PHASE DIAGRAMS Phase a chemically and structurally homogenous region of a material. Region of uniform physical and chemical characteristics. Phase boundaries separate two distinct phases. A single phase system is called homogeneous. A system with two or more phases is called heterogeneous. Phase Diagram a graphic representation showing the phase or phases present for a given composition, temperature and pressure. Component the chemical elements which make up the alloy. Solvent atoms: primary atomic species. Host atoms Solute atoms: the impurities. Normally the minor component Solubility Limit Maximum concentration of solute atoms that may dissolve in the solvent to form a solid solution. The excess of solute forms another phase of different composition. Example: water sugar
Phase Diagrams of Pure Substances Predicts the stable phase as a function of P total and T. Example: water can exist in solid, liquid and vapor phases, depending on the conditions of temperature and pressure. Characteristic shape punctuated by unique points. Phase equilibrium lines Triple Point (three different phases of water in equilibrium) Critical Point Example: In the pressure temperature (PT) phase diagram of water there exists a triple point at low pressure (4.579 torr) and low temperature (0.0098 o C) where solid, liquid and vapor phases of water coexists. Vaporization Line Liquid and vapor coexists Freezing Line Liquid and solid coexist. Sublimation Line Solid and vapor coexist
Phase Any portion including the whole of a system, which is physically homogeneous within it and bounded by a surface so that it is mechanically separable from any other portions.
Gibbs Phase Rule From thermodynamic considerations, J.W. Gibbs (1839 1903 American physicist University of Yale) derived the following equation: P + F C + 2 Where P number of phases which coexists in a given system F degrees of freedom C number of components in the system 2 one can vary temperature and pressure F 0 zero degrees of freedom. Neither P or T can be change (a point invariant point) F 1 one degree of freedom. One variable (P or T) can be changed independently (a line) F 2 two degrees of freedom. Two variables (P or T) can be changed independently (an area).
Schematic unary phase diagram for magnesium, showing the melting and boiling temperatures at one atmosphere pressure. C 1 for pure magnesium Point A: P 1 for pure liquid phase 2+CF+P 2+1F+1 F2 degrees of freedom change pressure and temperature in liquid Phase. Point B: P 2 for liquid and solid 2+CF+P 2+1F+2 F1 degrees of freedom change pressure or temperature (and the other variable is dependent to stay on the line).
Point X: P 3 (liquid, solid and vapor coexist) 2+CF+P 2+1F+3 F0 degrees of freedom pressure and temperature are fixed at the the single point called the triple point.
Example For pure substance where P and T can be changed P + F C + 2 1 + 2 3 Pure substance in a triple point, then C 1 (one component) P 3 (number of phases that coexist) The value of F is zero (zero degrees of freedom) the three phases coexist in a point. and For pure substance where P and T can be changed P + F 1 + 2 3 Pure substance in a freezing line, then C 1 (one component) coexist) and P 2 (number of phases that The value of F is one (one degree of freedom) the two phases (solid and liquid) coexist in a line.
Solubility: The amount of one material that will completely dissolve in a second material without creating a second phase. Unlimited solubility: When the amount of one material that will dissolve in a second material without creating a second phase is unlimited. Limited solubility When only a maximum amount of a solute material can be dissolved in a solvent material.
Solid Solution:
Solid solution strengthening Increasing the strength of a metallic material via the formation of a solid solution. Dispersion strengthening Strengthening, typically used in metallic materials, by the formation of ultra fine dispersions of a second phase. The effects of several alloying elements on the yield strength of copper. Resistance to dislocation motion (loss in ductility)
Microstructure The structure observed under a microscope Al Brake more than one phase Iron chromium alloy one phase (solid solution)
Phase Equilibria Free energy: a function of the internal energy of a system Equilibrium: a system is at equilibrium if its free energy is at a minimum Phase equilibrium: for a system which has more than one phase Phase Diagram is a diagram with T and Composition as axes. They define the stability of the phases that can occur in an alloy system at constant pressure (P). The plots consist of temperature (vertical) axis and compositional (horizontal) axis. Constitution: is described by (a) the phases present (b) the composition of each phase (c) the weight fraction of each phase
Isomorphous Phase Diagrams Binary phase diagram A phase diagram for a system with two components (C2). Ternary phase diagram A phase diagram for a system with three components (C3). Isomorphous phase diagram A phase diagram in which components display unlimited solid solubility. Liquidus temperature The temperature at which the first solid begins to form during solidification. Solidus temperature The temperature below which all liquid has completely solidified. Freezing range between the liquidus and solidus.
