This document defines key terms related to iron-carbon phase diagrams and alloy microstructures, including different crystal structures (body-centered cubic, face-centered cubic), phases that can form in iron-carbon alloys (ferrite, austenite, cementite, pearlite, bainite, martensite), and concepts important to understanding phase diagrams (eutectic, eutectoid, solubility, lever rule). Definitions are provided for over 30 relevant materials science and metallurgy terms.

1. Iron-carbon phase
diagram & basic
definitions

2. Muhammad Umair Bukhari
Engr.umair.bukhari@gmail.com
www.bzuiam.webs.com
03136050151

6. • Alloy: A metallic substance that is
composed of two or more elements
• Austenite: Face-centered cubic iron or an
iron 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 soluble
in the solid state

7. • 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.

8. • Cementite: The second phase formed
when carbon is in excess of the solubility
limit.
• 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.

9. • Constituents: Severing to form, compose,
or make up a unit or whole: component
• Critical point: Point where the densities
of 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. Exists
between 2802 and 2552 degrees F, has
BCC lattice structure and is magnetic

10. • 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

11. • 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 no
atom 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 and
diffusion is limited to shorter distances

12. • 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 one
phase is present in the system

13. • Hypereutectoid: Hypereutectoid systems
exist below the eutectoid temperature.
• Hypoeutectoid: Hypoeutectoid systems
exist above the eutectoid temperature
• Isomorphus: Having the same structure.
In the phase diagram sense, isomorphicity
means having the same crystal structure
or complete solid solubility for all
compositions.

14. • Incongruent phase transformation: This
is a phase transformation where at least
one of the phases involved undergoes a
change in composition
• Lamellae: A thin scale, plate, or layer of
phases
• Ledeburite: Eutectic of cast iron. It exists
when the carbon content is greater than 2
percent. It contains 4.3 percent carbon in
combination with iron.

15. • Lever rule: A mathematical expression
whereby the relative phase amounts in a
two-phase alloy at equilibrium may be
computed
• Liquidus Line: On a binary phase
diagram, that line or boundary separating
liquid and liquid + solid phase regions. For
an alloy, the liquidus temperature is that
temperature at which a solid phase first
forms under conditions of equilibrium
cooling

16. • Martensite: An unstable polymorphic phase
of iron which forms at temperatures below
the eutectoid because the face-centered
cubic 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

17. • 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

18. • 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

19. • Reference State: Usually an elements
most stable state at a given temperature
• Solidus Line: On a phase diagram, the
locus of points at which solidification is
complete upon equilibrium cooling, or at
which melting begins upon equilibrium
heating.
• Solubility: The amount of substance that
will dissolve in a given amount of another
substance.

20. • Tie line: A horizontal line constructed
across a two phase region of a binary
phase diagram; its intersections with the
phase 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 information
about the structure of the crystal

21. • Xray flourescence: A method of
determining the amount of an element
present in a substance by bombarding it
with electrons and measuring the intensity
of the charicteristic x-rays that are given
off by the sample.