Phase Transformation-sultan

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PHASE TRANSFORMATIONPhase Diagram : 

PHASE TRANSFORMATIONPhase Diagram

What’s in this microstructure? : 

What’s in this microstructure? What are the light coloured and dark coloured (lamellar like) things in the microstructure. Before we try to find out what are these there are some essentials which are required to be known

Review (Concept of solubility) : 

Review (Concept of solubility) The three forms of water – gas, liquid, and solid – are each a phase. Water and alcohol have unlimited solubility. Salt and water have limited solubility. Oil and water have virtually no solubility. Illustration of phases and solubility:

Dislocations : 

Dislocations

Edge and Screw Dislocations : 

Edge and Screw Dislocations

Metal Deformation : 

Metal Deformation The ability of a metal to deform depends on the ability of dislocations to move.

Dislocation Interaction : 

Two edge dislocations of the same sign and lying on the same slip plane exert a repulsive force on each other Edge dislocations of opposite sign and lying on the same slip plane exert an attractive force on each other Dislocation Interaction

Strengthening mechanisms in metals : 

Strengthening mechanisms in metals Restricting dislocation motion can make material stronger Mechanisms of strengthening in metals grain-size reduction strain hardening solid-solution alloying

Strengthening mechanismsGrain Size Reduction : 

Strengthening mechanismsGrain Size Reduction Grain boundaries act as barrier to dislocation motion The smaller the grain size, more the number of grain boundaries hence greater the strength

Strengthening mechanismsStrain Hardening : 

Strengthening mechanismsStrain Hardening If plastic strain has taken place then Proportional limit and elastic limit would increase because of strain hardening Point of rupture remains unchanged- ductility reduces In extreme cases may rupture without warning

Strengthening mechanisms Solid-Solution Strengthening : 

Strengthening mechanisms Solid-Solution Strengthening Interstitial or substitutional impurities cause lattice strain and interact with dislocation strain fields Alloys are usually stronger than pure metals When a solid solution becomes unstable — due to a lower temperature, the two components may separate into distinct phases. A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by the addition of solutes. Alloys are (Solid Solution) mixed with one or more other elements (metallic or not). The solute may incorporate into the solvent crystal lattice substitutionally, by replacing a solvent particle in the lattice, or interstitially, by fitting into the space between solvent particles.

Strengthening mechanisms Solid-Solution Strengthening : 

Strengthening mechanisms Solid-Solution Strengthening

Phase Diagram Introduction : 

Phase Diagram Introduction

Phase Diagram Metals : 

Phase Diagram Metals The phase diagrams provide fundamental knowledge of what the equilibrium structure of a metallic alloy is as a function of composition and temperature. The real structure may not be the equilibrium one, but equilibrium gives a starting point from which other non-equilibirium structures can be often be inferred. Phase Diagram shows how different phases develop or transform at various temperature and relative concentrations. A system of two components is known as a ‘binary system’ ; one with three components as a ‘ternary system’, and so on.

Phase Transformation : 

Phase Transformation Phase transformations (change of the microstructure) can be divided into three categories: Diffusion-dependent with no change in phase composition or number of phases present (e.g. melting, solidification of pure metal, allotropic transformations, recrystallization, etc.) Diffusion-dependent with changes in phase compositions and/or number of phases Diffusionless phase transformation - produces a metastable phase by cooperative small displacements of all atoms in structure.

What is diffusion?With regards to solid solutions : 

What is diffusion?With regards to solid solutions Diffusion is the process by which atoms move around the crystal lattice Phase diagram gives stable phases BUT Diffusion rate determines how fast new phases form (if at all)

How does Diffusion happen? : 

How does Diffusion happen? Diffusion Depend on two factors: is there somewhere for the atom to move to? does the atom have enough energy to jump to its new position? As a result Diffusion rates for interstitial solute atoms are 10-109 times greater than diffusion rates for substitutional solute atoms Diffusion rates increase with increasing temperature

Fe-C Phase Diagram : 

Fe-C Phase Diagram In their simplest form, steels are alloys of Iron (Fe) and Carbon (C). C is an interstitial impurity in Fe. It forms a solid solution with α, γ, δ phases of iron Maximum solubility in BCC α-ferrite is limited (max.0.022 wt% at 727 °C) - BCC has relatively small interstitial positions Maximum solubility in FCC austenite is 2.14 wt% at 1147 °C - FCC has larger interstitial positions The Fe-C phase diagram is a fairly complex one, but we will only consider the steel part of the diagram, up to around 7% Carbon

Fe-C Phase Diagram (contd) : 

