MIME 260 lecture 8

tie line(isotherm)

connects the phases in equilibrium with each other

at eutectic composition but below eutectic temperature

eutectic microstructure(lamellar), alternating layers of alpha and beta phases

intermetallic compound

exists as a line on the diagram not an area because of stoichiometry(cant have NaCl where there is 1.3 Na and 0.7 Cl)

eutectic

liquid transforms to two solid phases

eutectoid

one solid phase transforms to two other solid phases

peritectic

liquid and one solid phase transforms to a second solid phase

component

one of a collection of chemically independent constituents of a system

phase

a region of space throughout which all physical properties of a material are essentially uniform.

homogenous phase

only one phase in the system

heterogenous

multiple phases in the system

alloy

mixture of metals, or a metal combined with one or more other elements

components

pure metals or compounds

solute

dissolves in solvent

solubility limit

changes with temperature, maximum concentration for which only a single phase solution exists

solution

solid, liquid, or gas solutions, single phase

what is considered to be a phase

pure metal or compound; solid, liquid, or gaseous solution; different polymorphs(crystal structures)

if same crystal structure, similar electronegativity, similar atomic radii(Ni and Cu)…

totally soluble in one another for all proportions(isomorphous)

hypoeutectic

less wt% of element than at its eutectic reaction point

hypereutectic

more wt% of element than at its eutectic reaction point

pure iron undergoes __ allotropic transformations

2, (alpha-ferrite)BCC -> (gamma-austenite)FCC → (delta-ferrite)BCC

Fe3C or cementite

iron carbide containing 6.7% carbon

carbon forms _ solid solution with iron

interstitial

iron

<0.008% C

steels

0.008-2.14% C

cast irons

2.14-4.5% C

gamma austenite → alpha-ferrite + Fe3C reaction

fundamental to the heat treatment of steels, lamellar structure(pearlite)

pearlite

microconstituent mixture of alpha ferrite and cementite, looks like mother of pearl

pearlite forms in…

colonies, lamellae have same orientation, thick light areas are ferrite, cementite appears dark

mechanical properties of pearlite

mixture of soft, ductile, ferrite and hard, brittle cementite

hypereutectoid steel=

proeutectoid cementite + pearlite colonies

Alloying elements have strong effect on…

eutectoid temperature and composition

cooling rates influence microstructural constituents:

phase transformations not according to phase diagram, presence of metastable phases at room temperature(martensite)

Hypoeutectic steel=

Proeutectoid ferrite + pearlite