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Terms in this set (72)
Schottky Defects and what makes them up
- A point defect in ionically bonded materials. Unique to ionic materials. In order to maintain a neutral charge, a stoichiometric number of cation and anion vacancies must form.
1. equal number of cations and anions are missing from lattice sites
2. found in highly ionic compounds with HIGH coordination #s and where the cations and anions are of SIMILAR size
3. density of the solid DECREASES
Frenkel Defects and what makes them up
- A pair of point defects produced when an ion moves to create an interstitial site, leaving behind a vacancy.
1. a cation leaves the normal lattice site and occupies an interstitial site
2. found in ionic compounds with LOW coordination #s and where the anions are much LARGER in size than cations
3. density of the solid remains the SAME
For most technologically important metal alloys, the usual state or microstructure is:
a metastable one
- you can never reach equilibrium
In precipitation hardening, the solution heat treatment consists of (1) heating the alloy to a temperature such that all solute atoms are dissolved to form a single-phase solid solution, and (2) quenching to produce a nonequilibrium supersaturated solid solution. The alloy is then aged at a lower temperature to form a uniform distribution of small precipitates. Is this true or false?
- familiarize yourself with this
- is a heat treatment technique used to increase the yield strenth of malleable materials
- produces uniformly dispersed particles within a metal's grain structure
- These particles hinder dislocation motion and thereby strengthen the metal, particularly those that are malleable.
1. heat treatment or homogenization
2. quenching: The rationale behind the quenching process is to preserve the uniform solid solution structure of the alloy below the homogenization temperature.
Name the two general states that accompany a phase transformation
Nucleation and Growth
the formation of embryos of the new phase that are stable enough to growh proceeds until equilibrium is attained
the increase in size of the nucleated
General concepts behind phase transformations
1. rate increases as interfacial surface area increases & nuclei grow
2. maximum rate reached - now amount unconverted decreases so rate slows
3. transformation complete
Overall rate = product
Phase transformations that involve a change in the microstructure can occur through:
- Maintaining the type and number of phases (e.g., solidification of a pure metal, allotropic transformation, recrystallization, grain growth.)
- Alteration of phase composition (e.g., eutectoid reactions)
- Production of metastable phases
kinetics of phase transformations
- study of reaction rates of phase transformations.
To determine reaction rate - measure degree of transformation as function of time
- Change in composition implies atomic rearrangement, which requires diffusion
- the displacement of a given atom is proportional to the square root of time
- kinetics is meant by the time-dependence of the rate at which the reaction occurs
- S-shape form
"Coring," which represents a form of non-equilibrium solidification, results in variations in the composition of a solidifying alloy. Coring occurs because of rapid solidification (i.e., the alloy solidifies faster than diffusion can occur to equilibrate composition). Is this true or false?
- If you cool something rapidly, there won't be enough time for the system to reach equilibrium (via diffusion). This is the cause of coring
A eutectic reaction involves which of the following phases?
above line: L
below line: gamma + beta
one liquid and two solids
L --> S1 + S2
* at the eutectic invariant, rxn occurs
phases above point are transforming to phases below the point
A eutectoid reaction involves which of the following phases?
above line: gamma
below line: alpha + beta
S1 --> S2 + S3
solid phase into two different solids phase
A peritectic reaction involves which of the following phases?
above line: σ + L
below line: gamma
S1 + L --> S2
a solid phase and liquid phase will together form a second solid phase
A peritectoid reaction involves which of the following phases?
above line: alpha + beta
below line: micro
S1 + S2 --> S3
multiple solid phases to less solid phases
A binary composition vs. temperature phase diagram for an isomorphous system will be composed of regions that contain which of the following phases and/or combinations of phases?
b) Liquid + α
- A phase diagram in which the components display unlimited solid solubility.
- only one solid phase forms; the two components in the system display complete solid solubility.
- Pure components melt at a single temperature, while alloys in the isomorphous system melt over a range of temperatures*. I.e. for a given composition solid and liquid will coexist over a range of temperatures when heated.
A three-phase invariant reaction in which one liquid phase solidifies to produce two solid phases.
A three-phase invariant reaction in which one solid phase transforms to two different solid phases.
A three-phase reaction in which a solid and a liquid combine to produce a second solid on cooling.
A three-phase reaction in which two solids combine to form a third solid on cooling.
A solidus line separates which of the following phase fields?
α and liquid+α
Solidus Line and what it separates
- The solubility limit below which there is only solid present.
separates which phase fields:
alpha and liquid+alpha
Liquidus Line and what it separates
- The solubility limit above which there is only liquid present.
separates which phase fields:
liquid and liquid+alpha
Solvus Line and what it separates
- The solubility limit separating a single phase solid phase field from a multiphase solid phase field.
separates which phase fields:
alpha and alpha+beta
- The solvus line that contains the eutectic reaction. Occurs at a fixed temperature.
Isotherm line and what it separates
A line on a chart representing changes of volume or pressure under conditions of constant temperature
Tie line and what it separates
- A horizontal line drawn in a two-phase region of a phase diagram to assist in determining the compositions of the two phases.
- type of isotherm connecting two phase lines on a diagram
Once a system is at a state of equilibrium, a shift from equilibrium c result by alteration of which of
The general concepts concerning Equilibrium
Thermodynamic equilibrium is the lowest energy state at some specified combination of temperature, pressure, and composition. The characteristics of the system do not change with time (i.e., the system is "stable"). We use: Pressure, Temp, and Composition to adjust equilibrium.