Binary isomorphous systems Binary alloy: A mixture of two metals is called a binary alloy and constitute a two component system. Each metallic element in an alloy is called a separate component. [Sometimes a compound is considered a component, (e.g., iron carbide)] Isomorphous System: In some metallic systems, the two elements are completely soluble in each other in both the liquid and solid states. In these systems only a single type of crystal structure exists for all compositions of the components (alloy) and therefore it is called isomorphous system.
Example: Binary Isomorphous System (Cu Ni) T<1085 o C: Cu & Ni are mutually soluble in solid state complete solubility both have the same FCC structures, atomic radii and electronegativities are nearly identical similar valences isomorphous
Interpretation of Phase Diagrams Constitution: is described by (a) the phases present (b) the composition of each phase (c) the weight fraction of each phase
(a) Phases Present Point A: at T1100 o C 60wt% Ni 40wt% Cu Only phase is present Point B: at T 1250 o C 35wt%Ni 65wt% Cu Both & liquid phases are present at equilibrium (b) Composition of each phase Single phase: Point A: 60wt%Ni 40%Cu alloy at 1100 o C
Two phase region: Tie line: across the twophase region at the temperature of the alloy Point B: T1250 o C Composition of Liquid phase: C L 31.5wt%Ni 68.5%Cu Composition of phase: C 42.5wt%Ni 57.5wt%Cu
(c) Weight fraction of each phase Single phase: 100% Ex: Point A: 100% phase Two phase region: Ex: Point B LEVER RULE (Inverse Lever Rule) L L o L o L L C C C C W C C C C W S R R W S R S W + +
Example: Point B: C 0 35wt%Ni C 42.5%, C L 31.5% 68% or.... 32% or.... 0 68 31 5 42 5 35 42 5 0 32 31 5 42 5 31 5 35 L s o s L L s L o c c c c W c c c c W Volume fraction For an alloy consisting of and β phases, the volume fraction of the phase is defined as β β β β β β β β β ρ ρ ρ ρ ρ ρ v v v W v v v W V V v v v V + + + + ;, 1 Then, the weight fractions are Where ν and ν β are the volumes of and β β β ρ W ρ W ρ W V + β β β β β ρ W ρ W ρ W V +
Derivation of the lever rule 1) All material must be in one phase or the other: 2) Mass of a component that is present in both phases equal to the mass of the component in one phase + mass of the component in the second phase: 3) Solution of these equations gives us the Lever rule. W + W L W c + W c L L 1 c o W c c o c c L L W L c c c c o L
Equilibrium Cooling Development of Microstructure in Isomorphous Alloys Example: 35wt%Cu 65wt%Ni system Slow cooling from point a to point e
a: 1300 o C: complete liquid with 35wt%Cu- 65wt%Ni b: ~1260 o C: first solid begin to form (-46wt%Ni) c: ~1250 o C: -43wt%Ni, L- 32wt%Ni d:~1220 o C: last liquid to solidify e: 35wt%Cu 65wt%Ni solid phase
Nonequilibrium Cooling Development of Microstructure in Isomorphous Alloys Fast cooling Compositional changes require diffusion
Diffusion in the solid state is very slow. The new layers that solidify on top of the existing grains have the equilibrium composition at that temperature Formation of layered (cored) grains. Tie line method to determine the composition of the solid phase is invalid. The tie line method works for the liquid phase, where diffusion is fast. Solidus line is shifted to the right (higher Ni contents), solidification is complete at lower T, the outer part of the grains are richer in the low melting component (Cu). Upon heating grain boundaries will melt first. This can lead to premature mechanical failure.
Complete solidification occurs at lower temperature and higher Nickel concentration than equilibrium Solid can t freeze fast enough: solidus line effectively shifted to higher Ni concentrations. Shift increases with faster cooling rates, slower diffusion
Mechanical properties of isomorphous alloys Solid solution strengthening
The mechanical properties of coppernickel alloys. Copper is strengthened by up to 60% Ni and nickel is strengthened by up to 40% Cu.
