Exam 2 Flashcards
How to increase strength in materials by 5-30% ?
Solution Hardening
Alloying & Grain Size Control
How to increase strength in materials by 50-300% ?
Strain Hardening
Heat Treatment
When a material is plastically deformed by
Strain Hardening / Work Hardening
Yield, Tensile, Hardness strength ________ but Ductility ________
increases
decreases
Fully strain-hardened material yield strengths increase from ______ and tensile strengths increase _______
100-500%
50-100%
Dislocations and brittleness can be removed with new strain-free grains by _______ the material.
heating
___________ can be induced by rolling a polycrystalline metal
Anisotropy
In isotropic materials. _____________
grains are equiaxed & randomly oriented
Rolling in anisotropic materials affects ______
grain orientation and shape
Stronger material have more _____________
dislocations
Too many dislocations may lead to ___________
cracking
For Strain Hardening, Strength is higher at the _______ than the ________
surface
center
Why is Strain Hardening desirable?
Wear and high stresses at surface
How is metal cold worked?
by forging, stamping or rolling
Cold working does what to metal?
permanently changes its shape, (DEFORMED)
What allows for the overall change in shape of the metal?
dislocations or slips in the grain structure
How is warm working done?
same cold working process performed below recrystallization temperature but above room temperature
What does warm working accomplish?
Reduces the mechanical energy to deform
What is cold working?
working of material below its recrystallization temperature
What is Annealing?
used to remove effects of cold working (heat and then slow cooling to soften materials)
What is hot working?
working of material above its recrystallization temperature
During hot working, _________
material recrystallizes immediately – no strain hardening builds up
What is the advantage of hot working?
less forces required to deform material
What are the disadvantages of hot working?
High temp may oxidize surface
Dimensions harder to control
Tolerances harder to control
What happens during Recrystallization?
• New grains form that:
- - have low dislocation densities - - are small in size - - consume and replace parent cold-worked grains
All grains in cold-worked material have been _________
consumed/replaced
What are the effects of cold working?
Yield strength (YS) increases. Tensile strength (TS) increases. Ductility (%EL or %AR) decreases.
Strength is _______ by making dislocation
motion difficult.
increased
How to strengthen materials?
Dislocations
The ability of a metal to plastically deform depends on _________
the ability of dislocations to move
Restricting dislocation motion leads to
harder & stronger material
Grain boundary:
Point defect:
Dislocation:
Second phase:
grain size reduction
solid solution strengthening
strain hardening (cold work)
precipitation hardening
In the Plastic Deformation of Polycrystalline Materials, Because of the random crystallographic orientations of the different grains
the direction of slip varies from one grain to another.
During deformation, mechanical integrity is maintained along the
grain boundaries
Each individual grain is _________ to some degree by its neighboring grains
constrained
solute or impurity atoms replace or substitute for the host atoms
Substitutional Solid Solution
impurity atoms fill the voids or interstitials among host atoms
Interstitial Solid Solution
- Small interstitial positions
- Atomic size of an interstitial impurity must be small (H, N, & C).
completely soluble in one another at all proportions
Cu-Ni (SSS)
Solid Solution Hardening is also
alloying
What is the Solid Solution Hardening Technique?
adding atoms of one element (the alloying element) to the crystalline lattice of another element (the base metal)
Solvent is defined as
the base substance, wherein the solute is being dissolved
A solute is a
substance that is dissolved into the solvent
When both elements exist in the same ________, both elements in their pure form “should” be of the same __________ (for complete solubility)
crystalline lattice
crystal structure
Solvent and solute atoms must differ in ________ by less than __%
atomic size
15%
Strength is Solid Solution Hardening is dependent on
on ease of achieving dislocations in crystal lattice
Dislocations create ________ within material
stress fields
What is the process of Solid Solution Hardening?