The general concepts of Degrees of Freedom.
In response to a stimulus the ways in which the system can respond corresponds to the
degrees of freedom of the system
The Phase Rule: F = C - P + 2 or F - C + P = 2
F: Degrees of Freedom
C: Number of components
P: Number of phases
The degrees of freedom can be thought of as the difference between what you (can) control (C + 2) and what the system controls (P)
The surface energy of a single crystal depends on the crystallographic orientation with respect to the surface.
- Different crystal planes have different energies. Thus, surface energy will depend on crystal orientation.
What are the two fundamental types of dislocations? Better know this.
Small angle boundaries formed by edge dislocations are called tilt boundaries. Small angle boundaries formed by screw dislocations are called twist boundaries.
Small Angle Grain Boundary
- an array of dislocations that produces a small misorientation between the adjoining crystals
- Because the energy of the surface is less than that of a regular grain boundary, the small angle grain boundaries are not as effective in blocking slip.
- tilt and twist
- illustrated by slicing partway through a perfect crystal, spreading the crystal apart, and partly filling the cut with an extra half plane of atoms.
- A line defect that can be envisioned as an "extra half plane of atoms" inserted into the crystal.
Small angle boundaries formed by edge dislocations are called _____________ boundaries. Small angle boundaries formed by screw dislocations are called _______________ boundaries.
a. tilt, twist
- A line defect that can be envisioned as a spiral ramp in the crystal that was produced by skewing the crystal by one atomic spacing.
Increasing the number of grains or increasing the grain size of a metal material causes the strength of the
metal to decrease.
What do changes in grain size do to materials' properties?
- This is explained using the Hall-Petch relation.
- Smaller grains lead to higher strength or hardness.
- Larger grains leads to lower strength or hardness.
The solvent is an element or compound that is present in ________.
The greatest amount
The difference between the solute and solvent in solid solutions
- A solvent is a substance that dissolves a solute. A solute is a substance that is dissolved to make a solution. When a solute dissolves, it separates into individual particles. It is not possible to identify the solute and solvent as different substances in a solution.
- Solid solutions are not formed as solids. Instead, the solvent metal is heated until it melts and becomes a liquid. Then the solute is added, and the substances are thoroughly mixed together. When the mixture cools, it is a solid solution.
- i.e. Bronze consists of tin dissolved in copper.
Localized disruptions in otherwise perfect atomic or ionic arrangements in a crystal structure
Particularly useful in explaining deformation and strengthening in metallic materials.
- This equation relates a material's grain size to its yield strength.
- σy = σ0 + Kd-1/2
- σy is the yield strength, d is the average diameter of the grains, and and K are constants for the metal.
Line defects that, when a force is applied to a metallic material, move and cause a metallic material to deform.
Point or Zero-Dimensional Defects
Line or One-Dimensional Defects
Straight Dislocations (edge or screw)
Area or Two-Dimensional Defects
Volume or Three-Dimensional Defects
Impurity atoms in metals, ceramics, and alloys may form substitutional and interstitial solid
A charge balance is always required for every point defect introduced in an ionic solid.
_______________ refers to irreversible deformation that occurs when the force or stress that caused the deformation is removed.
BCC metals tend to have ___________ and __________ compared to FCC metals.
higher strengths, lower ductilities
The process of progressively breaking and reforming bonds requires far more energy than the energy that would be required to instantaneously break all of the bonds across a slip plane.
For self-diffusion, the activation energy is equal to the energy needed to create a(n) ________ and to cause the movement of the atom.
_________ refers to the net flux of any species, such as ions, atoms, electrons, holes, and molecules
- a way to relate temperature or thermal energy to the rate of atom or ion movement
Rate = Co exp(-Q/(RT)
In the Arrhenius equation Co is a constant, R is the gas constant, T is the absolute temperature (K), and Q is the activation energy required to cause one mole of atoms or ions to move.
At low temperatures, it will usually only take a large number of atoms a short time to diffuse to produce a uniform structure.
In _______ materials, a diffusing ion only enters a site having the same charge.
Activation energies are lower for interstitial diffusion than for vacancy diffusion.
In materials containing vacancies, atoms move from one lattice position to another. This process is known as ______________.
In Fick's first law the J is the ________, D is the ___________ and dc/dx is the _____________.
flux, diffusivity coefficient, concentration gradient
Fick's first law is given by the equation _____________.
J = -D((dc)/(dx))
The liquidus temperature is the temperature _____ which a material is completely liquid. The solidus temperature is the temperature ____ which the alloy is 100% solid.
Which of the following is not a criterion for unlimited solubility of an alloy system.
The atoms or ions must have a 50% or higher difference in atomic radius size
Chemicals added to molten metals to promote nucleation are known as .
The critical radius (r*) is the minimum size of a crystal that must be formed by atoms clustering together in the liquid before the solid particle is stable and can begin to grow.
The rate of nucleation is low for small undercoolings and increases for larger undercoolings. The growth rate of the new phase ______ continuously as the undercooling __________ .
The time-temperature-transformation (TTT) diagram from Figure 12-20 is used to predict the ______ treatment required to obtain certain transformations of steel.
A stoichiometric compound Fe3C is also known as _____ and forms when the solubility of carbon in solid iron is exceeded. The lamellar structure of α and Fe3C that develops in the iron-carbon system is called ________.
Tempering causes the decomposition of martensite in steels, which in turn causes the strength and _______ of the steel to decrease while the _______ and impact properties are improved.
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