Fe-C Phase Diagram (contd) α-ferrite - solid solution of C in BCC Fe Stable form of iron at room temperature. The maximum solubility of C is 0.022 wt% Transforms to FCC γ-austenite at 912 °C γ-austenite - solid solution of C in FCC Fe The maximum solubility of C is 2.14 wt %. Transforms to BCC δ-ferrite at 1395 °C Is not stable below the eutectic temperature(727 ° C) unless cooled rapidly δ-ferrite solid solution of C in BCC Fe The same structure as α-ferrite Stable only at high T, above 1394 °C Melts at 1538 °C Fe3C (iron carbide or cementite) This intermetallic compound is metastable, it remains as a compound indefinitely at room T, but decomposes (very slowly, within several years) into α-Fe and C (graphite) at 650 - 700 °C

Fe-C Phase Diagram (contd) : 

Fe-C Phase Diagram (contd) At room temperature it exists as ferrite or α iron BCC crystal structure Mostly iron with a little carbon Relatively soft Pure Iron upon heating experiences two changes in crystal structure. When we heat it upto 912 C it experiences a transformation to austenite or γ iron FCC crystal structure Unstable at room temperature Can accommodate more carbon At 1394 C austenite reverts back to a BCC phase called δ ferrite.

Fe-C Phase Diagram (contd)Eutectic and eutectoid reactions in Fe–Fe3C : 

Fe-C Phase Diagram (contd)Eutectic and eutectoid reactions in Fe–Fe3C (Pearlite)

Equilibrium and Non Equilibrium Phase Chase Change in Isomorphous 35-65 Ni-Cu Alloy : 

Equilibrium and Non Equilibrium Phase Chase Change in Isomorphous 35-65 Ni-Cu Alloy

Fe-C Phase Diagram (contd)Microstructure of Eutectoid steel : 

Fe-C Phase Diagram (contd)Microstructure of Eutectoid steel

Fe-C Phase Diagram (contd)Microstructure of Hypo-Eutectoid steel (composition < 0.76%) : 

Fe-C Phase Diagram (contd)Microstructure of Hypo-Eutectoid steel (composition < 0.76%) Micrograph of Hypo-eutectoid steel

Fe-C Phase Diagram (contd)Microstructure of Hyper-Eutectoid steel (composition < 0.76%) : 

Fe-C Phase Diagram (contd)Microstructure of Hyper-Eutectoid steel (composition < 0.76%)

Isothermal Phase Diagram : 

Isothermal Phase Diagram

Isothermal Phase TranformationEutectoid Steel : 

Isothermal Phase TranformationEutectoid Steel

Coarse and Fine Pearlite : 

Coarse and Fine Pearlite

Complete Isothermal Transformation Eutectoid Steel : 

Complete Isothermal Transformation Eutectoid Steel

Continuous Cooling Phase Transform : 

Continuous Cooling Phase Transform

Slide 31: 

Rapidly cool to 350 º C, hold for 104 seconds and then quench to room temperature (b) Rapidly cool to 250 º C, hold for 100 seconds and then quench to room temperature (c) Rapidly cool to 650 º C, hold for 20 seconds. Rapidly cool to 400 º C , hold for 103 seconds and then quench to room temperature

Bainite : 

Bainite A grain of Bainite-lower left to upper right corner Elongated and needle like particles of Cementite in Ferrite(α ) matrix Pearlite and Bainite transformations are competitive- Reheat required to change from one to other

Spheroidite : 

Spheroidite Pearlite or Bainite are heated to below eutectoid (700ºC ) and left for 18 to 24 hrs Sphere like particles of Cementite in Ferrite matrix No change of relative amounts of ferrite and cementite –only carbon diffusion

Martensite : 

Martensite Austenite rapidly quenched to room temperature Non equilibrium single phase structure-Needle like grains Diffusionless process Transformation competitive with Pearlite and Bainite If quenching is not rapid enough, then Ferrite and Cementite phase will also form

Slide 35: 

Rapidly cool to 350 º C, hold for 104 seconds and then quench to room temperature (b) Rapidly cool to 250 º C, hold for 100 seconds and then quench to room temperature (c) Rapidly cool to 650 º C, hold for 20 seconds. Rapidly cool to 400 º C , hold for 103 seconds and then quench to room temperature

Fe-C Phase Diagram (contd)Non-diffusional phase transformation- Martensite formation : 

Micrograph of Martensite – needle like structure very hard Fe-C Phase Diagram (contd)Non-diffusional phase transformation- Martensite formation

Mechanical Properties of Fe-C alloys : 

Mechanical Properties of Fe-C alloys

Mechanical properties of Fe-C alloys : 

Mechanical properties of Fe-C alloys

Questions : 

Questions

Pure Metals : 

Pure Metals Pure metals are very soft and therefore rarely used in engineering applications

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