Solute atoms introduced, then local stress fields are formed that interact with the dislocations, impeding their motion and causing an increase in the yield stress - increase in strength
Solute and solvent atoms differ in
size, local stress fields created
If the Solute atom size larger than solvent then____
field is compressive
If the Solute atoms are smaller than solvent atoms_____
field is tensile
Alloying with impurities form _______ or _______ solid solution.
Interstitial
substitutional
Impurity atoms _______
distort the lattice & generate stress
______ can produce a barrier to dislocation motion
Stress
Small impurities tend to concentrate at ________
dislocations (regions of compressive strains)
partial cancellation of dislocation compressive strains and impurity atom tensile strains
regions of compressive strains
Solid Solution Alloying_________ of dislocations and _______ strength
reduces mobility
increase
Large impurities tend to concentrate at __________
dislocations (regions of tensile strains)
Substitutional Rules: Conditions for substitutional solid solution
- Δr (atomic radius) < 15%
- Same crystal structure for pure metals
- Proximity in periodic table
i.e., similar electronegativities - Valency – “should” be identical
All else being equal, a (solvent) metal will have a greater tendency to dissolve a (solute) metal of higher valency than one of lower valency.
solute or impurity atoms replace or substitute for the host atoms
Substitutional Defects
Metals have a ___________
crystalline structure
When metal solidifies from the molten state
millions of tiny crystals start to grow
The longer metal takes to cool, _______
the larger the crystals grow
These crystals form the ______ in the solid metal
grains
Each grain is a distinct _____l with its own ________
crystal
orientation
adjacent grain planes at different orientations
Grain boundaries
Grain boundaries_________
prevent slip or makes slip more difficult
Slip occurs along a ______ of one atom
plane
Numerous grains ________
reduce amount of slip
When grain size decreases ______
strength increases
Strength is ________ related to grain area
inversely
Optical Microscopy is _________
Useful up to 2000X magnification
What is the Interception Method?
Draw a number of straight lines of same length so that the total interception of the lines with grain boundaries should be > 50
Atoms are bonded ________ along a grain boundary
less regularly
Energy is a function of the degree of _________, being ______for high-angle boundaries
misorientation
larger
The atoms are bonded ________ along a grain boundary
less regularly
Greater disorder leads to
grain boundary energy
More reactive_______
makes etching possible
Impurity segregation _________
affecting the materials properties
As the metal solidifies, __________ start to form first
small crystal nuclei
Upon completion of solidification, the colonies contact each other, forming _________
grain boundaries
What are grain boundaries?
Boundaries between crystals in a polycrystalline material
There is a change in __________ across grain boundaries
crystal orientation
Grain boundary hinders the dislocation motion or acts as a barrier to _________ motion (slip)
dislocation
A ___________ material is harder and stronger than one that is coarse grained, as more _______ are acting to block dislocation movement
finer-grained
barriers
Grain size reduction improves not only ________, but also the ________ of many alloys
strength
toughness
Defects affect _________
material properties
A knowledge and understanding of phase diagrams is important to the engineer as they relate to the design and control of ________
heat treating processes
What is the Solubility Limit?
Max concentration for which only a solution occurs
Solubility limit ________ with Temperature
increases
In SSS Atoms of the parent metal (or solvent metal) are _______ or ________ by atoms of the alloying metal (solute metal)
replaced
substituted
In SSS The atoms of the two metals in the alloy are of _______ size
similar
In ISS The atoms of the parent or solvent metal are _____ than the atoms of the alloying or solute metal
bigger
In ISS The smaller solute atoms fit into _________ or spaces between the large atoms
interstitial sites
For many alloy systems and at some specific temperature, there is a _________ of solute atoms that may dissolve in the solvent to form a _______
maximum concentration
solid solution
a homogeneous portion of a system that has uniform physical and chemical characteristics
phase
If more than one phase is present in a given system, each will have its own ________
distinct properties
A boundary separating the _____ exists across which there will be a discontinuous and abrupt change in ________
phases
physical and/or chemical characteristics
The physically and chemically distinct material regions that result
Phases in Materials
Solid solutions are commonly designated by
lowercase Greek letters (a , b, g, etc.)
