IE-114 Materials Science and General Chemistry Lecture-10 ‘’PHASE DIAGRAMS’’ IE-114 Materials Science and General Chemistry Lecture-10

Importance of Phase Diagrams There is a strong correlation between microstructure and mechanical properties. Phase diagrams provides information about: - The development of microstructure during heating or cooling can be understood from the phase diagrams. - Melting, casting, crystallization, etc.

Phase-Diagram of Water-Sugar System Solubility Limit At some specific temperatures , there is a maximum concentration of solute atoms that may dissolve in the solvent to form a solid solution; this is called solubility limit. Answer: 65wt% sugar. If Co < 65wt%  syrup If Co > 65wt%  syrup + sugar. Question: What is the solubility limit at 20oC? Solubility limit increases with T: T = 20oC, solubility limit = 65 wt% sugar. T = 100oC, solubility limit = 80 wt% sugar.

Aluminum-Copper Alloy Component: Components are pure metals/or compounds of which alloy is composed. e.g. Brass (Cu-Zn) ; components are Cu and Zn, Fe and C in carbon steel, H2O and NaCl in salted water Solid Solution: A solid solution consists of solute atoms, which occupy either substitutional or interstitial positions in the solvent lattice (, , etc,..) Phase: Homogenous portion of a system that has uniform physical and chemical characteristics Aluminum-Copper Alloy Every pure material is considered to be a phase; so also is every solid, liquid and gaseous solution For example if a substance can exist in two or more polymorphic forms (BBC and FCC) each of these structures is a separate phase because their physical properties are different.

Mixture or heterogeneous system: Homogeneous system: A single phase system Mixture or heterogeneous system: System of two or more phases. Most of the metallic alloys,ceramics, polymeric and composite systems are heterogeneous. Phase Equilibria: A system is said to be at equilibrium when the free energy, which is the internal energy and randomness of the atoms, is at minimum under some specified combination of temperature, pressure and composition.

Equilibrium Phase Diagrams Phase diagram is also called equilibrium or constitutional diagram. These diagrams define the relationship between the temperature and compositions or quantities of phases at equilibrium. External pressure could also be another parameter affecting the phase distribution but it remains constant at 1 atm in most of the applications. Isomorphous Binary Phase Diagrams Eutectic Binary Phase Diagrams

1) Isomorphous Binary Systems: Binary systems are composed of two components and they are isomorphous since there is a complete solubility of liquids and solids. Example: Cu-Ni Cu Ni wt.% Ni There are 2 phases : - L (liquid) - α-solid solution There are 3 phase fields : liquid, L+α, α Liquidus line:The line separating L and α+L phases. Solidus line: The line separating α+L and α phases is called.

1) Number and types of phases From the phase diagrams we can learn the followings: -Number and types of phases that are present at different temperatures for a fixed composition -Composition of the phases -Fractions of the phases 1) Number and types of phases If we know T and Co, then we know the number and types of phases present. Examples: 1) Cu-35Ni Alloy at 1250oC (Co = 35 wt.%Ni) 2 phases: α+L 2)Cu-60Ni Alloy at 1100oC (Co = 60 wt.%Ni) 1 phase: α

2) Composition of phases: If we know T and Co, then we know the composition of each phase. For Cu-35Ni Alloy (Co=35wt%Ni) at TA: -Phases: 1(only Liquid) -Composition of the alloy: CL = Co (=35 wt%Ni, 65 wt%Cu) at TB: -Phases: 2 (Liquid +α) CL = Cliquidus (32wt%Ni, 68 wt% Cu) Cα = Csolidus (43wt%Ni, 57wt%Cu) at TD: -Phases: 1 (only α) Cα=Co (35 wt%Ni, 65 wt%Cu)

3) Weight fractions (or percentage) of phases: If we know T and Co, then we know the amount of each phase (given in wt%). x100 x100 = 27wt%

Microstructural development during cooling a Cu-Ni alloy • Binary System (2 components) Isomorphous i.e., complete solubility of one component in another; a phase field extends from 0 to 100wt% Ni.

