Materials Science and Engineering an Introduction Chapter 9

austenite

Face-centered cubic iron; also iron and steel alloys that have the FCC crystal structure.

cementite

Iron Carbide (Fe3C)

component

A chemical constituent (element or compound) of an alloy that may be used to specify its composition.

congruent transformation

A transformation of one phase to another of the same composition.

equilibrium

The state of a system in which the phase characteristics remain constant over indefinite time periods. At equilibrium the free energy is a minimum.

eutectic phase

one of the two phases found in the eutectic structure

eutectic reaction

A reaction wherein, upon cooling, a liquid phase transforms isothermally and reversibly into two intimately mixed solid phases.

eutectic structure

A two-phase microstructure resulting from the solidification of a liquid having the eutectic composition; the phases exist as lamellae that alternate with one another.

eutectoid reaction

A reaction wherein, upon cooling, one solid phase transforms isothermally and reversibly into two new solid phases that are intimately mixed.

ferrite

Body-centered cubic iron; also iron and steel alloys that have the BCC crystal structure.

free energy

A thermodynamic quantity that is a function of both the internal energy and entropy (or randomness) of a system. At equilibrium, the free energy is at a minimum.

Gibbs phase rule (equation also)

for a system at equilibrium, an equation that expresses the relationship between the number of phases present and the number of externally controllable variables

hypereutectoid alloy

For an alloy system displaying a eutectoid, an alloy for which the concentration of solute is greater than the eutectoid composition.

hypoeutectoid alloy

For an alloy system displaying a eutectoid, an alloy for which the concentration of solute is less than the eutectoid composition.

intermediate solid solution

a solid solution or phase having a composition range that does not extend to either of the pure components of the system

intermetallic compound

A compound of two metals that has a distinct chemical formula. On a phase diagram it appears as an intermediate phase that exists over a very narrow range of compositions.

isomorphous

Having the same structure. In the phase diagram sense, isomorphicity means having the same crystal structure or complete solid solubility for all compositions.

lever rule (equation also)

a mathematical expression by which the relative phase amounts in a two-phase alloy at equilibrium may be computed

liquidus line

On a binary phase diagram, the line or boundary separating liquid- and liquid + solid-phase regions. For an alloy, the liquidus temperature is the temperature at which a solid phase first forms under conditions of equilibrium cooling.

metastable

A nonequilibrium state that may persist for a very long time.

microconstituent

An element of the microstructure that has an identifiable and characteristic structure. It may consist of more than one phase, such as with pearlite.

pearlite

A two-phase microstructure found in some steels and cast irons; it results from the transformation of austenite of eutectoid composition and consists of alternating layers (or lamellae) of -ferrite and cementite.

peritectic reaction

A reaction in which, upon cooling, a solid and a liquid phase transform isothermally and reversibly to a solid phase having a different composition.

phase

a homogeneous portion of a system that has uniform physical and chemical characteristics

phase diagram

A graphical representation of the relationships among environmental constraints (e.g., temperature and sometimes pressure), composition, and regions of phase stability, typically under conditions of equilibrium.

phase equilibrium

The state of a system in which the phase characteristics remain constant over indefinite time periods.

primary phase

a phase that exists in addition to the eutectic structure

proeuctectoid cementite

Primary cementite that exists in addition to pearlite for hypoeutectoid steels

proeutectoid ferrite

Primary ferrite that exists in addition to pearlite for hypoeutectoid steels.

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 limit

The maximum concentration of solute that may be added without forming a new phase.

solvus line

The locus of points on a phase diagram representing the limit of solid solubility as a function of temperature.

Terminal Solid Solution

A solid solution that exists over a composition range extending to either composition extreme of a binary phase diagram.

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 compositions of the respective phases at the temperature in question.