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Iron-carbon phase
diagram & basicdefinitions
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Alloy:A metallic substance that is
composed of two or more elements
Austenite: Face-centered cubic iron or aniron alloy based on this structure.
Bainite: The product of the final
transformation of austenite decomposition
Binary Eutectic: In this context, it means
a phase diagram that consists of two
elemwnts that are not completely solublein the solid state
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Binary Isomorphous: In this context, it
means a phase diagram that consists of
two elements that are completely soluble
in both the liquid and solid states.
Body-centered:A structure in which
every atom is surrounded by eight
adjacent atoms, whether the atom is
located at a corner or at the center of a
unit cell.
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Cementite: The second phase formed
when carbon is in excess of the solubilitylimit.
Component: Either a pure metal or a
compound of which an alloy is composed. Compositions: The concentrations of all
components or elements that are found in
an alloy.
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Constituents: Severing to form, compose,
or make up a unit or whole: component
Critical point: Point where the densitiesof liquid and vapor become equal and the
interface between the two vanishes.
Above this point, only one phase can exist Delta iron: The body-centered cubic
phase which results when austenite is no
longer the most stable form of iron. Existsbetween 2802 and 2552 degrees F, has
BCC lattice structure and is magnetic
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Dilatometer:An instrument used to
measure thermal expansion in solids,liquids, and gases.
Eutectic:A eutectic system occurs when
a liquid phase tramsforms directly to a
two-phase solid.
Eutectoid:A eutectoid system occurs
when a single-phase solid transforms
directly to a two-phase solid
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Face-centered:A structure in which there
is an atom at the corner of each unit cell
and one in the center of each face, but noatom in the center of the cube.
Ferrite: Body-centered cubic iron or an
iron alloy based on this structure
Fine pearlite:Results from thin lamellae
when cooling rates are accelerated anddiffusion is limited to shorter distances
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Free Energy:A function of the internal
energy of a system - a measure of entropy
of a system
Heterogeneous system: When a system
is composed of multiple phases
Homogeneous system: When only onephase is present in the system
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Hypereutectoid: Hypereutectoid systemsexist below the eutectoid temperature.
Hypoeutectoid: Hypoeutectoid systemsexist above the eutectoid temperature
Isomorphus: Having the same structure.In the phase diagram sense, isomorphicitymeans having the same crystal structureor complete solid solubility for allcompositions.
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Incongruent phase transformation: This
is a phase transformation where at least
one of the phases involved undergoes achange in composition
Lamellae:A thin scale, plate, or layer ofphases
Ledeburite: Eutectic of cast iron. It exists
when the carbon content is greater than 2percent. It contains 4.3 percent carbon in
combination with iron.
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Lever rule:A mathematical expression
whereby the relative phase amounts in a
two-phase alloy at equilibrium may becomputed
Liquidus Line: On a binary phase
diagram, that line or boundary separatingliquid and liquid + solid phase regions. For
an alloy, the liquidus temperature is that
temperature at which a solid phase firstforms under conditions of equilibrium
cooling
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Martensite:An unstable polymorphic phase
of iron which forms at temperatures below
the eutectoid because the face-centeredcubic structure of austenite becomes
unstable. It changes spontaneously to a
body-centered structure by shearing action,not diffusion.
Mass spectrometer:An instrument used to
separate charged particles from a prepared
beam by means of an electromagnetic field
and to photograph the resulting distribution
or spectrum of masses
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Microstructure: Structure of the phases
in a material. Can only be seen with an
optical or electron mircoscope.
Pearlite:A lamellar mixture of ferrite and
carbide formed by decomposing austenite
of eutectoid composition.
Phase:A homogeneous portion of a
system that has uniform physical and
chemical characteristics
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Phase diagram:A graphical
representation of the relationships
between environmental constraints,composition, and regions of phase
stability, ordinarily under conditions of
equilibrium. Polymorphic: The ability of a solid
material to exist in more than one form or
crystal structure. Quench: To rapidly cool
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Reference State: Usually an elements
most stable state at a given temperature
Solidus Line: On a phase diagram, thelocus of points at which solidification is
complete upon equilibrium cooling, or at
which melting begins upon equilibriumheating.
Solubility: The amount of substance that
will dissolve in a given amount of anothersubstance.
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Tie line:A horizontal line constructed
across a two phase region of a binary
phase diagram; its intersections with thephase boundaries on either end represent
the equilibrium conditions of the respective
phases at the temperature in question. Xray diffraction: The scattering of x-rays
by atoms in a crystal, producing a
diffraction pattern that yields informationabout the structure of the crystal
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Xray flourescence:A method of
determining the amount of an element
present in a substance by bombarding itwith electrons and measuring the intensity
of the charicteristic x-rays that are given
off by the sample.