3. Phase Diagrams and Alloying

Question 1

what is a solid solution? two types

Answer 1

like a salt water solution except the solvent is the parent lattice and the solute is the interstitial/substitute atoms. the two types are interstitial/substitutional solid solutions

Question 2

two rules for interstitial solutions

Answer 2

solute atoms must be small, there is always a limited solid solubility (saturation of interstitial atoms)

Question 3

three rules of substitutional solid solutions

Answer 3

substitutional atoms must have similar atomic radii, similar crystal structures for each (ie both FCC or both BCC by themselves) can have unlimited solubility

Question 4

what affects solubility limits for solid solutions?

Answer 4

TEMPERATURE

Question 5

what type of phase diagram exists for a system with unlimited/complete solid solubility

Answer 5

isomorphous (like the phase diagrams in 140)

Question 6

Liquidus

Answer 6

the line in a phase diagram above which the system exists in liquid only

Question 7

Solidus

Answer 7

the line in a phase diagram below which the system exists in solid only

Question 8

phase diagram of system with limited solid solubility

Answer 8

eutectic phase diagram

Question 9

three phases in a limited solid solubility system

Answer 9

alpha, silver in copper solid solution beta, copper in silver solid solution liquid phase

Question 10

two phases in unlimited solid solubility system

Answer 10

alpha, solid solution 0-100% liquid phase

Question 11

what is meant by the eutectic composition

Answer 11

the specific composition of metal % where the alloy melts at a single temperature, the lowest melting point for this alloy

Question 12

eutectic reaction

Answer 12

when a single liquid phase, on cooling, becomes two different solid phases. these two solid phases, alpha and beta, form at the same time

Question 13

what are crystallographically equivalent vectors?

Answer 13

when the atomic spacing along one vector is the same as another direction (ie you can rotate the system and have the points superimpose)

Question 14

if an alloy eutectic composition contains 70% silver, what are the names of the alloy compositions containing less than or more than 70% silver?

Answer 14

more: hypereutectic alloy. less: hypoeutectic

Question 15

when cooling down a noneutectic alloy, what is the name of the solid that forms first?

Answer 15

proeutectic solid

Question 16

Recall the Binary eutectic diagram (use your IMAGINATION)

Answer 16

Question 17

when a 100% alpha composition is cooled down and beta first forms, where does beta form?

Answer 17

on the grain boundaries of the alpha

Question 18

How do you find how much eutectic solid you form for a given alloy?

Answer 18

Use the binary eutectic phase diagram to find the amount of liquid just before solidifcation, this is equal to how much eutectic solid is formed.

Question 19

What is the eutectoid reaction of the iron-carbon alloy system?

Answer 19

gamma iron 0.8% carbon (austenite) cooling down to below (723) forming alpha iron (ferrite) and Fe_3C (Cementite)

Question 20

What unit cell structure does gamma iron/austenite and alpha iron/ferrite have respectively?

Answer 20

gamma/austenite : FCC; alpha/ferrite: BCC

Question 21

Proeutectoid solid for hypoeutectoid steel

Answer 21

alpha, it is the solid you form first from the gamma iron (austenite)

Question 22

Proeutectoid solid for a hypereutectoid steel

Answer 22

Fe₃C/cementite, this is the solid formed first from the gamma/austenite

Question 23

What is the special name given to the eutectoid structure of the iron-carbon alloy system?

Answer 23

Pearlite