Solidification of a Solid Solution Alloy Segregation The presence of composition differences in a material, often caused by insufficient time for diffusion during solidification.
Non Equilibrium Solidification and Segregation Coring Chemical segregation in cast products, also known as microsegregation or interdendritic segregation. Homogenization heat treatment The heat treatment used to reduce the microsegregation caused during nonequilibrium solidification. Macrosegregation The presence of composition differences in a material over large distances caused by nonequilibrium solidification.
Invariant Points in Binary Systems Binary alloys two components at ambient pressure. Gibbs rule states that P + F 2 + 1 3. If three phases coexists (P 3), they coexist at a point (zero degrees of freedom the invariant point, at a specific temperature and chemical composition Types of invariant points: eutectic, eutectoid, peritectic peritectoid, monotectic etc.
Five of the Most Important Three Phase Reactions (Invariant Points) in Binary Diagrams eutectic: Liquid/solid reaction eutectoid: solid/solid reaction
1150 o C: The in between point is at 15% B. δ + L are present above the point, γ is present below. The reaction is: δ + L γ, a peritectic 920 o C: This reaction occurs at 40% B: L1 γ + L2 a monotectic 750 o C: This reaction occurs at 70% B: L γ + β, a eutectic 450 o C: This reaction occurs at 20% B: γ + β, a eutectoid 300 o C: This reaction occurs at 50% B: + β μ or a peritectoid
Eutectic Systems The simplest kind of system with two solid phases is called a eutectic system. A eutectic system contains two solid phases at low temperature. These phases may have different crystal structures, or the same crystal structure with different lattice parameters. Examples: Pb(FCC) and Sn (tetragonal) solder systems Fe (BCC) and C (graphite hexagonal) cast irons Al (FCC) and Si (diamond cubic) cast aluminum alloys Cu(FCC) and Ag(FCC) high temperature solder
Cu/Ag Eutectic System Copper and Silver are both FCC, but their lattice parameters and atomic radii are very different, so they have limited solubility in the solid state. There are two solid stable phases and β, and at high temperatures there is a eutectic reaction where the solids, β and the liquid coexist. Heating L( CE ) ( CE ) + β ( CβE ) Cooling Hypoeutectic alloy An alloy composition between that of the lefthand side end of the tie line defining the eutectic reaction and the eutectic composition. Hypereutectic alloys An alloy composition between that of the righthand side end of the tie line defining the eutectic reaction and the eutectic composition.
Cu Ag System Cu: phase Ag: β phase Eutectic means easily melted in Greek Point E: invariant point (eutectic point) BG line: isotherm line
AB & FG: Solidus line BC & GH: Solvus line AE & EF: Liquidus line BEG: Solidus line, isotherm line T E : eutectic isotherm
Eutectic isotherm Invariant or eutectic point
Eutectic Reaction: For copper silver system: L Eutectic or invariant point Liquid and two solid phases co exist in equilibrium at the eutectic composition C E and the eutectic temperature T E. Eutectic isotherm horizontal solidus line at T E. L Heating ( C ) ( C ) + β ( C ) E Cooling Heating ( 71.9wt% Ag) ( 8.0wt% Ag) + β ( 91.2wt% Ag) Cooling E βe
Binary Eutectic System Eutectic reaction transition from liquid to mixture of two solid phases, + β at eutectic concentration C E. At most two phases can be in equilibrium. Three phases (L,, β) may be in equilibrium only at a few points along the eutectic isotherm. Single phase regions are separated by 2 phase regions.
