describe the freezing range.
difference between liquidus and solidus
Describe how to determine the composition in both a single-phase diagram and a two-phase diagram.
- single-phase: equals the alloy composition
- two-phase: draw tie line (horizontal line to help determine the compositions of the two phases) at temp of interest, draw a perpendicular from the intersection of the tie line and the phase boundaries, and read the composition of each phase on the x-axis
Describe how to determine the composition of the first solid to form and the last liquid to freeze.
create “Z” shape
- first solid to form: point farthest left
- last liquid to freeze: point farthest right
What do the initial liquid and final solid make up?
the overall alloy composition
Describe how to determine the phase fraction in both a single-phase diagram and a two-phase diagram.
- single-phase: equals 100%
- two-phase: (opposite arm of lever / total length of tie line) * 100%
Describe coring.
nonuniform composition produced by nonequilibrium solidification over short distances
Describe the homogenization heat treatment.
heating the casting to a temp below the nonequilibrium solidus to reduce the interdendritic segregation and problems with hot shortness
Define segregation.
possessing different compositions, usually present in most castings from alloys
What does the inverse level rule find?
the proportions of each phase at a given temp
