3. Phase Diagrams and Alloying Flashcards
what is a solid solution? two types
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
two rules for interstitial solutions
solute atoms must be small, there is always a limited solid solubility (saturation of interstitial atoms)
three rules of substitutional solid solutions
substitutional atoms must have similar atomic radii, similar crystal structures for each (ie both FCC or both BCC by themselves) can have unlimited solubility
what affects solubility limits for solid solutions?
TEMPERATURE
what type of phase diagram exists for a system with unlimited/complete solid solubility
isomorphous (like the phase diagrams in 140)
Liquidus
the line in a phase diagram above which the system exists in liquid only
Solidus
the line in a phase diagram below which the system exists in solid only
phase diagram of system with limited solid solubility
eutectic phase diagram
three phases in a limited solid solubility system
alpha, silver in copper solid solution beta, copper in silver solid solution liquid phase
two phases in unlimited solid solubility system
alpha, solid solution 0-100% liquid phase
what is meant by the eutectic composition
the specific composition of metal % where the alloy melts at a single temperature, the lowest melting point for this alloy
eutectic reaction
when a single liquid phase, on cooling, becomes two different solid phases. these two solid phases, alpha and beta, form at the same time
what are crystallographically equivalent vectors?
when the atomic spacing along one vector is the same as another direction (ie you can rotate the system and have the points superimpose)
if an alloy eutectic composition contains 70% silver, what are the names of the alloy compositions containing less than or more than 70% silver?
more: hypereutectic alloy. less: hypoeutectic
when cooling down a noneutectic alloy, what is the name of the solid that forms first?
proeutectic solid
Recall the Binary eutectic diagram (use your IMAGINATION)
when a 100% alpha composition is cooled down and beta first forms, where does beta form?
on the grain boundaries of the alpha
How do you find how much eutectic solid you form for a given alloy?
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.
What is the eutectoid reaction of the iron-carbon alloy system?
gamma iron 0.8% carbon (austenite) cooling down to below (723) forming alpha iron (ferrite) and Fe3C (Cementite)
What unit cell structure does gamma iron/austenite and alpha iron/ferrite have respectively?
gamma/austenite : FCC; alpha/ferrite: BCC
Proeutectoid solid for hypoeutectoid steel
alpha, it is the solid you form first from the gamma iron (austenite)
Proeutectoid solid for a hypereutectoid steel
Fe3C/cementite, this is the solid formed first from the gamma/austenite
What is the special name given to the eutectoid structure of the iron-carbon alloy system?
Pearlite