Binary Eutectic System Compositions and relative amounts of phases are determined from the same tie lines and lever rule, as for isomorphous alloys demonstrate A B C
Example For Point C: 40wt%Sn 60wt%Pb alloy at 150 o C a) What are the phases present? b) What are the compositions of the phases present? c) Mass fraction? d) Volume fraction at 150 o C? Knowing that the densities of Pb and Sn are 11.23 and 7.24g/cm 3, respectively
a) At C, and β phases coexist b) Draw Tie Line at 150 o C: For phase: C 10% 10wt%Sn 90wt%Pb For β phase: C β 98% 98wt%Sn 2wt%Pb C
c) Mass fraction: d) volume fraction: where 0.34 10 98 10 40 0.66 10 98 40 98 1 1 β β β β C C C C W C C C C W 0.43 0.57 1 1 0.57 7.39 0.34 10.64 0.66 10.64 0.66 + + + β β β β ρ ρ ρ ν ν ν V V W W W V 3 3 3 3 3 3. 7.29. 11.23 2. 7.24 98 100 100. 10.64. 11.23 90. 7.24 10 100 100 + + + + cm g cm g cm g C C cm g cm g cm g C C Pb Pb Sn Sn Pb Pb Sn Sn ρ ρ ρ ρ ρ ρ β β
L L+ Development of microstructure in eutectic alloys (I) Several types of microstructure formed in slow cooling an different compositions. Cooling of liquid lead/tin system at different compositions. In this case of lead-rich alloy (0-2 wt% of tin) solidification proceeds in the same manner as for isomorphous alloys (e.g. Cu-Ni) that we discussed earlier.
L Development of microstructure in eutectic alloys (II) +L +β At compositions between room temperature solubility limit and the maximum solid solubility at the eutectic temperature, β phase nucleates as the solid solubility is exceeded at solvus line.
Development of microstructure in eutectic alloys (III) Solidification at the eutectic composition (I) No changes above eutectic temperature, T E. At T E liquid transforms to and β phases (eutectic reaction). L +β
Development of microstructure in eutectic alloys (IV) Solidification at the eutectic composition (II) Compositions of and β phases are very different eutectic reaction involves redistribution of Pb and Sn atoms by atomic diffusion. Simultaneous formation of and β phases results in a layered (lamellar) microstructure:called eutectic structure. Formation of eutectic structure in lead tin system. Dark layers are leadreach phase. Light layers are the tin reach β phase.
Development of microstructure in eutectic alloys (V) Compositions other than eutectic but within the range of the eutectic isotherm Primary phase is formed in the + L region, and the eutectic structure that includes layers of and β phases (called eutectic and eutectic β phases) is formed upon crossing the eutectic isotherm.
L +L +β
Development of microstructure in eutectic alloys (VI) Microconstituent element of microstructure having a distinctive structure. For case described on previous page, microstructure consists of two microconstituents, primary phase and the eutectic structure. Although the eutectic structure consists of two phases, it is a microconstituent with distinct lamellar structure and fixed ratio of the two phases.
Compositions of and β phases are very different eutectic reaction involves redistribution of Pb and Sn atoms by atomic diffusion. Simultaneous formation of and β phases results in a layered (lamellar) microstructure:called eutectic structure. Formation of eutectic structure in lead tin system. Dark layers are leadreach phase. Light layers are the tinreach β phase.
Relative amounts of microconstituents? Eutectic microconstituent forms from liquid having eutectic composition (61.9 wt% Sn) Treat the eutectic as if it were a separate phase and apply lever rule to find relative fractions of primary phase (18.3wt% Sn) and eutectic structure (61.9wt% Sn): W e W P ( P + Q) Q ( P + Q)... eutectic... primary Terminal solid solution: a solid solution that exists over a composition range extending to either composition extremity of a binary phase diagram.
(a) A hypoeutectic lead tin alloy. (b) A hypereutectic lead tin alloy. The dark constituent is the lead rich solid, the light constituent is the tin rich solid β, and the fine plate structure is the eutectic (x400).
The effect of the chemical composition and strengthening mechanism on the tensile strength of lead tin alloys.
Eutectic colonies and interlamellar spacing
Equilibrium Diagrams Having Intermediate Phases or Compounds Copper-zinc Intermediate solid solution: β γ δ ε and η: two terminal solid solution β, γ, δ, & ε are intermediate phases
Intermetallic Compounds Ex: magnesiumlead phase diagram: Intermetallic compound: Mg 2 Pb can exist by itself only at the precise composition of 19wt%Mg 81wt%Pb
Eutectoid Reaction (Invariant Point E at 560 o C) Copper zinc δ cooling heating γ + ε Eutectoid reaction
Peritectic Reaction (Invariant Point P at 598 o C) Copper zinc δ + L cooling heating ε Peritectic reaction