Equilibrium conditions may be defined
as slow heating and/or cooling of the many materials to permit any phase change to occur
The relationships between ________ and the _______ & the quantities of phases at equilibrium
temperature
compositions
For a binary system of known composition (C0) & temperature (T) that is at equilibrium, the following information can be obtained from the phase diagram:
The phases that are present
The composition of each phase
The amount (or percentage) of each phase
Line indicates the temperature at which the first solid appears upon cooling and the temperature at which the last solid disappears on heating. In few instances liquid separates into two liquids, the temperature at which the separation starts.
Liquidus Line
Line indicates temperature at which the last liquid disappears upon cooling.
Solidus Line
The line indicates the solubility limits
Solvus line
Line connecting three phases that are in equilibrium at a specific temperature in a binary system. Appears as a horizontal line because at constant temperature.
Invariant Reaction Line
connects the phases in equilibrium with each other – also sometimes called an isotherm
Tie line
liquid transforms to two solid phases
Eutectic
one solid phase transforms to two other solid phases
Eutectoid
liquid and one solid phase transform to a second solid phase
Peritectic
In a Eutectic Reaction, Upon cooling, _______ phase is transformed into the _______ phases (a and b) at the ______ temperature TE; upon heating, the opposite reaction occurs
one liquid
two solid
eutectic
For alloy of composition
C0 = CE
In a Eutectoid Reaction, Upon cooling, _____ phase is transformed into the ______ phases at the _____ temperature TE; upon heating, the opposite reaction occurs
one solid
two solid
eutectoid
one liquid phase transforms into two solid phase at a constant temperature
Eutectic reaction
one solid phase transforms into two solid phase at a constant temperature
Eutectoid reaction
Both steels and cast irons, primary structural materials in every technologically advanced culture, are essentially
iron-carbon alloys
the art and science of controlling thermal energy for the purpose of altering the properties of metals and alloys
Thermal processing or heat treating
a-Fe
Ferrite
y-Fe
Austenite
iron carbide Fe3C
Cementite
Pure Fe (room temperature to 912C), a single-phase BCC solid solution
Ferrite
a single-phase FCC solid solution (a ferrite transforms from BCC to FCC at 912C)
Austenite
The maximum solubility of carbon in austenite
2.14 wt% at 1147C.
Austenite is not stable below ______
727 degrees
intermediate phase with the chemical formula Fe3C
Cementite
Cementite is
brittle and hard
One eutectic reaction exists for the iron-iron carbide system,
at 4.30 wt% C and 1147C
One eutectoid reaction exists for the iron-iron carbide system
at 0.76 wt% C and 727C
The microstructure of the eutectoid steel that is slowly cooled consists of alternating layer or lamellae of the two phases (ferrite and cementite) that form simultaneously during the transformation
Pearlite
The thick light layers are the ______ phase, and the ______ phase appears as thin lamellae most of which appear dark
ferrite
cementite
Pure iron
From the phase diagram, it is composed almost exclusively of the ferrite phase at room temperature
Steels
In most steels the microstructure consists of both a and Fe3C phases.
Carbon concentrations in commercial steels rarely exceed 1.0 wt%.
Cast irons
Commercial cast irons normally contain less than 4.5 wt% C.
Phase diagrams are useful tools to determine:
-the number and types of phases
-and the composition of each phase
for a given T and composition of the system
one liquid phase transforms into two solid phase at a constant temperature
Eutectic reaction
one solid phase transforms into two solid phase at a constant temperature
Eutectoid reaction
Steels and cast irons are based on binary Fe-C system and offer a wide range of properties due to different microstructures
Hypoeutectoid & hypereutectoid steels
What are the Four strengthening mechanisms?