Binary Eutectic Systems Eutectic reaction: Liquid (71.9%) (8%Ag) +  (91.2% Ag) heating cooling Eutectic composition: 71.9wt%Ag, 28,1wt%Cu Eutectic temperature: 779oC max. solubility of Cu in Ag (8.8 wt%) Eutectic line. This line shows the minimum temperature for the liquid phase existence. Max. Solubility of Ag in Cu 8 wt% Ag at 7790C SOLVUS SOLIDUS LIQUIDUS Melting point of pure Cu Melting point of pure Ag There are three phases : Liquid,  and  Cu Ag

For a 40wt%Sn-60wt%Pb alloy at 150oC, find: 1) the phases present 2) the compositions of the phases

For a 40wt%Sn-60wt%Sn alloy at 150oC, find: --the phases present: a + b --the compositions of the phases: Ca = 11wt%Sn, 89wt%Cu Cb = 99wt%Sn, 1wt%Cu --the relative amounts of each phase: x100

Microstructural Development During Cooling of Pb-Sn Alloys

• 18.3wt%Sn < Co < 61.9wt%Sn • Room temperature microstructure: a crystals and a eutectic microstructure

Cooling of an Alloy Having Eutectic Composition The alloy having eutectic composition is called eutectic alloy alternating layers of  and  crystals.

Hypoeutectic and Hypereutectic Alloy

Intermetallic Compounds For some alloy systems, discrete intermediate compounds rather than solid solutions may be observed in phase diagrams. For example; Mg-Pb system. These are called intermetallic compounds. The compound Mg2Pb is shown as a vertical line on the diagram rather than a phase region since it exists precisely at the composition defined.

Eutectoid and Peritectic Reactions Consider Cu-Zn system. Eutectoid reaction: 5600C and 74 wt% Zn-26 wt% Cu  γ +  cooling heating Peritectic reaction: 5980C and 78.6 wt% Zn-21.4 wt% Cu  + L  cooling heating

Congruent and Incongruent Phase Transformation Phase transformations can be classified according to whether or not there is any change in composition. Phase transformations in which there is no changes in composition are called as congruent transformations. The opposite is incongruent transformation. Allotropic transformations are congruent as well as melting pure metals. Eutectic, eutectoid or melting alloy systems are incongruent transformations.

Iron Carbon System This is the most important system in manufacturing since primary structural materials are essentially Fe-C alloys, such as, steel and cast iron. STEEL IS an ALLOY OF;

Iron-Iron Carbide Equilibrium Phase Diagram Phases and phase mixtures present in iron alloys; Ferrite (α) Cementite (Fe3C) Pearlite (ferrite + cementite) Austenite (γ) -ferrite Ledeburite (austenite + cementite)

Definition and Properties of Phases 1) Ferrite : -iron, Solid Solution, max. Carbon solubility 0.022%wt. at 727oC BCC structure, SOFT 2) Cementite : Iron carbide(Fe3C), contains 6.67% wt. C Orthorhombic structure, HARD and BRITTLE 3) Pearlite : Phase mixture (ferrite+cementite), Lamellar structure, contains ~0.8% wt. C Produced from austenite decomposition 4) Austenite : -iron, Solid solution, stable at higher temperatures (>727oC) Max. Carbon solubility is 2.14%wt. at 1147oC, FCC structure HIGH TOUGHNESS 5) Ledeburite: Eutectic phase mixture(austenite+Fe3C), seen in cast irons Contains 4.3 %wt. Carbon, forms at 1147oC 6) -ferrite : Solid solution, max. carbon solubility is 0.1%wt. At 1493oC

Invariant reactions in Fe-Fe3C Phase diagram 1) At 1493oC, 0.18 %wt C (PERITECTIC REACTION) Liquid(l, 0.5%C)+-ferrite(,0.1%C) Austenite(, 0.18%C) cooling heating 2) At 1147oC, 4.30 %wt C (EUTECTIC REACTION) Liquid(l, 4.30 %C) Austenite(, 2.14 %C) + Cementite(Fe3C,6.67%C) cooling heating LEDEBURITE 3) At 727oC, 0.77 %wt C (EUTECTOID REACTION) cooling heating Austenite(, 0.77 %C) Ferrite(, 0.022 %C) + Cementite(Fe3C,6.67%C) PEARLITE

STEELS CAST IRONS Hypereutectoid Steel Hypoeutectoid Steel

Eutectoid Steel

Hypoeutectoid Steel

Hypereutectoid Steel

Effect of Carbon on Mechanical Properties

EXAMPLE1: For an annealed (cooled in equilibrium conditions after austenitization) hypoeutectoid steel; Determine the composition of the steel (C wt.%) if quantitative metallographic analyses revealed 22% secondary ferrite (assume solubility of carbon in ferrite is nil) Calculate the weight fraction ratio of secondary ferrite and cementite present in pearlite. Determine the total amount of carbon dissolved in secondary ferrite at room temperature for the corresponding steel if the total weight of the steel part in kgs is equal to 80.(Room temperature solubility of carbon in ferrite is 0.008%) EXAMPLE2: The mass fraction of eutectoid cementite in iron carbon alloy is 0.109. On the basis of this information, is it possible to determine the composition of the alloy? If so, what is its composition?