Materials- Metals: Phase Diagrams Flashcards
What is an alloy?
A metallic substance composed of two or more elements
What is a solid solution?
A homogeneous crystalline phase that contains two or more chemical species. The solute atoms occupy either the substitutional or interstitial positions in the solvent lattice and the crystal structure of the solvent is maintained.
Define solute
One component or element of a solution present in a minor concentration dissolved in the solvent
Define solvent
The component of a solution present in the greatest amount. It dissolves the solute
Define solubility limit for an alloy system
The maximum concentration of solute atoms that may dissolve in the solvent to form a solid solution at some specific temperature.
What happens if you add solute atoms in excess of the solubility limit?
It results in the formation of another solid solution or compound that has a distinctly different composition
What is a phase?
A homogenous portion of a system that has uniform physical and chemical characteristics.
Define homogeneous and heterogeneous
Homogenous: a single phase system
Heterogeneous: a system composed of two or more phases (mixture)
How does the formation of a solid solution depend on atomic size factor, crystal structure and electronegativity?
ASF- the difference in atomic radii between the two atoms must be less than about +/-15% otherwise the solute atoms will create substantial lattice distortions and a new phase will form.
CS- crystals structures for metals of both atom types must be the same
E- greater difference in electronegativity means greater likelihood that an intermetallic compound forms instead of substitutional solid solution
What is a multiphase alloy?
An alloy that consists of two or more solid crystal types and/or composition due to more solute atoms being added than can be in solid solution with the solvent
How is an alloy made for industrial use?
Melt and mix elements in proportions desired. Cast the mixture to form an ingot. Then there is homogenisation (heat treatment) by hot and cold rolling. This forms a plate or sheet of the alloy.
Describe the boundary separating phases in a multi phase system
Represents a discontinuous and abrupt change in physical and/or chemical characteristics
What is a binary phase diagram?
A map that represents the relationships between temperature and the compositions and quantities of phases at equilibrium, which influence the micro structure of an alloy. Pressure is held constant (normally 1atm).
What is ferrite?
α iron. BCC crystal structure. Forms below 910°C
What is austenite?
γ iron. FCC crystal structure. Forms above 910°C up to 1400°C.
Describe the binary phase diagram for the simplest alloy system
Y axis of temperature. X axis of composition (in % of one component). Has a liquidus line above which the material is completely liquid. Has a solidus line below which the substance is completely solid. Between the two lines (which meet at 0 and 100%) is the liquid+solid solid field where the alloy exists as a mixture of liquid and solid.
What do lower case Greek letters represent on a phase diagram?
Different solid solutions
For a single phase region of the binary phase diagram, what is composition of the phase?
The same composition as the alloy at that temperature
How to find the compositions of the two phases of a two-phase region of a binary phase diagram at a specified temperature and initial alloy composition, C0
Draw a tie line (isotherm) across the two-phase region at the temperature of the alloy. Note the intersections of the tie line and the phase boundaries on either side. Perpendiculars a dropped from these intersections to the horizontal composition axis from which the composition of each of the respective phases is read.
How to find the relative phase amounts in a two-phase region of a binary phase diagram for a known temperature and initial alloy composition (C0)
Construct a tie line across the two-phase region at the temperature of the alloy. The overall alloy composition is located on this tie line. The fraction of one phase is computed by taking the length of the tie line from the overall alloy composition (C0) to the phase boundary for the other phase and dividing by the total tie line length. If the composition axis is scaled in weight %, the answer will be a mass fraction. This is the lever rule
What is different about a binary eutectic phase diagram?
There is more than one type of solid solution phase possible in the alloy. The solvus line is the solubility limit line separating two different solid solution phases.
What is an invariant point on a binary eutectic phase diagram?
The point through which if the temperature changes (with the specific composition at that point), a eutectic reaction occurs. That is, upon cooling, a liquid phase is transformed into two or more solid phases. The opposite occurs upon heating.
Example of an equation representing a eutectic reaction
L(CE)α(CαE)+β(CβE) All letters after C are subscript CE represents the eutectic composition L represents liquid α and β represent different solid phases CαE and CβE are respective compositions of α and β at the eutectic temperature (written as 61.9wt% Sn for example)
What happens if you cool a eutectic alloy so it crosses the solvus line?
The solubility limit for one solid phase is exceeded, resulting in the formation of small particles of a different solid phase. With continued cooling, these particles grow in size.
What happens when you cool a eutectic alloy so it passes through an invariant point?
Upon crossing the eutectic temperature at the eutectic composition, the liquid transforms to the two solid phases simultaneously. During this transformation there is a redistribution of the two components (because the α and β phases have different compositions to each other and the liquid). This is accomplished by atomic diffusion just ahead of the eutectic-liquid interface. The microstructure of the solid formed consists of alternating layers (lamellae) of the α and β phases.
What is a eutectic structure?
A two-phase microstructure resulting from solidification of a liquid having the eutectic composition. The phases exist as lamellae that alternate with one another.
What is an 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
What is a eutectoid reaction?
A reaction wherein, upon cooling, one solid phase transforms isothermally and reversibly into two new solid phases that are intimately mixed
What is a peritectic reaction?
A reaction wherein upon cooling, a solid and a liquid phase transform isothermally and reversibly to a solid phase having a different composition