Grain size refinement
Solid-solution strengthening
Strain hardening
Precipitation hardening
The development of microstructure in both single- and two-phase alloys ordinarily involves some type of __________
phase transformation
an alteration in the number and/or character of the phases
phase transformation
The _________ is fundamental to the development of microstructures in steel alloys
eutectoid reaction
_______ is the microstructural product of this transformation
Pearlite
The percentage of the transformation product is related to the __________ and __________
holding temperature
holding time
For Fe-C alloys with compositions other than eutectoid composition, a _________ phase (either ferrite or cementite) will coexist with ________
proeutectoid
pearlite
Pearlite has mechanical properties between the soft, ductile ________ and the hard, brittle _______
ferrite
cementite
a change in the number and/or character of the phases that constitute the microstructure of an alloy
Phase Transformation
Nucleation
formation of very small particles, or nuclei, of the new phase
favorable nucleation sites
imperfection sites,
e.g. grain boundaries)
increase of the nuclei in size. Some volume of the parent phase disappears
Growth
The thickness of the ferrite/cementite layers in pearlite depends on the ___________
temperature
With decreasing temperature, the layers of pearlite become progressively __________
thinner
At temperatures just below eutectoid __________
relatively thick layers → coarse pearlite
In the vicinity of 540°C__________
relatively thin layers
→ fine pearlite
Pearlite forms above the nose ________; bainite forms below the nose ________
(540 ~ 727°C)
(215 ~ 540°C)
Bainite consists of ______ and _______ phases
ferrite
cementite
Bainite forms as ________ or _______, depending on the temperature of the transformation
needles
plates
Unlike pearlitic transformation, martensitic transformation is _____________
instantaneous
The horizontal lines indicate that
- The transformation is independent of time;
- It is a function only of the temperature to which the alloy is quenched or rapidly cooled.
What is the beginning of this Martensite transformation?
M(start)
Two other horizontal and dashed lines, labeled ______ and _____, indicate percentages of the austenite-to- martensite transformation.
M(50%)
M(90%)
Martensite is formed when _______ Fe-C alloys are rapidly ________ (or quenched) to a relatively _____ temperature (in the vicinity of the ambient)
austenitized
cooled
low
A transformation product that is competitive with pearlite
Martensite
Martensite is a transformation of FCC to
BCT (body-centered tetragonal)
Martensite occurs _______
instantaneously → time-independent
The martensite grains ________ and grow at a very rapid rate
nucleate
< 0.6 wt%C,
long/thin plates, side by side, aligned parallel to one another
Lath
> 0.6 wt%C,
lenticular or
platelike appearance
Lenticular
Cementite (Fe3C) is much ______ but more _____ than ferrite (α)
harder
brittle
Fe3C ↑
⇒ strength↑, ductility↓
Fine pearlite is ______ and ______ than coarse pearlite but less ______
harder
stronger
ductile
Hardest and strongest, and most brittle
Martensite
Volume change in Martensite
crack formation during quenching
In the as-quenched state, martensite, in addition to being ______, is so _____ that it cannot be used for most applications
very hard
brittle
Any internal stresses that may have been introduced during quenching have a _________ effect
weakening
The ductility and toughness of martensite may be enhanced and internal stresses relieved by a heat treatment know as
Tempering
Tempering is accomplished by
heating the martensitic steel to a temperature below the eutectoid (normally, between 200-650°C) for a specified time period to allow microstructure/ property change through diffusion
By diffusional processes: Martensite (BCT, single phase)_______
→ Tempered Martensite (α + Fe3C)
Tempered martensite may be nearly as hard and strong as martensite, but with substantially enhanced _______
& _________
ductility
toughness
Tempered martensite consists of _________
extremely small cementite particles embedded within a ferrite matrix
_________ is involved in the transformation
Carbon diffusion
Heat treatment variables are _______
temperature and time
most treatments are
constant- temperature processes
