Ch. 9 - Phase Diagrams Flashcards
Component
Pure metals and/or compounds of which an alloy is composed.
Solvent
The base.
Solute
The thing that is being dissolved.
Solubility limit
The maximum concentration of solute atoms that may dissolve in the solvent to form a solid solution.
Adding any solute in excess of this limit results in the formation of another solid solution or compound that has a distinctly different composition.
Equilibrium
When a system’s free energy is at a minimum under some specified combination of temperature, pressure, and composition. Characteristics of the system do not change with time but persist indefinitely.
Free energy
Function of the internal energy and the randomness of the atoms or molecules (entropy).
Phase equilibrium
Equilibrium as it applies to systems in which more than one phase may exist. Reflected by a consultancy with time in the phase characteristics of a system.
Metastable
A state of true equilibrium is never completely achieved because the rate of approach is extremely slow. The system is in nonequilibrium. This state may persist indefinitely, experiencing only extremely slight and almost imperceptible changes as time progresses.
Isomorphous
As in the nickel-copper system. The two elements are nearly completely soluble within one another. This is due to the fact that the two elements have the same crystal structure (FCC), nearly identical atomic radii and electronegativities, and similar valences.
Tie line
An isotherm that extends across the two-phase region and terminates at the phase boundary lines on either side. Perpendiculars to the intersections are dropped and the wt% of the solute at the boundary can be determined.
Lever rule / inverse lever rule
Tie line is constructed. The overall alloy composition is located on the tie line. The fraction of one phase is computed by taking the length of the tie line from the overall alloy composition to the phase boundary for the OTHER phase and dividing this length by the total tie length.
Equilibrium cooling
Extremely slow cooling. Sufficient time must be allowed at each temperature for the appropriate compositional readjustments.
Nonequilibrium cooling
In practice, it is impossible to cool slow enough for compositional readjustments to fully realize. When cooling from a liquid to a solid, this type of solidification results in grains that have higher concentrations of the solid at their center. The AVERAGE concentration of the grain will be approximately what is expected based on the intersection of the tie line. The degree of displacement of the new solidus curve from the equilibrium solidus curve depends on the rate of cooling. Less time given to cooling means greater displacement.
Generally results in a cored structure, which gives rise to less than optimal properties. Grain boundaries will melt faster because they are less rich in the higher-melting component. Results in a sudden loss of mechanical integrity
Segregation
Concentration gradients are established across the grains as a result of nonequilibrium cooling.
Solvus line
Solid solubility limit line separating the A solid and the A solid + B solid phase regions.
Solidus line
Boundary between the solid region and the solid + Liquid regions. Basically represents the lowest temperature at which a liquid can exist.
Liquidus line
The line between The Liquid region and the Liquid + solid region.
Invariant point
The point corresponding to the eutectic concentration and temperature.
Eutectic reaction
When cooling, the Liquid turns into two different solids, A and B. This occurs at the lowest temperature at which the liquid will still be liquid. Handy for solder.
Eutectic structure
Microstructure that occurs at a temperature just below the eutectic point, and it consists of alternating layers (lamellae) of the two solids, A and B. Pearlite.
Two forms of A (alpha) solid
When the hypoeutectic microstructure forms, A grains form within the pearlite. The solid A grains are known as primary A and the A that exists in the pearlite is known as Eutectic A. The eutectic A will have the same weight percentage as it did immediately before it crossed the eutectic isotherm. The pearlite will have the same weight percentage as the liquid.
Microconstituent
An element of the microstructure having an identifiable and characteristic structure.
Terminal solid solutions
Solid solutions of a phase diagram that exist only at the extreme horizontals of the diagram.
Intermediate solid solutions
A solid that exists within a phase diagram somewhere other than at one of the two horizontal extremes.
Eutectoid reaction
When cooled, a solid phase turns into two different solid phases.
Peritectic reaction
When cooled, a solid + liquid phase turn into a single solid phase.
Congruent transformations
Those phase transformations for which there are no compositional alterations.
Eutectic and eutectoid reactions are INcongruent transformations.
Ferrite
Part of the iron-iron carbide phase diagram. The stable form that exists at room temperature, also known as alpha iron. Has a BCC crystal structure.
Austenite
Part of the iron-iron carbide phase diagram. Occurs when ferrite experiences a polymorphic transformation to an FCC structure at 912°C. Also known as gamma (y) iron.
Cementite
Part of the iron-iron carbide phase diagram. Formed at 6.70 wt% C. Fe3C. Prior to this phase, the iron-carbon system is considered to be “iron rich”.
Delta ferrite
Part of the iron-iron carbide phase diagram. Occurs when austenite (gamma ferrite) reaches 1394°C and reverts back to a BCC structure.
Eutectic of iron-iron carbide system
L –> austenite + cementite
Carbon concentrations in iron
Iron: <0.008 wt% C
Steel: 0.008 to 2.14 wt% C
Cast iron: 2.14 to 6.70 wt% C
Pearlite
Microstructure of eutectoid steel that is slowly cooled through the eutectoid temperature. Consists of alternating layers or lamellae of the two phases (ferrite and cementite). Carbon atoms diffuse away from the 0.022 wt% ferrite and goes to the 6.70 wt% cementite layers.
Hypoeutectoid alloy
When cooling occurs to the left of the eutectoid, between 0.022 and 0.76 wt% C. Consists of grains of pearlite (which form from the austenite) and grains of ferrite (which begin growing in the ferrite + austenite phase region). The ferrite in the pearlite is known as proeutectoid ferrite.
Hypereutectoid alloys
Those microstructures that are formed to the right of the eutectoid point, between 0.76 and 2.14 wt% C. Consists of grains of pearlite (which form from the austenite) and grains of cementite (which begin growing in the cementite + austenite phase region). The cementite in the pearlite is known as proeutectoid cementite.
